On the stability of [Pb(Proline)]2+ complexes. Reconciling theory with experiment

Chemical Physics Letters 598 (2014) 91–95

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On the stability of [Pb(Proline)]2+ complexes. Reconciling theory with experiment Fernando Aguilar-Galindo, M. Merced Montero-Campillo ⇑, Manuel Yáñez, Otilia Mó Departamento de Química, Módulo 13, Universidad Autónoma de Madrid, Campus de Excelencia (UAM-CSIC), Cantoblanco, 28049 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 23 December 2013 In final form 4 March 2014 Available online 12 March 2014

a b s t r a c t Salt-bridge and canonical charge solvated complexes of neutral proline (Pro) and Pb(II), [Pb(Pro)]2+, are discussed for the first time. Although thermochemically stable with respect to their corresponding deprotonated forms [Pb(Pro-H)]+, the dicationic complexes are not observed experimentally. Indeed, for the deprotonated complexes a disagreement between IRMPD results and theoretical calculations was reported. We perform an exhaustive DFT assessment to correctly predict the experimental findings, and to rationalize why [Pb(Pro)]2+ complexes are not observed. The deprotonation is likely to occur through a highly exergonic proton transfer between [Pb(Pro)]2+ and a water molecule resulting in the observed [Pb(Pro-H)]+ singly charged ion. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Lead is one of the chemical elements whose effects for human health have been studied for a very long time [1]. Its harmful activity has a direct relationship, among other factors, with the interaction that this metal is able to establish with amino acids, the building elements of the protein architecture [2–5]. The study of the interaction of metal cations with aminoacids is widely represented in the literature, many papers being devoted to complexes between proline (Pro) and mono and dications from an experimental point of view [6–12]. From the whole family of natural aminoacids, Pro presents some unique characteristics, some of them related to its quite rigid structure and its secondary amine group [13]. Theoretical and computational chemistry has offered many insights about Pro-cation complexes, as is the case of alkaline monocations (Li+, Na+, K+, Rb+, Cs+), alkaline-earth dications (Be2+, Mg2+, Ca2+), or transition metals (Cu2+, Mn2+, Zn2+, Ag+, Pb2+) [14– 19]. As far as the interactions between Pro and Pb(II) are concerned the first significant finding is that Electrospray Ionization/Mass Spectrometry (ESI–MS) experiments showed that in these complexes Pro is deprotonated, so only [Pb(Pro-H)]+ singly charged species are detected [20]. Similar deprotonation processes have been described also in the literature when Pb2+ interacts with other relevant biochemical systems, such as thiouracil derivatives [21] or uridine-50 -monophosphate [22] among many others. In some cases, suitable mechanisms for these deprotonation process have been also suggested [21,22]. Also very recently the structure of ⇑ Corresponding author. E-mail address: [email protected] (M.M. Montero-Campillo). http://dx.doi.org/10.1016/j.cplett.2014.03.006 0009-2614/Ó 2014 Elsevier B.V. All rights reserved.

bare and hydrated [Pb(Pro-H)]+ monocations have been characterized using Infrared Multiple Photon Dissociation Spectroscopy (IRMPD) [20,23]. Quite importantly, however, theoretical calculations performed by the same authors seem not to be in good agreement with the experiments, since the theoretically predicted most stable structure has an IR spectrum which does not match the experimental one. In this Letter we aim at providing a better knowledge of the Pb(II)-Pro systems and a reliable theoretical protocol to study them. We will focus first our attention on the structure of [Pb(Pro)]2+ complexes and on their stability with respect to the different coulomb explosions they can undergo. To the best of our knowledge this question has never been investigated before, and it would help to explain why these doubly charged species were never detected in the gas phase. Secondly, we will try to solve the apparent dichotomy between experiment and theory with regards to the most stable conformation of the [Pb(Pro-H)]+ system. Electronic structure calculations of heavy elements are often challenging due to both relativistic effects and large electron correlation contributions, so that the appropriate choice not only of the method but of the basis set may be crucial, as our group has recently shown [24,25]. Hence unavoidably, the first step trying to solve the aforementioned dichotomy, requires an assessment of the theoretical model. 2. Computational details Electronic structure calculations were carried out with the GAUSSIAN 09 code [26]. All stationary points of the potential energy surface were fully optimized and have been proved to be minima or first-order saddle points through the corresponding analytical

