Molecular structure of tetraaqua adenosine 5′-triphosphate aluminium(III) complex: A study involving Raman spectroscopy, theoretical DFT and potentiometry

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 88–101

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Molecular structure of tetraaqua adenosine 50 -triphosphate aluminium(III) complex: A study involving Raman spectroscopy, theoretical DFT and potentiometry Thaís Tenório a,⇑, Andréa M. Silva a,b, Joanna Maria Ramos c, Camilla D. Buarque a, Judith Felcman a,1 a Department of Chemistry, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente, 225, Edifício Cardeal Leme, 6° andar, sala 671L, 22453-900 Rio de Janeiro, RJ, Brazil b Department of Chemistry, Instituto Federal de Educação, Ciência e Tecnologia do Rio de Janeiro, Rua Lúcio Tavares, 1045, 26530-060 Nilópolis, RJ, Brazil c Department of Inorganic Chemistry, Institute of Chemistry, Universidade Federal do Rio de Janeiro, Av. Athos da Silveira Ramos, 149, Bloco A, 6° andar, sala 630, 21941-909 Rio de Janeiro, RJ, Brazil

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

" AlATP complex participates in the

"

"

"

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pathogenic process of Alzheimer’s disease. Raman and potentiometry show that ATP acts as a bidentate ligand in [Al(ATP)(H2O)4]. The ATP´s donor atoms are one oxygen of the phosphate b and one of the phosphate c. Molecular modeling of the complex suggested a distorted octahedral structure. Complete analysis of vibrational spectra and DFT investigations confirm the result.

a r t i c l e

i n f o

Article history: Received 15 September 2012 Received in revised form 1 December 2012 Accepted 6 December 2012 Available online 14 December 2012 Keywords: Aluminium(III) Adenosine 50 -triphosphate Raman spectroscopy DFT calculations Stability constants

a b s t r a c t The Alzheimer’s disease is one of the most common neurodegenerative diseases that affect elderly population, due to the formation of b-amyloid protein aggregate and several symptoms, especially progressive cognitive decline. The result is a decrease in capture of glucose by cells leading to obliteration, meddling in the Krebs cycle, the principal biochemical route to the energy production leading to a decline in the levels of adenosine 50 -triphosphate. Aluminium(III) is connected to Alzheimer’s and its ion provides raise fluidity of the plasma membrane, decrease cell viability and aggregation of amyloid plaques. Studies reveal that AlATP complex promotes the formation of reactive fibrils of b-amyloid protein and independent amyloidogenic peptides, suggesting the action of the complex as a chaperone in the role pathogenic process. In this research, one of complexes formed by Al(III) and adenosine 50 -triphosphate in aqueous solution is analyzed by potentiometry, Raman spectroscopy and ab initio calculations. The value of the log KAlATP found was 9.21 ± 0.01 and adenosine 50 -triphosphate should act as a bidentate ligand in the complex. Raman spectroscopy and potentiometry indicate that donor atoms are the oxygen of the phosphate b and the oxygen of the phosphate c, the terminal phosphates. Computational calculations using Density Functional Theory, with hybrid functions B3LYP and 6-311++G(d,p) basis set regarding water solvent effects, have confirmed the results. Frontier molecular orbitals, electrostatic potential contour surface, electrostatic potential mapped and Mulliken charges of the title molecule were also investigated. Ó 2012 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +55 21 3527 1824; fax: +55 21 3527 1637. 1

E-mail addresses: [email protected] (T. Tenório), [email protected] (J.M. Ramos), [email protected] (C.D. Buarque). Deceased.

1386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2012.12.019

T. Tenório et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 88–101

Introduction Aluminium ion has the oxygen atom as the main binding site in biological systems so carboxylate, phenolate, catecholate and phosphate are the best ligands for this cation [1,2]. Studies [3,4] emphasize the Al(III) influence in the functioning of phosphate biomolecules, indicating that they may inhibit the action of the cholinergic enzyme acetylcholinesterase [3], and interfere in phosphate transfer reactions induced by enzymes containing magnesium(II) ion [4]. The aluminium concentration in human brain is also related to neurodegenerative diseases [5–9] such as Alzheimer’s disease that has a high incidence among older population [5], and is characterized by proliferation of neurofibrillary structures and senile plaques in brain regions [5,10] as hippocampus [11–13]. The aluminium(III) provides the neurolipofuscingenesis and changes behavior of neural activity, causing reduction in antioxidants concentration as superoxide dismutase, catalase and glutathione peroxidase, reducing the total concentration of proteins and lipids, and increasing concentration of carbonyl protein, lipid peroxidation and lipofuscin [14]. Adenosine 50 -triphosphate (ATP), Fig. S1, is a nucleotide with high-energy phosphate bonds present in all cells. It constitutes the most important form for energy storage of cells. The energy produced by the byproducts formation from the breakdown of this molecule initiates muscle contraction and attends many cytological processes such as active transport, synthesis and secretion of substances, division, among others [15,16]. One of the first papers [17] suggesting the relation between Alzheimer’s and adenosine 50 -triphosphate pointed A1-adenosine receptors accumulation in neurodegenerative structures that mediate the processing of amyloid precursor protein, and the Tau phosphorylation. A1-adenosine receptors amount is both increased in early and advanced disease, however, no difference is found through pathologic progression. Furthermore, are also raised the A2A-adenosine receptors. Although there is an accumulation of adenosine receptors, rise of bonds and levels of protein expression of the receptors are not associated with increased levels of mRNA that encodes these receptors, showing that probably there is awareness of the corresponding transduction pathways [18]. In Alzheimer’s patients, there are significant decrease in the density of agonists and antagonists of adenosine receptors in the binding sites of the molecular layer of the dentate gyrus [19]. Research [20] using immunostaining in autopsies of Alzheimer’s patients and controls showed that adenosine receptors in the hippocampus and in the cerebral cortex presented a change in expression pattern and there is receptors redistribution in these brain areas. Moreover, neurons in culture analysis reveal the evidence of the relationship between oxidative stress and changes in glucose transport. Noting that beta-amyloid protein (Ab) alters the capture of glucose, leading a decrease in ATP levels. Probably this occurs through conjugation of 4-hydroxynonenal, aldehyde derived from lipid peroxidation of n-6 polyunsaturated fatty acids, to the glucose transporter protein GLUT3, interfering in glucose transport for the Krebs cycle [21,22]. The glucose metabolism change also restricts the synthesis of acetylcholine, glutamate, aspartate, aminobutyric acid and glycine [23]. It is also known that Ab generates free radicals in Alzheimer’s and these reactive species cause lipid peroxidation, protein oxidation and loss of membrane integrity, resulting in inhibition of ATPases, waste of calcium homeostasis, inhibition in uptake system of glutamate sodium dependent into glial cells, metabolic pathways adulteration, transcription factors activation and, finally, apoptosis [21].

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The diffusion in the body of free ATP and of ATP bonded in the metal should be similar, since the molecular size of the complex is not much larger than the free ATP [24]. In Alzheimer’s and other amyloidosis etiology, the formation of amyloid fibrils has an important role. Study [25] shows that the AlATP complex promotes the formation of beta-amyloid reactive fibrils and of amyloidogenic independent peptides. Induce of fibrils formation is followed by complexation of AlATP by one or more monomers of the peptides. However, the complex formed cannot be identified directly, suggesting the action of AlATP as a chaperone in the amyloid fibrils formation, in other words, the AlATP would assist the fibrils unfolding. The effect of AlATP is not mimicked by AlADP nor by AlAMP [25]. Brain homeostasis of aluminium has an impact on neural functions, and changes in brain functions involve the potentiation of the neurotransmitters activities through the action of AlATP in brain receptors of ATP [26]. Studies on binary complexes of aluminium(III) and phosphate ligands represent a rich field of research and despite the fact that some works have been published in the literature, there is still need for deepening the knowledge on the subject. Studies on complexes formed by aluminium(III) and phosphate ligands such as adenosine-50 -monophosphoric acid [27], guanosine-50 -monophosphoric acid [27] and adenosine-50 -diphosphoric acid [28], among others can be found in the literature. ATP has been a commonly studied ligand, mainly because it is the most important form for energy storage. Some studies with ATP and other metallic ions as Ca(II) [29], Co(II) [29,30], Zn(II) [31] and La(III) [32] can be found in the literature. An attempt to widen the knowledge about the complexes formed between adenosine 50 -triphosphate and aluminium(III) is important in living organism. Investigations were performed in aqueous solution, using techniques such as potentiometry, Raman spectroscopy and also theoretical calculations for one of the complexes formed by aluminium(III) and adenosine 50 -triphosphate system ligand to metal ratio of 1:1. Material and methods Reagents All chemicals used came from analytical-reagent grade. Adenosine 50 -triphosphate disodium was purchased from Sigma–Aldrich (Missouri, USA) and used as received. Al(NO3)39H2O, HCl, NaOH, ammonium hydroxide, solochrome black indicator, KNO3, standard solutions of carbonate-free KOH 0.1 mol L1 and of EDTA Titriplex III 0.1 mol L1, standard buffer solutions of pH 4.0 and 7.0 were purchased from Merck (Darmstadt, Germany). The solutions were prepared freshly each day with ultrapure and CO2-free water. Equipments Potentiometric data were obtained using a system consisted by a titrando automatic microburette (model 809, Metrohm, Herisau, Switzerland), a stirrer (model 801, Metrohm), a dosing system (800 Dosino, Metrohm) and a combined glass electrode. This system was coupled with personal computer and operated using software Tiamo. Raman spectroscopic analysis were made using a Raman Spectrometer Model 400 (Perkin Elmer, Massachusetts, USA) with a high performance stabilized 785 nm diode laser with signal-tonoise >40 dB and stability of 0.1 nm. The instrument was coupled with personal computer running with software Spectrum. Theoretical calculations were performed using Gaussian 03 W program [33], GaussView molecular visualization programs [34,35] and ChemCraft program [36].

