Spectral–luminescent properties of laurdan molecule

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 64–69

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Spectral–luminescent properties of laurdan molecule Tatyana Yu. Titova ⇑, Victor Ya. Artyukhov, Oksana M. Zharkova, Julia P. Morozova National Research Tomsk State University, Tomsk-50, Lenina 36, Russian Federation

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

 The laurdan optimized geometries for

a r t i c l e

i n f o

Article history: Received 20 March 2013 Received in revised form 27 December 2013 Accepted 27 December 2013 Available online 7 January 2014 Keywords: Fluorescent probe Laurdan TDDFT method INDO method MEP method

ethanol

Wavelength, nm

S0 and S1 states were obtained by ab initio and semiempirical methods.  Analysis of the distribution charges on atoms was held.  The MEP calculations show that molecule of laurdan at the ground state has two proton-acceptor centers.  The rate constants of radioactive and non-radioactive processes were calculated for considered object.

isopropanol

480

acetonitrile 440

tetrahydrofuran ethylacetate

400

isooctane

0,0

0,2

EN T

0,4

0,6

a b s t r a c t Quantum-chemical calculations of ground and excited states of fluorescent probe (laurdan) by ab initio and semiempirical methods were performed. The laurdan optimized geometries of S0 and S1 states were obtained. The influence of laurdan nonrigidity structure on dipole moments and location of energy levels were studied. The specific solvation centers of laurdan were obtained. The rate constants of photoprocesses and fluorescence quantum yield of laurdan in non-polar solvent were calculated. Ó 2014 Elsevier B.V. All rights reserved.

Introduction Fluorescent probes are used in the study of biological materials which do not have intrinsic fluorescence. After infiltration probe can be obtained necessary and useful information about the molecular system [1–4]. Laurdan molecule (9-dodecanoyl-1-dimethylamino) (Fig. 1) is considered as a fluorescent probe. The laurdan molecule can be located at definite points of complex organic

⇑ Corresponding author. Address: National Research Tomsk State University, Lenina St. 49, Titova, 634000 Tomsk, Russian Federation. Tel.: +7 3822533426, mobile: +7 8 923 405 15 34. E-mail address: [email protected] (T.Yu. Titova). 1386-1425/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.097

structures. This can lead to different characteristics which cause show information concerning with dynamics of the environment from picoseconds up to tens of nanoseconds [2,5,6]. Laurdan and prodan (6-propionyl-2-dimethylamino naphthalene) [2,7] are a derivative of naphthalene with presence of acceptor and donor groups. Both molecules are widely applied in medical and biochemical research. The molecules have attracted a considerable interest due to their extremely large shift in the fluorescence spectra and large change in quantum yield by variation the solvent polarity. There are data about the geometry of the ground and fluorescent state, dipole moments, energies, transitions oscillator strengths, the rate constants of photoprocesses, quantum yield of fluorescence for the prodan molecule in [7–13].

T.Yu. Titova et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 64–69

65

Fig. 1. Numeration of atoms for the laurdan molecule.

Laurdan molecule is widely used [3,5,14–20]. There are experimental data on the absorption and fluorescence spectra of molecule. A value of the laurdan dipole moments varies in the range (3.8–4.0 D) for the ground and (7.6–10.2 D) for the excited state [14,15]. But for laurdan there are no data of quantum-chemical studies (structure of the ground and excited states, the rate constants of photophysical processes and fluorescence quantum yields). For laurdan molecule there are several of experimental and theoretical data on the energy levels, transitions oscillator strengths and the dipole moments [3,5,14,16,17,19–23]. Energies and dipole moments of the ground and first excited states depending on torsion angle (u) of methyl groups were calculated. The changes in the dipole moments depending on the angle u from the equilibrium value are most significant in the ground state [15]. In [23], we have shown that the laurdan has the non-rigidity structure and exists as a planar and in the non-planar conformations. The possible conformations of the probe should be involved in the formation of the solvation shell and to determine the structure of the absorption and fluorescence spectra in solution. There are specific requirements for the fluorescent probes [3], namely the solution of the probe is a heterogeneous system (a), the monomer molecules are capable of take various conformations (b), each of which is characterized by change in the dipole moment of the transition from the ground state to an excited state (c), and the possibility of various conformations differently solvated by surrounding solvent molecules (d). To confirm the requirement (d) for laurdan necessary to consider different conformations of the molecule and evaluate the specific interaction centers in the ground and excited states for each probe structure. For this reason, we found it interesting to carry out a thorough study by the theoretical and experimental methods.