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second-derivatives calculation. A series of different density functionals, namely M06, M06-L, B3LYP, BLYP, LC-BLYP, B2PLYP, X3LYP, O3LYP, B97-D, PBE0, MPW1PW91, B3PW91 and BP86 were used, plus some additional MP2 calculations [27–41]. This set contains several types of DFT functionals (pure, hybrid, double-hybrid, long-range corrected, and dispersion corrected). Lead active electrons have been represented by different valence basis sets plus small-core effective relativistic potentials as defined in the GAUSSIAN 09 code and the EMSL basis set library (LANL2DZ, SDD, cc-PVTZ-PP, aug-cc-PVTZ-PP, aug-cc-PVQZ-PP DEF2-TZVPPD) [42,43]. Hydrogen, carbon, nitrogen and oxygen atoms were defined by 6-31G(d,p) and 6-311g(d,p) double and triple-zeta Pople’s basis set, respectively when LANL2DZ and SDD were used for lead. In all other cases the same type of basis set was used for all the atoms. The nature of bonding on a given molecule can be studied by looking at the topology of the electron density, by locating its critical points, among which the BCP values (Bond Critical Points) are of particular relevance. The density at BCPs provides information about the strength of the bond and it allows to identify and quantify weak interactions that are not evident by just looking at the geometry. Also, the so-called natural atomic charges coming from the Natural Bond Analysis (NBO) approach were used [44]. NBO offers a Lewis-picture of a given molecule by describing bonding in the system in terms of lone pairs and localized hybrids.

3. Results and discussion 3.1. On the structure of the [Pb(Pro-H)]+ and [Pb(Pro)]2+ complexes Pro aminoacid has two predominant conformations, ‘‘puckerup’’ (Cc-exo) and ‘‘pucker-down’’ (Cc-endo) [45]. These two structures are separated by 2 kJ/mol at B3LYP/6-311+G(d,p) level [17]. Only the ‘‘pucker-up’’ complexes will be presented here for simplicity. On complexing with Pb2+ cation, the most stable conformers are those shown in Figure 1, which contains both [Pb(Pro)]2+ and [Pb(Pro-H)]+ complexes. The first row on Figure 1 corresponds to complexes in which Pro remains intact. These complexes can be classified as charge-solvated and salt-bridge depending on Pro being in its canonical or in its zwitterionic structure in which a proton transfer from the acidic function to the amino group took place. The deprotonated complexes [Pb(Pro-H)]+, shown in the second row of Figure 1, have been labeled as pCS(X,Y) or pSB(X,Y) where X, Y denote nitrogen or oxygen atoms depending on the coordination of the cation, and pCS and pSB stand for pseudocharge-solvated and pseudosalt-bridge. We decided to use the prefix pseudo, as they