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Procedures The purity of adenosine 50 -triphosphate was verified by determining the exact concentrations of their solutions using the Gran method [37]. The concentrations of Al(III) solutions were obtained via back-titration with standard solution of EDTA 0.1 mol L1 using solochrome black indicator and ammonia buffer solution [38]. Potentiometry In potentiometry, the glassware used to prepare solutions was cleaned with alcoholic KOH, HCl 1:20 v/v solution or NaOH 50% w/w solution. The ionic strength of all measured solutions was adjusted to 0.1 mol L1 with KNO3 in acidic medium. Aluminium(III) nitrate solution (0.01 mol L1) and adenosine 50 triphosphate solution (0.001 mol L1) were prepared by dissolving reagents in ultrapure and CO2-free water. For the determination of adenosine 50 -triphosphate acid dissociation constants, an aqueous solution of protonated ligand (0.001 mol L1) was titrated with 0.1 mol L1 KOH. For the determination of the stability constants of the aluminium(III) complexes, samples with solutions containing adenosine 50 -triphosphate (0.001 mol L1) and Al(III) (0.01 mol L1) were titrated at metal–ligand ratio (M:L) of 1:1 with 0.1 mol L1 KOH under the same conditions of the ligand titration. All titrations were performed at 25.0 ± 0.1 °C by coupling the titration cell to a thermostatic bath. The stability constants of the ligand and complexes were determined using the Hyperquad 2000 program [39]. Validation curves and species distribution diagrams were determined using the Hyss 2006 program [40]. The ionic product of water was found to be pKw = 13.77. Hydrolysis constants for Al(III) were obtained from literature [41,42] and employed in the calculations. The constants values were: 5.33 for AlOH, 10.91 for Al(OH)2, 17.3 for Al(OH)3, 13.13 for Al3(OH)4 and 107.47 for Al13(OH)32.

B3LYP/6-311G(d) [48], by 0.9619 for B3LYP/6-311G(d,p) [47], and by 0.9580 for B3LYP/6-311G+(d,p) and B3LYP/6-311G++(d,p) [49]. The scaling factors were employed to compensate the systematic error caused by neglecting harmonicity and electron correlation. In the paper only the results calculated in DFT:B3LYP/6311G++(d,p) were exposed. The results calculated by other bases can be found in Tables S1 and S2. The Raman scattering activities (Si) calculated with Gaussian [33] were converted to relative Raman intensities (Ii) using the relationship derived from the theory of Raman scattering [50,51].

Ii ¼

f ðm0  mi Þ4 Si mi ½1  expðhcmi =kTÞ

where m0 is the laser exciting wavenumber (cm1), mi the vibrational wavenumber of the ith normal mode (cm1), h, c and k are universal constants (Planck constant, speed of light and Boltzmann constant), f is a suitably chosen common normalization factor for all peak intensities, and T is temperature (Kelvin). The scaled calculated Raman spectra of the ligand and the complex were made using pure Lorentizian band shape with a bandwidth of 10 cm1 along Raman spectra. In addition, frontier molecular orbitals (FMO’s), molecular electrostatic potential (MESP) and others properties were performed with the DFT:B3LYP/6-311++G(d,p) for the [Al(ATP)(H2O)4]. The FMO’s and MESP of the ligand were performed with the same level of theory. Efforts to synthesize some complex of Al(III):ATP system in solid state were done, but unfortunately the successful could not be achieved yet. Results and discussions Geometrical optimization

Raman Spectroscopic analyses All measurements were carried out at room temperature (25.0 ± 0.1 °C) in the dark under the following conditions: 100 mW of laser power, 2 cm1 of resolution, 20 s exposure time and accumulation of 100 scans. The binary system had ligand to metal ratio of 1:1. Several spectra with different resolutions and number of scans were made in order to select the most suitable one for analysis. Subsequently, the procedure was repeated with the analysis conditions established. All the spectra were baseline corrected by subtracting smoothed backgrounds. Aluminium(III) nitrate and adenosine 50 -triphosphate solutions, all in 0.5 mol L1, were prepared by dissolving reagent in ultrapure and CO2-free water. Measurements were made on a 4.0 mL quartz cuvette. All Raman spectra were obtained focusing the laser directly on the sample aqueous solutions. The Raman spectra from ATP (aqueous solution) and from Al(III):ATP solution at a 1:1 M:L ratio were made in solutions under a specific pH chosen according to the speciation studies. Raman spectrum from the solid ligand was also obtained. DFT calculations In this study quantum chemical calculations were made using density functional theory (DFT) with B3LYP (Becke’s three parameter hybrid functional for the exchange part and the Lee–Yang– Parr (LYP) correlation function) [43–46] hybrid functions and the bases 6-311G, 6-311G(d), 6-311G(d,p), 6-311+G(d,p) and 6311++G(d,p) for structures of ligand and binary system. All calculations have considered the molecules in water solvent with polarizable continuum model (PCM). The optimizations and calculated vibrational spectra were made in the same procedure. The calculated vibrational wavenumbers of the optimized molecules were scaled by 0.9613 for B3LYP/6-311G [47], by 0.9614 for

The geometric parameters results for partially deprotonated adenosine 50 -triphosphate and [Al(ATP)(H2O)4] complex can be seen in Table 1. These data were obtained using DFT:B3LYP/6311++G(d,p) procedure for ATP3 and for the complex. The comparison of the bond lengths of atoms in the ATP3 with that in the complex indicated small discrepancies, with higher differences in values of atoms triphosphate region due to the conformational change occurred in the ligand structure for the formation of a ring with the metal ion. In general, bond lengths showed differences no higher than 0.010 Å. The more discrepant bond lengths, indicated in bold in the Table 1, presented differences that exceed 0.010 Å and were O(38)AP(37), O(40)AP(37), O(36)AP(37), O(41)AP(35), O(39)AP(35), O(34)AP(35), O(2)AP(1), O(3)AP(1), O(4)AP(1) and O(4)AC(5), and correspond to a relative error of 2.66%, 3.63%, 1.09%, 2.59%, 1.73%, 2.55%, 0.92%, 0.80%, 0.97% and 0.97% respectively, all being below 4.00%. The O(40)AP(37) shows the greatest difference, of 0.060 Å. Al(III) bonds are those with the oxygen of the c phosphate (1.848 Å), with the oxygen of the b phosphate (1.847 Å) and with the oxygen from coordinated water (average of 1.949 Å). Bonds between aluminium ion and phosphates have slight discrepancy, being the bond with the b phosphate 0.001 Å shorter than the bond with the c phosphate, suggesting a stronger bond through the oxygen atom of the b phosphate. It is also noticed that the average of AlAO bonds (coordinated water) is 0.102 Å greater than the average of AlAO bonds (phosphates). Regarding bonds of aluminium ion with the oxygen of coordinated water, it can be seen that the AlAO(46) bond is 1.893 Å, being the smallest of the bonds of this type existing in the complex, it should be noted that the water molecule in question is near to oxygen of a phosphate.