are not real charge-solvated or salt-bridge structures, but this nomenclature is useful to establish an analogy with the corresponding neutral forms. It should be noted that deprotonated complexes were already studied by Burt and collaborators, whereas neutral complexes are reported here for the first time. As it could perhaps be easily anticipated, the global minima of the [Pb(Pro)]2+ PES corresponds to structure SB(O,O) in which the doubly-charged metal ion bridges between the two oxygen atoms of the carboxylate group of the zwiterionic form of Pro, whereas the different CS structures lie significantly higher in energy (see Table 1). It is interesting to note that Pb2+ is not symmetrically bound to both oxygen atoms in SB(O,O), because as it is shown by the corresponding molecular graph (see Figure 2) the oxygen atom closer to the amino group acts as a hydrogen bond acceptor and consequently is a weaker electron donor with respect to Pb2+. This picture is also consistent with the second order NBO interaction energies which show that the charge donation form this oxygen atom towards Pb2+ is lower than from the other oxygen atom (see Table S1 of the Supporting information). Also interestingly, the attachment of Pb2+ to the two oxygens of the carboxylate group does not lead to the most stable structure for the [Pb(Pro-H)]+ complexes, the global minimum being conformer pCS(NH,O), in which the metal interacts simultaneously with the amino group and one of the O group of the carboxylate group. Note that a similar structure is not possible for [Pb(Pro)]2+ because the amino nitrogen is necessarily protonated. Rather close in energy there is a second pCS structure, namely pCS(N,O). The large stability of this complex can be understood because, although the C@O group of the carboxylic acid is a poorer electron donor than that of the carboxylate group, the Lewis basicity of the deprotonated amino N is significantly enhanced with respect to that of the NH group. These differences are clearly seen in the electron densities at the corresponding BCPs and in values of the NBO second order interaction energies, between the lone pairs of the NH basic site and the empty p orbital of Pb2+ (see Table S1 of the Supporting information). 3.2. Thermochemical stability Our group has studied through ab initio and DFT calculations in the recent years several biomolecule-cation charged systems in comparison with experimental findings. Their stability is strongly correlated to the nature of the interacting cation and the possibility of proton transfer reactions in the media. With ESI–MS techniques as those used for proline–Pb(II) systems, it was found for example

Figure 1. Most stable conformers of non-deprotonated [Pb(Pro)]2+ and deprotonated [Pb(Pro-H)]+ complexes of proline with Pb2+ cation at the B3LYP/6-31+G(d,p)/SDD level.

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F. Aguilar-Galindo et al. / Chemical Physics Letters 598 (2014) 91–95 Table 1 Relative Gibbs energy of deprotonated and neutral Pro complexes with Pb(II) at the B3LYP/6-31+G(d,p)/SDD level of theory in kJ/mol.

a

[Pb(Pro-H)]+

DG a (kJ/mol)

[Pb(Pro)]2+

DG (kJ/mol)

pCS(N,O) pCS(NH,O) pSB(O,O) pCS(N,OH)

1.4 0.0 33.7 53.1

SB(O,O) CS1(N,O) CS2(N,O) CS(O,O)

0.0 57.4 139.9 189.1 Scheme 1. Generic potential energy surface of a AB2+ system.

These values agree with those reported in Ref. [20].

that uracil-Ca2+ complexes are detected, but only deprotonated [(uracil-H)(uracil)-Cu]+ together with UracilH+ have been observed when the interaction involves Cu2+ rather than Ca2+ [46–49]. The fact that uracil-Cu2+ is not observed depends on its relative stability with respect to coulomb explosions and on the barriers that the systems should overcome to reach the products (see Scheme 1). Considering [Pb(Pro)]2+ system, our calculations at B3LYP/631+G(d,p)/SDD level of theory (see Table 2) show that the free energy of dissociation of the most stable SB(O,O) complex into Pro+Pb2+ is highly positive (reaction [1]) and that the expected coulomb explosions (reactions [2] and [3]) are clearly endergonic and therefore should not be observed in the gas phase. This means that [Pb(Pro)]2+ is thermodynamically stable, and not only kinetically stable, with respect to all these fragmentation reactions. Note that in particular the deprotonation process yielding the most stable [Pb(Pro-H)]+ complex through the reaction SB(O,O) ? pSB(O,O) + H+ is the most endergonic at 298 K. Then, it is reasonable to ask why only the singly-charged deprotonated species are observed but not the doubly-charged complexes. It is obvious that the strong charge transfer from Pro towards Pb2+ implies a drastic reorganization of the electron density of Pro and a significant increase of its intrinsic Bronsted acidity. As a consequence, a proton transfer from [Pb(Pro)]2+ towards either a second Pro molecule or to a solvent water molecule in the declustering process occurring during the electrospray ionization are both highly favored (See Table 1), yielding the [Pb(Pro-H)]+ singly charged complex. Both processes are compatible with usual observation of the protonated base in the source [21,22]. It must be noted however, that in the electrospray experiments the residual pressure is very small and, in principle, the probability of observing reaction [4] is rather small, so under the experimental conditions in which the experiments of Ref. [20] have been carried out,

Table 2 Calculated Gibbs energies of different reactions involving Pb(II)-Pro complexes. SB(O,O) is the reference for [Pb(Pro)]2+ and pCS(N,O) is the reference for [Pb(Pro-H)]+ at B3LYP/6-31+G(d,p)/SDD. The "*" denotes Proline radical.

[1] [2] [3] [4] [5]

Reaction

DG (kJ/mol) – B3LYPa

[Pb(Pro)]2+ ? Pro + Pb2+ [Pb(Pro)]2+ ? [Pb(Pro-H)]+ + H+ [Pb(Pro)]2+ ? Pb+ + Pro+⁄ [Pb(Pro)]2+ + Pro ? [Pb(Pro-H)]+ + ProH+ [Pb(Pro)]2+ + H2O ? [Pb(Pro-H)]+ + H3O+

+560.1 +586.6 +14.8 335.4 69.3

a M06 with same basis set results for reactions [2–4] with the same basis set are +583.9, +14.3 and 330.3 kJ/mol, respectively.

reaction [5] is the most likely to occur. The high exergonicity of reaction [4] indicates however that should the [Pb(Pro)]2+ be produced in the gas-phase through different techniques, the probability of observing these doubly charged species should be still negligible. 3.3. Experiment-theory dichotomy Since only [Pb(Pro-H)]+ can be found in the gas phase as discussed in the previous section, this one is devoted to try to solve the apparent disagreement between theory and experiment reported in Ref. [20] which led to the authors to conclude that theory was not reliable for this particular problem: ‘The fact that simply changing a basis set produces different minimum energy structures demonstrates the importance that experiments have in revealing structural information. One cannot solely rely on calculations’. According to the calculated energy gap between pCS(NH,O) and pCS(N,O) reported in that reference (See Table 1) both species

Figure 2. Molecular graphs of the [Pb(Pro)]2+ and [Pb(Pro-H)]+ complexes. Electron densities are in a.u.

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Table 3 Free energy difference between pCS(NH,O) and pCS(N,O) complexes in kJ/mol calculated with B3LYP and different basis sets.a The total number of primitive GAUSSIAN basis functions is also provided.

a

Table 5 Free energy gaps (kJ/mol) between pCS(NH,O) and pCS(N,O) complexes calculated with M06 and different basis sets. The total number of primitive GAUSSIAN basis functions is also provided.

Basis set

Number of basis functions

DG (kJ/mol)

Basis set

Basis functions

DG (kJ/mol)

LANL2DZ (Pb), 6-31+G(d,p) SDD (Pb), 6-31+G(d,p) SDD (Pb), 6-311+G(d,p) DEF2-TVZPPD (Pb), 6-311+G(d,p) cc-pVTZ-pp (all atoms) aug-cc-pVTZ-pp (all atoms)

192 197 237 280 391 607

5.5 1.4 1.3 0.2 3.8 3.1

SDD (Pb), 6-31+G(d,p) SDD (Pb), 6-311+G(d,p) DEF2-TVZPPD (Pb), 6-311+G(d,p) cc-pVTZ-pp (all atoms) aug-cc-pVTZ-pp (all atoms) cc-pVQZ-pp (all atoms)

197 237 280 391 607 1178

4.2 4.7 4.8 7.1 7.1 7.6

See also ‘‘comment on Table 3’’ (Supp. info).