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T. Tenório et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 88–101 Table 1 Calculated geometrical parameters (bond lengths in Å and bond angles in degrees) by the DFT:B3LYP/6-311++G(d,p) for the [Al(ATP)(H2O)4] complex and partially deprotonated adenosine 50 -triphosphate (ATP3). Bond length (Å)

[Al(ATP)(H2O)4]

ATP3

Al(45)AO(38) Al(45)AO(41) Al(45)AO(46) Al(45)AO(47) Al(45)AO(48) Al(45)AO(49) O(38)AP(37) O(42)AP(37) O(40)AP(37) O(36)AP(37) O(36)AP(35) O(41)AP(35) O(39)AP(35) O(34)AP(35) O(34)AP(1) O(2)AP(1) O(3)AP(1) O(4)AP(1) O(4)AC(5) C(5)AC(8) C(8)AC(27) C(29)AC(27) C(29)AC(11) O(10)AC(11) O(10)AC(8) C(27)AO(32) C(29)AO(31) C(11)AN(13) C(14)AN(13) C(26)AN(13) C(26)AC(17) C(14)AN(16) N(16)AC(17) C(26)AN(25) C(23)AN(25) C(18)AC(17) C(18)AN(22) C(23)AN(22) C(18)AN(19) O(46)AH(50) O(46)AH(51) O(47)AH(52) O(47)AH(53) O(48)AH(54) O(48)AH(55) O(49)AH(56) O(49)AH(57) O(40)AH(43) C(5)AH(6) C(5)AH(7) C(8)AH(9) C(27)AH(28) C(29)AH(30) C(11)AH(12) C(14)AH(15) C(23)AH(24) N(19)AH(20) N(19)AH(21)

1.848 1.847 1.893 1.970 1.958 1.975 1.544 1.501 1.594 1.634 1.651 1.546 1.481 1.604 1.660 1.520 1.492 1.635 1.442 1.523 1.528 1.531 1.537 1.410 1.459 1.425 1.421 1.469 1.382 1.376 1.394 1.312 1.381 1.349 1.342 1.411 1.352 1.332 1.342 0.982 1.033 0.967 0.967 0.971 0.967 0.994 0.966 0.967 1.091 1.093 1.093 1.092 1.091 1.095 1.078 1.085 1.008 1.007

– – – – – – 1.503 1.507 1.654 1.652 1.653 1.506 1.507 1.646 1.655 1.506 1.504 1.651 1.428 1.518 1.532 1.532 1.546 1.406 1.458 1.428 1.412 1.471 1.379 1.375 1.395 1.314 1.382 1.344 1.336 1.411 1.350 1.337 1.347 – – – – – – – – 0.965 1.094 1.096 1.093 1.093 1.093 1.096 1.078 1.085 1.008 1.007

Bond angles (degrees) O(38)AAlAO(41) O(38)AAlAO(46) O(38)AAlAO(47) O(38)AAlAO(48) O(38)AAlAO(49) O(41)AAlAO(46) O(41)AAlAO(47) O(41)AAlAO(48) O(41)AAlAO(49) O(46)AAlAO(47) O(46)AAlAO(48) O(46)AAlAO(49) O(47)AAlAO(48)

[Al(ATP)(H2O)4] 94.04 93.20 93.30 174.16 88.81 95.39 172.53 88.31 94.83 85.61 91.90 169.42 84.25

ATP3 – – – – – – – – – – – – –

Table 1 (continued) Bond length (Å)

[Al(ATP)(H2O)4]

ATP3

O(47)AAlAO(49) O(48)AAlAO(49) AlAO(38)AP(37) AlAO(41)AP(35) P(37)AO(36)AP(35) P(35)AO(34)AP(1) O(34)AP(1)AO(4) P(1)AO(4)AC(5) O(4)AC(5)AC(8) O(10)AC(8)AC(27) C(27)AC(29)AC(11) C(11)AN(13)AC(26) N(13)AC(26)AN(25) C(26)AN(25)AC(23) C(26)AN(13)AC(14) N(13)AC(14)AN(16) C(26)AC(17)AC(18) N(25)AC(23)AN(22) N(19)AC(18)AN(22)

83.90 85.66 123.47 139.00 127.00 137.85 95.01 121.67 111.47 105.41 100.96 126.10 128.55 111.88 105.91 113.40 116.60 128.17 118.83

– – – – 135.04 135.96 97.92 120.38 109.61 105.85 101.54 125.45 128.26 112.07 106.04 113.39 116.29 128.02 118.83

Regarding the Al(III) coordination with the ATP ligand, several studies indicate that coordination takes place almost exclusively through the terminal phosphates (b and c) [28,52,53] such as occurred in this complex in study which the ligand acts as bidentate through the oxygen atoms of the terminal phosphates b and c. With respect to the skeleton, in complex with adenosine 50 -triphosphate, aluminium ion acts with coordination number six, adopting octahedral structure the same arrangement found in other studies [52,53]. In chelate is formed only a six-membered ring with the metal ion and the structure is distorted octahedral, as shown by bond angles in bold displayed in Table 1. Planar fragments centered on Al(III) that define the distorted octahedral are O(38)AO(41)AO(48)AO(47), O(38)AO(46)AO(48)AO(49) and O(41)AO(46)AO(47)AO(49). The sums of the angles on the plane are, respectively, 346.69°, 343.58° and 341.95°. The structural geometries of adenosine 50 -triphosphate and [Al(ATP)(H2O)4] are present in Fig. 1a and b. This species was particularly studied because of its great predominance in the acidic pH range before starting an intense hydrolysis. Potentiometric studies The adenosine 50 -triphosphate macroscopic dissociation constants are known and have been recalculated in the experiment conditions used. The values of macroscopic dissociation constants of ligand (as pKa values) are 4.24 ± 0.01 for pK1 and 6.81 ± 0.01 for pK2, which are in concordance with values, 4.25 and 6.50, found in literature [54]. The pK1 refers principally to N1 of the adenine base deprotonation and pK2 refers mostly to terminal phosphate group (phosphate c). The dissociation constants values are relatively close, but is already established [28,52,55] the predominant participation of each group in the dissociations constants. In potentiometry, the stability of the considered binary complexes can be estimated by the general equilibrium, Eq. (1), formed in complexation: 3þ

a½Al  þ b½Hþ  þ c½ATP $ Ala Hb ATPc

bAlaHcATPc

ð1Þ

The stability constants of the binary complexes can be presented through Eq. (2): 3þ

log babc ¼ ½Ala Hb ATPc =½Al a ½Hþ b ½ATPc

ð2Þ

Aluminium(III):adenosine 50 -triphosphate system at metal–ligand ratio of 1:1 was investigated through numerous models calculated

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Fig. 1. Molecular modeling of structures and atoms numbering of the adenosine 50 -triphosphate ligand (a) and of the tetraaqua adenosine 50 -triphosphate aluminium(III) complex, [Al(ATP)(H2O)4] (b).

Table 2 Stability constants of formation (log b) of binary complexes (AlIII:ATP 1:1) at 25 °C and I = 0.1 mol L1 (KNO3). Species

Log b

AlATPH2 AlATPH AlATP AlATPOH Al2ATP2(OH)

15.68 ± 0.02 13.04 ± 0.01 9.21 ± 0.01 2.80 ± 0.02 16.16 ± 0.01

by Hyperquad 2000 [39] and the possibility of complexion formation was evaluated. The model with better results for this system was found with stability constants of proton (AlATPH2 and AlATPH), metal (AlATP) and hydroxy (AlATPOH and Al2ATP2OH) complexes with values indicated in Table 2. Other species models regarding AlATP2 and AlATP3 were proposed, but these did not show a good concordance. The Al(III):ATP system has some researches [28,52,56,57] in literature including those using potentiometry, but constantly divergent values has been found, and study [32] which examinee several constant values for different systems not pointed recommended values for any complex of this system. The KAlATP values discovered for other researchers [28,52,56,57] have a range of 2.40–8.40, considering the 25 °C of temperature and 0.1 mol L1 of strength ionic. The KAlATP found in this work was 9.21 ± 0.01, higher than the values already discovered for the complex, but it must be considered that the values previously found vary widely. Besides that, it was not observed turbidity in the system, so the titrations were performed until the base volume of 3.50 mL, reaching to end of the titration, with pH around 8.0. In some studies already mentioned, titrations considered stopped until pH 4.5, when the turbidity in the system began. The best model of stability constant calculations also regards the Al2ATP2OH species that is detected according to studies [53], in the concentrations similar were used in this work, by 1H NMR at neutral pH together with the AlATP species M:L ratio 1:1, beyond other hydrolyzed species, the 31P NMR analysis reveals that Al2ATP2OH species present each aluminium(III) ion coordinated with ligand in a bidentate form by oxygen atoms of b and c phosphates. This dimmer would form through effect of bases attraction. Titration curves of adenosine 50 -triphosphate ligand and binary system with aluminium(III) are seen together in Fig. S2. It noticed that curves have totally distinct patterns as expected and, when titration starts, the binary species of complex are already present. It happens because at pH that titration begins, above pH 4.00, some amount of ATP is already deprotonated and the donor atoms from