should be observed on the experimental IR spectrum, the latter being the dominant one (64%). However, the experimental spectrum does not show the characteristic N–H stretching mode, which should be located on the 3410–3440 cm1 range, indicating that the pCS(N,O) is not present in the sample, and that therefore the theoretically predicted relative stabilities should not be correct. Taking into account that similar theoretical models normally reproduce quite accurately the main features of IRMPD spectra of similar molecular ions, [22] it seems necessary to analyze in further detail the possible origin of the malfunction of the theoretical model. Besides the fact that the energy gap between both structures is likely small, two possible sources of error to get the right stability order can be envisaged, the basis set and the density functional used. In order to have a first estimate of the possible effect of the basis set we have recalculated this energy gap using increasingly large basis sets with the same functional. For this survey the B3LYP functional was chosen and the results obtained (see Table 3) indicate that at least a triple-zeta basis set for small atoms plus a better representation of the lead atom is needed in the optimization process, to predict pCS(N,O) to be more stable than pCS(NH,O). This seems to point out that one of the possible deficiencies of the model was the use of a too small SDD/6-31+G(d,p) basis set. In order to analyze the possible effect of the functional, in a second test we decided to explore the performance of the rather complete series of functionals mentioned in the Computational Details section, maintaining the relatively small SDD/6-31+G(d,p) basis set used in Ref. [20]. Table 4 shows that indeed the energy gap changes within a rather small range (10.8 kJ/mol). The use of the small base leads to the wrong stability order when some functionals, such as B97-D, LC-BLYP. . . are used, whereas some others as O3LYP, B3PW91. . . correctly reproduce it even with this small basis set. Taking into account that the sign

Table 4 Relative Gibbs energy gap (kJ/mol) between pCS(N,O) and pCS(NH,O) complexes using 6-31+G(d,p)/SDD basis set with different DFT functionals. Functional

DG (kJ/mol)

B97-D C-BLYP BLYP B2PLYP B3LYP X3LYP BP86 O3LYP M06-L B3PW91 mPW1PW91 PBE0 M06

6.2 5.6 2.5 2.3 1.4 0.4 1.3 1.9 2.6 2.8 2.8 2.9 4.2

of the gap changes with the size of the basis set when B3LYP functional is used, we decided to study what would be the predictions of these latter functionals if the basis set used was more flexible. For this last test we have chosen the M06 functional because it is the one that predicts a larger positive gap when the small basis set is used. The results obtained in this scrutiny have been summarized in Table 5. These values clearly indicate that the energy gap seems to converge to a value around 7 kJ/mol, since negligible variations are observed at the cc-pVTZ level when diffuse functions are included in the basis set, and the increase of the gap is only 0.5 kJ/mol on going from the aug-cc-pVTZ-pp to the cc-pVQZ-pp basis set. A free energy gap of 7.6 kJ/mol would be consistent with the dominance of the pCS(N,O), since, assuming a Boltzman type distribution at 298.2 K a mixture of 98% of pCS(N,O) and only a 2% of pCS(NH,O) in the gas phase should be expected, in nice agreement with the experimental evidence. 3.4. Infrared spectra Since the experimental characterization of the [Pb(Pro-H)]+ was based on the characteristics of their IRMPD spectra, and in particular on the position of the OH stretching mode, we have investigated in this section which of the functionals included in our assessment better reproduce the experimental value without a posteriori corrections. Not surprisingly, the values summarized in Table 6 reveal that pure functionals such as BLYP and BP86 give excellent results on predicting the position of the main peak with almost negligible errors (see also Figure S1). Interestingly, BP86 functional was able to predict Hg-H stretching modes with similar small errors [24]. To

Table 6 Raw frequency (m, cm1) values of the O–H stretching mode peak in pCS(N,O) complex with SDD/6-31+G(d,p) basis set and different DFT functionals, along with their relative error with respect to the experimental value (3570.0 cm1). Also, scaled values are provided. Functional

m

Erel (%)

m scaleda

Erel (%)