molecule are free to coordinate with metal ion. The species model was validated by the titration simulation with the stability constants of all the species proposed using the Hyss 2006 program [40]. This calculated titration curve and the respective experimental curve to binary system, Fig. S3, presented a good agreement, but a little deviation can be observed close to pH 7.5, which is due to hydrolyzed species presence, since the aluminium ion is easily hydrolysable, and at pH next to 5.5 in system hydrolyzed species are predominant – making hard to determine the species accurately. Considering the value obtained for log KAlATP of 9.21, adenosine 50 -triphosphate should acts as a bidentate ligand in the chelate, binding to the metal by one oxygen atom of the b phosphate and by one oxygen atom of the c phosphate. In Fig. 2a it is noticed that the speciation curve for the Al(III):ATP system M:L 1:1 in the potentiometry solution concentrations, and can be observed that the species which predominates at the beginning of the titration (pH 3.7) is the AlATPH whose proton can be associated to the phosphate group. It is also observed that the formation of the AlATP in start of the titration, in pH range 3.9–5.2, this species is predominant. The hydrolyzed species can be seen shortly after the beginning of the titration in small proportion and become predominant from pH 5.5. In physiological pH there is the coexistence of hydrolyzed species of the complex with hydrolyzed species of the metal ion. Vibrational results Raman spectra have been obtained from solutions at specific pH value. The adenosine 50 -triphosphate ligand was analyzed at pH 2.5, in order to obtain a higher concentration of totally protonated ligand while the Raman spectra from binary system was performed at pH 3.0 since in this concentration had a greater amount of AlATPH complex. The species distribution diagram as a function of pH for AlIII:ATP binary system in the M:L ratio 1:1 made with concentration solution of Raman spectroscopy can be seen in Fig. 2b. The Raman spectra of the adenosine 50 -triphosphate ligand in aqueous solution, in solid state and the calculated one are seen in Fig. 3. In Fig. 3.2a it is shown the Raman spectrum of [Al(ATP)(H2O)4] in aqueous solution at pH = 3.0. In Fig. 3.2b it can be seen the Raman spectrum calculated DFT:B3LYP/6311++G(d,p) of [Al(ATP)(H2O)4] in water solvent while Fig. 3.2c shows the 3500–2900 cm1 region of the Raman spectrum of [Al(ATP)(H2O)4] and Fig. 3.2d shows the 1800–100 cm1 region of the Raman spectrum of [Al(ATP)(H2O)4] (in both spectra the deconvolution analysis were made). In Fig. 3.2e it is observed the 3500– 2500 cm1 region of the Raman spectrum of [Al(ATP)(H2O)4] (in black) and the second derivative band positions (in gray).

T. Tenório et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 105 (2013) 88–101

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Fig. 2. Diagrams of distribution of species as a function of pH for AlIII:ATP binary system in the M:L ratio 1:1. Solutions in the concentration of potentiometry (a) and solutions in the concentration of Raman spectroscopy (b).

Fig. 3.2f indicates the 1800–100 cm1 region of the Raman spectrum of [Al(ATP)(H2O)4] (in black) and the second derivative band positions (in gray). In Table 3 there are experimental and calculated Raman vibrational spectra data for the complex [Al(ATP)(H2O)4]. The experimental and calculated wavenumbers of spectra presented good agreement. The normal modes 3n  6 = 165 of [Al(ATP)(H2O)4] can be divided into 71 stretching (including coordinates of redundancy), 173 bending (including coordinates of redundancy) and 13 torsions. Except for stretching OH, NH and CH in the region of high vibrational 3700–2300 cm1 and for some adenine stretching, stretching can be found as coupled modes, because there are other internal coordinates describing the vibrational mode. Overlapping bands are expected in the vibrational spectrum, so to assist in the bands characterization, deconvolution analysis, calculation of second derivative and curve-fitting deconvolution analysis were used. The curve-fitting deconvolution of the Raman spectrum of [Al(ATP)(H2O)4] in the 1450–1200 cm1 region can be seen in Fig. S4a. The correlation coefficient obtained was R2 = 0.999993. Fig. S4b shows the curve-fitting deconvolution of the Raman spectrum of [Al(ATP)(H2O)4] in the 600–300 cm1 region. The correlation coefficient obtained in this curve-fitting was R2 = 0.999999. The peaks observed with the curve-fitting deconvolution analysis were 1469 cm1, 1426 cm1,1402 cm1, 1380 cm1, 1365 cm1, 1349 cm1, 1303 cm1, 1244 cm1, 582 cm1, 548 cm1, 529 cm1, 509 cm1, 488 cm1, 468 cm1, 440 cm1, 418 cm1, 383 cm1, 363 cm1, 331 cm1 and 261 cm1. Both manual subtractions of ultrapure water spectra and solution of aluminium nitrate were made. Through both the manual subtractions ultrapure water and solution of aluminium nitrate spectra of the complex were attributed to the water or nitrate present in solution, in the experimental spectrum of the complex the band 3410 cm1. In the deconvoluted experimental spectrum of the complex the bands were 3335 and 3380 cm1. In second derivative of the complex experimental spectrum the bands were 1640 and 3335 cm1. Assignments of the bands Vibrational modes m(OH), m(NH) and m(CH). In the region between 3700 and 2400 cm1 vibrational representation presented 21 vibrational modes (There are eleven AOH groups, one ANH2 group, one ACH2 group and six ACH group) that are subdivided as COH = 11 A, CNH = 2 A and CCH = 8 A.

Vibrational modes m(OH)(H2O), m(OH)(Pc) and m(OH)(ribose). As in the complex there are four coordinated water molecules, one phosphate group AOH and two ribose groups AOH, as a whole there are eleven stretching of AOH group. In Fig. 3.2b of binary system Al:ATP spectrum there are more bands in the region 3700–2400 cm1 that in Fig. 3.1c of ligand spectrum, indicating the presence of coordinated waters in the complex, which is confirmed from the calculated wavenumbers scaled with DFT:B3LYP/6-311++G(d,p) which represent stretching of coordinated water at 3686, 3663, 3661, 3624, 3443, 3338, 3039 and 2430 cm1. These bands are observed in the second derivative of the spectrum of the complex at 3468, 3217, 3186, 2942 and 2546 cm1. It can be seen that there are vibrational modes m(OH)(H2O) at 2546 and 2942 cm1 which are seemingly a discrepancy as the band of wavenumbers cited by the references [58–60] – in other words, these values would not be plausible. However, by analyzing the atoms of these modes it shows they suffer interference, respectively, of the oxygen of a phosphate and of the oxygen of c phosphate. This way, justified the difference between the wavenumbers 2546 and 2942 cm1 and the range of AOH group stretching. Regarding the m(OH)(P) of complex is observed one stretching concerning of the c phosphate group, m(OH)(Pc). The scaled calculated wavenumber, there is 3613 cm1. In the experimental spectrum of the complex this band is not noticed. In the second derivative of the spectrum there is 3429 cm1. In relation to the m(OH)(ribose) in the scaled calculated spectrum of the complex these are found at 3564 and 3136 cm1. In the deconvolved spectrum of the complex can be observed two bands at 3495 and 3205 cm1. Vibrational modes m(NH). The m(NH)(NH2) has as attribute two bands, an asymmetrical and other symmetrical [61]. These can be seen in the scaled calculated spectrum at 3548 and 3641 cm1. In the deconvolved spectrum of the complex is observed only a band relative on the symmetric stretching at 3417 cm1. Vibrational modes m(CH)(adenine), m(CH)(ribose) and m(CH)(CH2). With respect to the m(CH) is expected to see eight bands related to ATP ligand in the complex, two corresponding to m(CH)(adenine), two relative to the m(CH)(CH2) and four of the m(CH)(ribose). The values of scaled calculated spectrum for the bands of m(CH) are 3249, 3121, 2976, 2965, 2954, 2948, 2924 and 2907 cm1. These can be correlated to the bands found in the experimental spectrum at 3122, 3019, 2960, 2880, 2831, 2790 and 2670 cm1 and in the deconvoluted experimental spectrum at 3242, 3157,

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Fig. 3. 3.1 Raman spectra of the adenosine 50 -triphosphate ligand in solution (a), in solid state (b) and calculated (c) and 3.2 Raman spectrum of [Al(ATP)(H2O)4] in solution at pH = 3.0 (a), Raman spectrum calculated (DFT:B3LYP/6-311++G) of [Al(ATP)(H2O)4] in water solvent (b), Raman spectrum of [Al(ATP)(H2O)4] in the 3500–2900 cm1 region with band deconvolution analysis (c), in the 1800–100 cm1 region with band deconvolution analysis (d), Raman spectrum of [Al(ATP)(H2O)4] (in black) in the region of 3500–2500 cm1 showing the second derivative band positions (in gray) (e) and in the 1800–100 cm1 region showing the second derivative band positions (in gray) (f).

3080, 3007, 2970 and 2941 cm1. The DFT calculations have good agreement with the experimental values. Vibrational modes m(CN)(adenine). Study indicates that in cyclic conjugated systems, the absorption position of C@N is not clear and m(C@N) would be in a range from 1660 to 1480 cm1 [62]. In the scaled calculated spectrum of the complex are observed five bands belonging to the m(CN)(adenine). They are located at 1542, 1527, 1457, 1317 and 1262 cm1. In the deconvolved spectrum it can be observed only two bands at 1512 and 1480 cm1. Notice that the values of the stretching m(CN) of adenine are lower than expected for the m(C@N). It happens because of the

double bond resonance at adenine, which makes the C@N bond weakens so their wavenumbers appear in a region lower than expected for the m(C@N). This was confirmed by DFT calculations for the vibrational spectrum. A Raman study [63] with the ATP in aqueous solution indicated that m(CN)(adenine) are found between 1600 and 1220 cm1 and in this work these bands were found between 1590 and 1290 cm1, exhibiting a good agreement with previous research. This also shows that adenine does not participate in coordination of the complex, because comparing ligand and complex does not have displacement.

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Table 3 Experimental and calculated [DFT:B3LYP/6-311++G(d,p)] Raman vibrational spectra for the complex [Al(ATP)(H2O)4]. Mode nos.

Experimental wavenumbers (cm1) Observed

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72

3458 3260 3174 3122 3019

2960 2880 2831 2790 2670 2335 1754 1682 1610 1566

Deconvolution

3495 3417 3315 3289 3242 3205 3157 3118 3080 3007 2970 2941

1765 1733 1700 1671 1556

1510

1512 1480

1386

1379

3468 3429 3397 3280 3217 3186 3125 3057 2993 2942 2885 2850 2793 2740 2691 2625 2546 1750

1671 1585 1524 1490 1440

1235

1187

1202

1147 1112

1116

1054

1068

1028

978

917

SRaa

Approximate assignments

3686 3663 3661 3641 3624 3613 3564 3548 3443 3338 3249 3136 3121 3039 2976 2965 2954 2948 2924 2907 2430 1631 1590 1585 1574 1566 1542 1527 1457 1436 1432 1406 1383 1380 1368 1345 1342 1334 1317 1300 1296 1289 1278 1262 1257 1239 1222 1209 1204 1181 1171 1171 1144 1128 1083 1080 1072 1052 1042 1038 1030 1023 1006 998 983 970 957 947 947 925 924 892

41.98 114.31 87.27 139.97 146.52 152.86 93.72 460.77 432.14 198.85 504.75 160.13 150.74 239.94 176.12 175.64 218.64 411.31 209.36 304.74 159.41 0.85 7.82 5.95 24.91 1.92 18.44 140.79 149.60 85.20 16.70 10.64 49.54 13.31 2.64 38.78 2.10 34.40 138.24 9.49 19.85 45.53 33.21 46.35 89.77 16.10 17.34 67.33 42.69 9.66 29.63 6.38 31.14 12.09 3.88 7.98 11.26 3.58 6.26 18.90 14.16 10.43 18.14 7.58 4.75 12.29 5.73 10.58 2.46 2.09 3.29 9.45

mas(OH)(H2O) m(OH)(H2O) mas(OH)(H2O) mas(NH) ms(OH)(H2O) m(OH)(Pc) m(OH)(ribose) ms(NH) ms(OH)(H2O) m(OH)(H2O) m(CH)(adenine) m(OH)(ribose) m(CH)(adenine) m(OH)(H2O) mas(CH)(CH2) m(CH)(ribose) m(CH)(ribose) m(CH)(ribose) m(CH)(ribose) ms(CH)(CH2) m(OH)(H2O)

2nd derivative

1286

1191

Scaled theoretical wavenumbers (cm1)

927

d(HOH)sciss d(HOH)sciss d(HOH)sciss d(HNH)sciss + m(CANH2) d(HOH)sciss d(HNH) + m(CC)(adenine) + m(CN)(adenine) m(CC)(adenine) + m(CN)(adenine) m(CN) + m(CC)(adenine) + m(CN)(adenine) d(HCH)sciss d(CH)(adenine) d(OH)(ribose) d(CH)(ribose) d(CH)(ribose) d(CH)(ribose) + d(CH)(adenine) x(HCH) d(adenine) + d(CH)(adenine) d(CH)(ribose) m(CN)(adenine) + d(CH)(ribose) d(CH)(ribose) d(OH)(ribose) + d(CH)(ribose) d(OH)(ribose) + d(CH)(ribose) d(HCH)twist + d(OH)(ribose) + d(CH)(ribose) d(CH)(ribose) + m(CN)(adenine) d(CH)(ribose) d(adenine) + q(NH2) d(HCH)twist + d(CH)(ribose) d(adenine) d(OH)(ribose) + d(CH)(ribose) d(adenine) + q(NH2) m(CO)(ribose) + d(adenine) q(OH)(H2O) d(ribose) + q(OH)(H2O) d(ribose) + x(HOH) d(adenine) + d(ribose) q(CH2) + d(OH)(Pc) d(OH)(Pc) m(CC)(ribose) + m(OPb) d(ribose) q(OH)(H2O) + m(OPa) + d(ribose) q(OH)(H2O) + m(CC)(ribose) m(OPa) + x(HOH) q(CH2) + m(CO)(ribose) + x(HOH) q(NH2) + d(adenine) m(OPc) + q(OH)(H2O) m(CC)(ribose) + m(PAOAC) + q(CH2) d(CH)(adenine) m(PaOPb) + d(OH)(Pc) + q(OH)(H2O) m(OPa) + q(OH)(H2O) m(PbO) + m(PcO) + q(OH)(H2O) d(ribose) d(adenine) (continued on next page)

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Table 3 (continued) Mode nos.

Experimental wavenumbers (cm1) Observed

73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145

859

815

721

Deconvolution

859

815

873

820

768

783

721

740

660

675

631

628

584

577

505

515

472

461

433

407

400

375 360

360 342

313

306

281

248

223

190

SRaa

Approximate assignments

887 881 873 846 839 833 819 815 774 766 746 710 703 687 683 681 673 657 655 652 648 628 607 597 585 559 555 552 543 537 521 516 514 503 482 465 454 446 432 431 415 408 396 384 376 367 357 346 341 331 320 314 310 308 301 290 289 284 271 263 251 245 244 243 235 226 224 217 214 207 198 191 186

14.12 11.88 3.52 14.30 11.97 4.87 4.75 5.55 5.35 4.32 23.01 42.06 14.58 10.09 8.34 1.58 4.13 0.50 4.00 13.62 8.24 1.17 7.79 10.80 7.69 0.38 0.50 2.16 9.69 1.59 7.26 1.11 4.60 2.31 2.58 2.90 3.70 3.37 5.22 7.06 2.69 0.73 1.74 4.29 2.86 1.17 2.88 2.98 4.76 1.36 1.10 1.22 2.34 1.00 3.37 4.38 2.77 2.05 0.98 3.10 1.29 2.14 2.36 2.09 0.24 1.38 0.82 1.41 0.52 1.51 3.00 0.38 5.01

m(PaOPb) + m(PbOPc) + q(OH)(H2O) m(Pa@O) + m(Pb@O) + m(Pc@O)

2nd derivative

552

444

Scaled theoretical wavenumbers (cm1)