B97-D LC-BLYP BLYP B2PLYP B3LYP X3LYP BP86 03LYP M06-L B3PW91 mPW1PW91 PBE0 M06

3637.6 3785.4 3565.8 3735.2 3715.9 3723.3 3589.8 3738.9 3748.6 3747.3 3779.0 3773.7 3769.9

1.9 6.0 0.1 4.6 4.1 4.3 0.6 4.7 5.0 5.0 5.9 5.7 5.6

– – 3557.7 – 3585.1 – 3567.9 3607.3 – 3598.2 3602.1 3602.8 –

– – 0.3 – 0.4 – 0.1 1.0 – 0.8 0.9 0.9 –

a Values corrected according to scale factors given in Ref. [50] for 6-31+G(d,p) basis set for the corresponding functionals.

F. Aguilar-Galindo et al. / Chemical Physics Letters 598 (2014) 91–95 Table 7 Wavenumbers (m, cm1) of the N–H and O–H stretching mode peaks on SB(O,O) and CS1(N,O) complexes with SDD/6-31+G(d,p) basis set and BLYP functional. B3LYP corrected values (scale factor 0.9648) are given in parentheses for comparison.

m

SB(O,O)

CS1(N,O)

N–H stretch

3300.0 (3282.2) 3379.4 (3359.1)

3367.3 (3354.1)

O–H stretch

–

3513.9 (3529.4)

obtain a similar result with B3LYP, an a posteriori scale factor of 0.955 is used by Burt et al. Table 6 also includes scaled frequency values, which is a standard approach for harmonic calculations. Although Pb atom is represented by a pseudopotential, we chose scale factors obtained for 6-31+G(d,p) basis set, used in our calculations for C, N, O and H atoms [50]. As long as scale factors for BLYP and B386 are very near to 1, corrected values are quite similar and still those with the smallest errors. However, corrections are crucial for the rest of the functionals, and particularly for B3LYP. For the sake of completeness we present in Table 7 the main peaks of the IR spectra of the two more stable [Pb(Pro)]2+ complexes obtained by means of the BLYP functional which was the one exhibiting a better performance as far as Pb(Pro-H)]+ complexes were concerned. B3LYP corrected values are also provided for comparison. 4. Conclusions Salt-bridge and canonical charge solvated complexes of neutral Pro and Pb2+ are discussed for the first time. The relative stability of the deprotonated Pb(Pro-H)]+ complexes is correctly reproduced by several functionals, provided that a flexible enough basis set is used for both the base and the metal dication. It is important to emphasize that, although is not surprising to find variations in the stability order of systems with similar stability, this does not invalidate the theoretical approach and only indicates that the model should be appropriately assessed. Of course the availability of experimental information can be very helpful indeed in this assessment, but not strictly necessary if it is proved that the theoretical predictions converged, in the sense that substantial improvements in the basis set have a negligible effect on the calculated values. We have shown that this is the case in particular when the M06 functional was used to describe Pb(Pro-H)]+ complexes. We have also found that pure functionals such as BLYP and BP86 gave infrared data for this system with very small errors without needing an a posteriori scaling correction. B3LYP gives a very similar result using the corresponding scale factor. Theoretical calculations allowed to explain the absence of the most stable [Pb(Pro)]2+ species in the experiments, which is likely due to a proton transfer between the complex and a water molecule resulting in the observed deprotonated complex [Pb(Pro-H)]+. The high exergonicity of the proton transfer between [Pb(Pro)]2+ and Pro indicates that even assuming that the doubly charged species could be generated in the gas phase by different experimental techniques, the probability of observing it should be negligible. Acknowledgements This work has been partially supported by the Ministerio de Economía y Competitividad (Project No. CTQ2012-35513-C0201), by the CMST COST Action CM1204, by the Project MADRISOLAR2, Ref.: S2009PPQ/1533 of the Comunidad Autónoma de Madrid, and by Consolider on Molecular Nanoscience CSC200700010. F.A.-G. thanks Ministerio de Educación, Cultura y Deporte for his undergraduate Collaboration Research Grant. M.M.M.-C.

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