234

d(CH)(adenine)

m(phosphates) + d(OH)(Pc) + q(OH)(H2O) q(OH)(H2O) + d(CH)(adenine) + d(OH)(ribose) m(phosphates) + d(CH)(adenine) + q(OH)(H2O) d(HCH)twist + m(CO)(ribose) d(adenine) + d(HOH)twist

q(OH)(H2O) + d(OH)(ribose) d(HOH)twist + q(OH)(H2O) x(HOH) d(ribose) + x(HOH) m(PcOH) + m(PbOPc) + q(OH)(H2O) m(PbOPc) + q(OH)(H2O) m(adenine) x(HOH) + q(OH)(H2O) + m(PcOH) d(adenine) d(adenine) + d(ribose) d(adenine) Coupled modes x(HOH) d(adenine) + d(ribose) + d(CHC) Coupled modes m(H2OAAl) + d(AlAOH2) + x(HNH) m(AlAO)(Pc) + d(adenine) + x(HNH) m(AlAO)(Pb) + x(HNH) + x(HOH) d(HNH)twist d(HNH)twist q(NH2) + d(AlAOH2) x(HNH) m(H2OAAlAO)(Pc) + d(H2OAAlAOH2) + x(HOH) m(adenine) d(AlAOH2) + m(ribose) d(H2OAAlAO)(Pc) + d(H2OAAlAO)(Pb) + d(H2OAAlAOH2) d(AlAO)(Pc) + d(AlAO)(Pb) + d(PbOPc) d(HOH)twist + x(HOH) d(ring) m(H2OAAlAO)(Pc) + m(H2OAAlAO)(Pb) + m(H2OAAl) + x(HOH) d(ring) + d(phosphates) + d(ribose) Coupled modes m(AlAOH2) + d(AlAOH2) + d(HOH)twist d(HOH)twist d(AlAO)(Pc) + d(OPc) + d(OPa) + d(O@Pa) d(PaOPb) + d(OH)(ribose) m(AlAOH2) + d(AlAO)(Pb) + d(AlAO)(Pc) m(AlAOH2) + d(ring) d(OPa) + d(OPb) + d(OPc) + d(PbOPc) d(AlAOH2) + d(ring) + d(H2CAO) d(H2CAO) + d(HOH)twist Coupled modes d(HOH)twist m(AlAO)(Pc) + d(AlAO)(Pb) + d(AlAOH2) d(H2OAAlAOH2) + q(CH2) Coupled modes d(H2OAAl) + d(AlAO)(Pb) d(CANH2) d(adenine) + d(H2OAAlAOH2) + d(H2OAAlAO)(Pb) + d(H2OAAlAO)(Pc) d(OH)(Pc) + d(H2OAAl) + d (ring) d(OH)(Pc) + d(H2OAAl) Coupled modes Coupled modes Coupled modes Coupled modes Angular distortion (ring) d(OH)(Pc) + d(OH)(ribose) Coupled modes d(AlAOH2) + d(OH)(ribose) d(adenine) Angular distortion (ring)

s d(OH)(Pc) + d(OH)(ribose) d(PcAOH) + d(OH)(Pc) Coupled modes

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Table 3 (continued) Mode nos.

Experimental wavenumbers (cm1) Observed

146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 a

Deconvolution

108

111

SRaa

Approximate assignments

177 173 162 148 145 140 133 124 118 116 92 80 75 65 60 58 45 36 25 15

1.49 1.56 0.23 0.21 0.36 0.40 0.16 0.59 0.74 0.59 0.31 0.13 1.94 1.09 4.43 5.28 5.04 5.75 1.07 3.71

Coupled modes Angular distortion (ring) Angular distortion (ring) Coupled modes s(H2O)

2nd derivative

165

136

Scaled theoretical wavenumbers (cm1)

s s(H2O) s Coupled modes Coupled modes

s s s s s d(adenine) d(adenine)

s s s

Raman activities are calculated in A4 amu.

Vibrational modes m(CANH2). Only a stretching relevant to the CANH2 bond is displayed in the complex. The m(CANH2) of adenine is located around 1600 in both the spectrum of the complex, as the spectrum of the ligand. In the scaled calculated spectrum, it can be found for the ATP in 1650 cm1 and for the complex at 1574 cm1, both with medium activity. This is seen in the deconvoluted experimental spectrum of the complex at 1671 cm1 and in the experimental spectrum of the ligand in the solid state at 1610 cm1. Vibrational modes m(CC) and m(CO). The CO and CC stretching have a great physical mixing of internal coordinates. For the [Al(ATP)(H2O)4] complex these vibrational modes are found between 1590 and 840 cm1 and have a strong degree of coupling, the assignments may be noted in Table 3. In the second derivative spectrum these modes are coupled, showing m(CC)(adenine) at 1524 and 1490 cm1 and m(CC)(ribose) at 1068 cm1. Vibrational mode m(PAOAC). Accordingly literature [64], the m(PAOAC) is found between 1050 and 970 cm1 what is in a good agreement with the m(PAOAC) shown in the scaled calculated spectrum at 970 cm1. Vibrational modes m(P@O). Normal modes for the phosphates have a high degree of coupling. In the scaled calculated spectrum, the m(Pa@O) presents coupled to m(Pb@O) and m(Pc@O) at 881 cm1. These are in agreement with literature [65] that indicates the phosphate vibrations between 1125 and 680 cm1. Vibrational modes m(PO) and d(PO). In the scaled calculated spectrum is observed that m(PaO), m(PcO) and m(PbO) are in the range 1060–680 cm1. The localization of these vibrations has good concordance with the range found in previous work [66] with ATP in aqueous solution that indicates phosphate vibrations in the range 1125–680 cm1. The d(PO) are situated in the vibrational spectrum of the complex in the range 500–360 cm1 what is in agreement with literature [62] that indicate the d(PO) between 600 and 250 cm1. Vibrational modes m(PcAOH) and d(PcAOH). The m(PcAOH) is located in the scaled calculated spectrum of the complex at 703 and 681 cm1 while the d(PcAOH) is situated at 191 cm1. Vibrational modes d(OH)(P). In the scaled calculated spectrum note that the d(OH)(Pc) are in the range 190 to 1080 cm1. This can be seen in Table 3. Vibrational modes d(HCH) and d(HNH). In the complex, deformations of HNH and HCH groups in general appear as coupled modes.

In the scaled calculated spectrum of [Al(ATP)(H2O)4] there assignments of d(HCH), d(CH)(adenine) and d(CH)(ribose). It read some wavenumbers in the experimental, deconvoluted and second derivative spectra at 1286 cm1 of the d(CH)(ribose), at 873 cm1 of the d(CH)(adenine) and at 1440 cm1 of the d(HCH)sciss. The curve-fitting deconvolution analysis exposed the d(CH)(adenine) at 1426 cm1 and 1365 cm1, and the d(CH)(ribose) at 1380 cm1, 1365 cm1 and at 1303 cm1. In relation d(HNH) in the deconvoluted experimental spectrum are observed the d(HNH)sciss at 1671 cm1, q(NH2) at 1235 cm1 and x(HNH) at 584 cm1. A study with ATP and ternary complexes [59] found only one band referent to d(HNH) at 1709 cm1 probably this correspond to a d(HNH)sciss that can be seen at 1671 cm1. Vibrational modes d(HOH). Deformations of the HOH groups have, in general, coupled modes. In the deconvoluted experimental spectrum of the complex it can be noticed assignments of (HOH)sciss, x(HOH), d(HOH)twist, q(OH)(H2O) and s(H2O). It looks at the wavenumbers of the deconvoluted experimental spectrum of the complex at 1700 cm1 for d(HOH)sciss, at 1147 cm1 for q(OH)(H2O), at 815 cm1 for d(HOH)twist, at 472 cm1 for x(HOH) and 136 cm1 for s(H2O). Skeletal framework vibrations Al(O)4O2. The structural skeleton Al(O)4O2 had their normal modes strictly analyzed by studying of distorted geometry of normal modes exposed through ab initio procedure DFT:B3LYP/6-311++G(d,p), identifying normal modes through bond lengths and bond angles that had the greatest participation in the wavenumbers. In the Fig. S5 it can be observed the distorted geometries of [Al(ATP)(H2O)4] normal modes complex with their displacement vectors. The figures were obtained using the program Chemcraft [36]. In this complex, it is believed that ligand acts as bidentate through the oxygen atom of the b phosphate and the oxygen atom of the c phosphate. The vibrational modes of the Al(O)4O2 structural skeleton are observed in the region between 600 and 220 cm1 in the calculated spectrum. The wavenumbers associated to the bond between aluminium ion and the oxygen atom of the b phosphate in the scaled calculated spectrum are at 559, 503, 482, 446, 376, 314 and 301 cm1, which correspond to vibrational modes m(AlAO)(Pb), d(H2OAAlAO)(Pb), d(AlAO)(Pb) and m(H2OAAlAO)(Pb). In the deconvoluted experimental spectrum of the complex is found the

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wavenumber 375 cm1 related to the d(AlAO)(Pb). The curve-fitting deconvolution analysis showed the wavenumber 488 cm1 related to the d(H2OAAlAO)(Pb). The wavenumbers related to the bond between aluminium ion and the oxygen atom of the c phosphate in the scaled calculated spectrum are at 585, 521, 503, 482, 446, 396, 376, 314 and 289 cm1, being represented by the vibrational modes m(AlAO)(Pc), m(H2OAAlAO)(Pc), d(AlAO)(Pc) and d(H2OAAlAO)(Pc). In the deconvoluted experimental spectrum of the complex is found the wavenumber 584 cm1 for the m(AlAO)(Pc). In the curve-fitting deconvolution analysis it can be seen at 582 cm1. Regarding the bond between the metal ion and the coordinated waters is noted in the deconvoluted experimental spectrum of the complex the wavenumber at 281 cm1 corresponding to the vibrational mode d(H2OAAl). In the scaled calculated spectrum these are at 284 cm1. A study with tetraaqua serine aluminium(III) cation complex [67] found one band referent to d(H2OAAlAOH2) at 261 cm1. The band corresponding to this vibrational mode was found at 297 cm1 in the [Al(ATP)(H2O)4]. The ab initio calculated spectrum scaled and the respective distorted geometries of the complex are able to predict the assignments of the vibrational modes. The analysis of the spectrum below 600 cm1 reveal the wavenumbers and their assignments are at 597 and 415 cm1 of m(H2OAAl) + d(AlAOH2), 585 cm1 of m(AlAO)(Pc), 559 cm1 of m(AlAO)(Pb), 543, 514, 346, 284, 271 and 224 cm1 of d(AlAOH2), 521 cm1 of m(H2OAAlAO)(Pc) + d(H2OAAlAOH2), 503 and 289 cm1 of d(H2OAAlAO)(Pc) + d(H2OAAlAO)(Pb) + d(H2OAAlAOH2), 482 cm1 of d(AlAO)(Pc) + d(AlAO)(Pb), 446 cm1 of m(H2OAAlAO)(Pc) + m(H2OAAlAO)(Pb) + m(H2OAAl), 396 cm1 of d(AlAO)(Pc), 376 cm1 of m(AlAOH2) + d(AlAO)(Pb) + d(AlAO)(Pc), 367 cm1 of m(AlAOH2), 314 cm1 of m(AlAO)(Pc) + d(AlAO)(Pb) + d(AlAOH2), 310 cm1 of d(H2OAAlAOH2), and 301 cm1 of d(H2OAAl) + d(AlAO)(Pb). These suggest the conformity with the coordination mode proposed. The bands mentioned were not found in the spectrum of the ligand. The vibrational assignments for all metal–ligand modes show a strong degree of coupling between different internal coordinates in each vibrational mode. Vibrational modes of torsion. The torsion modes of the complex are observed in the 210–10 cm1 region in the calculated spectrum scaled. Vibrational modes m(OH)(Pc) and d(OH)(Pc) of ligand. It is emphasized that vibrations related to the AOH bond of the c phosphate still occur in the complex, considering that the ATP has two OH bonds in this phosphate. In the complex one of these bonds no longer exists, to come up the bond with the metal ion, though the other remains. The wavenumbers of AOH bond of Pc are shown in Table 3 refer to the bond existing in the complex. In calculated spectrum scaled of the ligand these are observed the attributions d(OH)(Pc) at 1181 cm1 and m(OH)(Pc) at 2342 cm1 due to AOH bond of the c phosphate, however, in complex these are not localized. In the experimental spectrum of the ligand in aqueous solution is just note d(OH)(Pc) at 1127 cm1. The AOH bond of the c phosphate existing in the complex and in the ligand can be represented by m(OH)(Pc) and d(OH)(Pc) at 3556 and 1070 cm1 in the calculated spectrum scaled of the ligand which correspond at 3495 and 1046 cm1 in experimental spectrum of the ligand in solid state. Vibrational modes m(OH)(Pb) and d(OH)(Pb) of ligand. The vibrations related to the AOH bond of b phosphate do not occur in the complex because in this phosphate of ATP there was only one AOH bond that breaks down in the complex, to emerge the bond with the metal ion.

In the calculated spectrum scaled of the ligand are the vibrational modes of the AOH bond of Pb, m(OH)(Pb) at 3298 cm1 and d(OH)(Pb) at 1181 and 653 cm1. In the experimental spectrum of the ligand in solution is observed the stretching at 3212 cm1 and the deformation at 1127 cm1. In the experimental spectrum of the ligand in solid state, stretching at 3304 cm1 and deformation is not visualized. In the complex cannot be found the vibrational modes related to this bond. Frontier molecular orbitals (FMOs) of deprotonated ATP3 ligand The highest occupied molecular orbital (HOMO) and the lowestlying unoccupied molecular orbital (LUMO) are known as frontier molecular orbitals (FMOs). The Fig. S6 shows some molecular orbitals for ATP3 ligand: the second highest and highest occupied and the lowest and the second lowest unoccupied molecular orbitals which are HOMO1, HOMO, LUMO and LUMO + 1, respectively. The HOMOs correspond to the ability to donate an electron while LUMOs act as an electron acceptor representing the ability to gain an electron [68]. The two highest occupied molecular orbitals are localized on the phosphates region and on the adenine. The second highest molecular orbital is localized mainly on the oxygen atoms of the phosphates which present mostly affinity for electrons. This suggests the possibility of beta and gamma phosphates react with the metal cations such as aluminium(III). Notice also that HOMO1 and HOMO orbitals are completely separated spatially. After the orbitals of the phosphates are filled, the adenine base begins to be filled too. Therefore the HOMO is located on the base adenine. The LUMO and LUMO+1 are localized on the adenine region. Thus, compared to the others atoms of the molecule, this region could act an electron acceptor too. Molecular electrostatic potential (MESP) map of deprotonated ATP3 ligand In the graphic of total electron density surface mapped with the electrostatic potential, the sign of the electrostatic potential in a surface region is determined by the predominance of negative charges contribution or positive charges contribution. Accordingly, it is possible to identified regions more susceptible to approximation of electrophilic molecules or nucleophilic molecules, so the molecular electrostatic potential map is commonly used as a reactivity map [69]. To predict regions more susceptible to approximation of either electrophiles or nucleophiles, MESP was calculated at the B3LYP/6311++G(d,p) optimized geometry in water solvent and shown in Fig. S7. The importance of total electron density surface mapped with the electrostatic potential lies in the fact that it simultaneously displays molecular size, shape, as well as positive or negative electrostatic potential regions in terms of color grading and is very useful in research of molecular structure with its physiochemical property relationship [70]. The different values of the electrostatic potential are represented by different colors. The range values for the color scale of the mapped MESP should be symmetrical to allow easily identification of negative (red) and the positive (blue) potential regions. The use of a symmetrical potential scale values eases the recognition of positive, zero or negative regions. In GaussView 5.0.8 visualizing program [35] the following spectral color scheme is used. So potential increases in the order: red < orange < yellow < green < cyan < blue. Therefore red indicates negative regions, blue indicates positive regions, while green appears over zero electrostatic potential regions.

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It is accepted that the negative (red) and the positive (blue) potential regions in the mapped MESP represent regions susceptible to approach electrophilic molecules or nucleophilic molecules, respectively. In fact, the mapped MESP over a single surface does not suffice to point out which ligand’s regions are more prone to oncoming electrophilic species. To really find out these regions one should analyze MESP’s contour map of the ligand. Spatial regions denser in MESP’s contour lines present stronger electrostatic fields them region with less contour lines. Also, electrostatic field planar projection points orthogonally towards decreasing MESP contours. Therefore, in general, the red regions depictured in the total electron density surface mapped with the MESP indicate the occurrence of inward electrostatic fields, which favor the approach of electrophilic species and repel nucleophlic ones. As can be seen from Fig. S7, the phosphates of the ATP3 are the region of most negative potential (red) and the oxygen atoms of the phosphates favor the nearing of positive charges. The electrostatic potential is less negative (yellow) around the ribose and the adenine. Regions with potential close to zero (green) are also found on the adenine. Aluminium(III) has high positive charge and is a good electrophile. Consequently it is expected to approach the phosphates region. Frontier molecular orbitals of [Al(ATP)(H2O)4] The energy gap between the frontier molecular orbitals is essential in determining the molecule chemical activity. The HOMO–LUMO energy gap established the kinetic stability, chemical reactivity and, optical polarizability and chemical hardness– softness of a molecule [68,71]. A small HOMO–LUMO energy gap implies low kinetic stability, because it is energetically favorable to add electrons to a low-lying LUMO and to receive electrons from a high-lying HOMO [72–74]. The calculations indicate that the compound has 155 occupied molecular orbitals. Fig. S8 presents the energies and distributions of the second highest and highest occupied molecular orbitals (HOMO1 and HOMO), and the lowest and the second lowest unoccupied molecular orbitals (LUMO+1 and LUMO) computing at B3LYP/6311++G(d,p) level for the title complex. The energies values and the dipole moment are presented in Table 4. One can observe that in the HOMO1 and HOMO orbitals, electrons are localized mainly in the ribose, non attached phosphate and adenine of the ligand molecular part, meanwhile in the LUMO and LUMO+1 orbitals, electrons are localized mainly over the adenine and coordinated waters. The HOMO–LUMO energy gap value obtained was 5.26 eV. This large energy gap suggests that the molecule has high kinetic stability and low chemical reactivity, so the structure is quite stable.

Table 4 Calculated energy values and dipole moment of [Al(ATP)(H2O)4] in H2O. Energy values and dipole moment

DFT:B3LYP/6-311++G(d,p)

Etotal (Hartree) EHOMO1 (eV) EHOMO (eV) ELUMO (eV) ELUMO+1 (eV) DEHOMO–LUMO gap DEHOMO1–LUMO+1 gap Dipole moment (Debye)

3214.1180 7.6173 6.7030 1.4373 1.0008 5.2657 6.6165 26.3975

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Electrostatic potential contour surface of [Al(ATP)(H2O)4] The MESP’s contour lines illustrate a plane section in particular of equipotential surfaces. The GaussView program [35] adopts the follow colors scheme: yellow contour lines for positive electrostatic potential and red lines for negative potential. Spatial regions denser in MESP’s contour lines have more intense potential variations and, consequently, stronger electrostatic fields. In this case, a major force is applied on the charges in the proximity. Also, electrostatic field planar projection points orthogonally towards decreasing MESP contours. Fig. S9a shows contour lines in a plane that divide the complex, and Fig. S9b and Fig. S9c present contour lines in two planes centered on Al(III). It can be seen that the oxygen atoms of the phosphates have an electrostatic field relatively intense despite the interaction with the metal. The adenine’s nitrogen(22) and ribose’s oxygen(10) also have a intense potential variation. Around aluminium(III) the potential is positive, yielded by an electron deficient region. The signal alteration of the potential closest to the oxygen atoms of the phosphates suggests that this region concentrates negative charges. It can be noted in the Fig. S9b and c. Molecular electrostatic potential map of [Al(ATP)(H2O)4] The MESP map displays a surface that at the same time shows electrostatic potential value, and molecular size and shape. The molecule presents positive and negative potential surfaces. For example, the electrostatic potential contour surface of [Al(ATP)(H2O)4], Fig. S10a, highlighted two equipotential surfaces where the orange is negative and the yellow is positive. Nevertheless, the electrostatic potential differences are not large. This can be better appreciated in the total electron density surface mapped with the electrostatic potential of [Al(ATP)(H2O)4], Fig. S10b. The molecular electrostatic potential map shows that the complex has many nearly zero electrostatic potential regions (green). The molecule has regions with electrostatic potential more positive among adenine (bluish-green), and with electrostatic potential more negative (yellowish-green) among oxygen of the phosphates groups. The presence of large ‘‘neutral’’ (green) regions in the electrostatic potential map for complex suggests that the molecule will be soluble in lipids [75]. This fact summed with molecule’s zero total charge points to a probable ability to transpose plasmatic membrane. Mulliken charge distribution of [Al(ATP)(H2O)4] The Mulliken populations show one of the simplest pictures of charge distribution. The Mulliken charges provide net atomic populations in the molecule while electrostatic potentials yield the electric field out of the molecule produced by the internal charge distribution. Thus, in the reactivity studies, Mulliken populations and MESP are complementary tools, and correlation between the schemes is expected [76]. However, Mulliken population analysis require very careful because problems as large changes of the calculated atomic charges with small changes in the bases used and the overestimation of the covalent character of a bond are common. So, in general, the absolute magnitude of the atomic charges has little physical meaning, on the other hand, their relative values can offer valuable information. The Mulliken charge distribution of the molecule was calculated on B3LYP level with 6-311++G(d,p) basis set. The calculated values of the charges of the molecule are given in Fig. S11a. Mulliken

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Conclusion

Fig. 4. Correlation graphic of calculated [DFT:B3LYP/6-311++G(d,p)] versus experimental wavenumbers of the [Al(ATP)(H2O)4] complex.

charge distribution structurally and graphically is shown in Fig. S11b. For Mulliken charge distribution, the GaussView [35] adopts the follow colors scheme: bright red for more negative charge and bright green for more positive charge. The red hues illustrate negative charges while green hues expose positive charges. The charge distribution of the compound shows that the phosphorous atoms attached with oxygen atoms have positive charges. All the hydrogen atoms have positive Mulliken charges and all the oxygen atoms have negative Mulliken charges. The atom Al45 has the highest Mulliken charge (3.528) when compared to other atoms. The oxygen atoms attached with aluminium are much more negative than the others oxygen atoms of the complex and present the lowest Mulliken charges compared to other atoms. The smallest Mulliken charge value (1.057) was obtained for O41 atom. The Mulliken charge distribution and the MESP informations are concordant. Bases analysis: Error analysis of different vibrational theoretical calculations of [Al(ATP)(H2O)4] The root mean square deviation is small for all bases and these showed a uniform deviation after scaling. The calculated correlation coefficients (R2) values are similar for all bases and the relations between the calculated and experimental wavenumbers are linear. The correlation graphic for DFT:B3LYP/6-311++G(d,p) which described harmonies between the calculated and experimental wavenumbers was plotted (Fig. 4). This was described by the following equation:

mcalc ¼ 1:02150mexp  20:1810 ðR2 ¼ 0:99760Þ The difference between the experimental and theoretical spectra probably occurs by the environment and because the DFT calculations neglect the anharmonicity effects [69,77].

Complexes formed among aluminium(III) and adenosine 50 -triphosphate seems to be involved in the Alzheimer’s pathogeneses, disease very common in the actuality. Understand the chemical details about species connected with this pathology can help in the prevention and treatment besides support biological and medical studies. The aluminium(III):adenosine 50 -triphosphate system M:L ratio of 1:1 was studied in aqueous solution through potentiometry, Raman spectroscopy and theoretical calculations of geometrical optimization and vibrational spectra. The complexation was confirmed by these techniques and many complexes were formed according pH of solution. The log KAlATP value found indicated that adenosine 50 -triphosphate ligand acts as bidentate in AlATP complex adopting a octahedral geometry. In addition, the molecular modeling of the [Al(ATP)(H2O)4] complex suggested a distorted octahedral structure. Raman spectroscopy, potentiometry and complete analysis of vibrational spectra employing computational calculations regarding water solvent effect show that donor atoms are the oxygens of the terminal phosphates b and c in the [Al(ATP)(H2O)4] complex. The frontier molecular orbitals and the MESP map of ATP3 ligand also indicates the phosphates region as susceptible to approaching of aluminium(III). The Raman spectroscopy, in aqueous solutions, was useful in determining the complex’s vibrational modes, and helpful in the coordination mode identification. This research illustrates that the Raman calculated vibrational spectra agree well with the Raman experimental spectrum according correlation graphics and that vibrational assignments indicate a strong degree of coupling for all metal–ligand modes. The energy gap of the frontier molecular orbitals indicates that [Al(ATP)(H2O)4] has high kinetic stability and low chemical reactivity. Electrostatic potential contour surface and electrostatic potential mapped of the complex show regions with potential more positive among adenine, and with potential more negative among oxygen of the phosphates groups. The Mulliken charge distribution and the MESP information are concordant. The presence of large ‘‘neutral’’ regions in the electrostatic potential map for complex suggests that the molecule will be soluble in lipids. Acknowledgements This study was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio). The authors also thank the Professor André Tenório of the Instituto Federal do Rio de Janeiro for his excellent advices. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.saa.2012.12.019. References [1] F.A. Cotton, G. Wilkinson, C.A. Murillo, M. Bochmann, Advanced Inorganic Chemistry, sixth ed., John Wiley & Sons, New York, l999. [2] P. Rubini, A. Lakatos, D. Champmartin, T. Kiss, Coord. Chem. Rev. 228 (2002) 137–152. [3] G.L. Eichhorn, J.J. Butzow, P. Clark, H.P. Von Hahn, G. Rao, J.M. Heim, E. Tarien, D.R. Crapper, S.J. Karlik, in: A.E. Martell (Ed.), Inorganic Chemistry in Biology and Medicine, American Chemical Society, Inc., Washington, 1980, pp. 75–88.

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