Experimental and computational details The electron absorption spectrum of laurdan was registered with dual beam spectrophotometer Cary 5000 (Varian). The fluorescence spectra were registered on the SDL-2 spectrofluorometer. For luminescent measurements, the photon counting method was used. The excitation wavelengths were within 250–380 nm. For spectra excitation the HAMAMATSU lamp was used. The 10 mm cell was used. All spectral measurements were carried out at the ambient temperature of 298 K. Hexane, ethanol, triton and water were used as solvents. All using solvents were of spectroscopic grade and were obtained from joint-stock company Ekros. The laurdan was obtained from Fluka company. The solvents were used without further purification after confirming the absence of

absorption and fluorescent impurities. The laurdan concentrations are 3.5  105  6  104 M. The laurdan geometry optimization of S0 was performed in three stages. We started from a planar laurdan structure. At the first stage of laurdan optimization was performed by the molecular mechanics method MM2 (software package Chem Office 10.0). At the second and third stages the laurdan was optimized by TDDFT (Time-Dependent Density Functional Theory methods [24] with hybrid functional B3LYP [25] (GAMESS US software package) [26]. Using algorithms were QA (Quadratic approximation) and NR (Straight Newton–Raphson) methods of optimization (GAMESS US software package) [26]. All ab initio calculations were performed with system on the bases of processors Intel Xeon 5150 2.667 GHz and grid QLogic InfiniPath of interregional calculating center of Tomsk State University [27]. Imitation of molecule motions was studied by the molecular dynamics method (Chem Office 10.0). Semiempirical quantum-chemical calculations of the laurdan molecule were performed using the software package that reproduced the method of intermediate neglect of differential overlap (INDO) with specific spectroscopic parameterization [28,29]. The molecular electrostatic potential (MEP) methods was used to estimate the possibility of specific interaction of the molecule with the proton-donor solvent. The MEP method is realized in a semiempirical software package, reproduced on the basis of INDO method [29]. The search of laurdan geometry for S1 excited state has been carried out in two ways. We started from the laurdan optimized geometry of S0. At the first way we used QA and NR methods of optimization (GAMESS US software package). At the second way we used a procedure described in [30]. This procedure is based on knowledge of atoms charges distribution in a molecule and an established linear dependence between bond length and bond population. Results and discussion Geometric laurdan structure in the ground state In the initial step of research was determined geometric structure of the laurdan in ground state. At the first stage the laurdan structure was optimized by MM2 method. Planar geometric structure with deviation of dimethylamino group less than 1° has been obtained. The authors indicate that the orientation of methyl groups of laurdan in the excited state the similar to the plane of the naphthalene ring [15]. The investigation of laurdan dynamic

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T.Yu. Titova et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 64–69

behavior was realized for molecule structure optimized by MM2 method. The maximum rotation angle of methyl groups of dimethylamino group is ±60° on both sides of the molecule plane. Deformation of aromatic rings and rotation of C–C bonds in hydrocarbon chain of the laurdan molecule are observed. The averaged data obtained by molecular dynamics method showed a planar laurdan structure. For the ground state laurdan structure optimized by MM2 method calculations are carried by the INDO method [30]. In the second step in optimization of the S0 laurdan geometry was held by TDDFT/B3LYP method (software package GAMESSUS [26]). Received S0 laurdan geometric structure is planar (Fig. 1). This structure is confirmed by frequency analysis. All the calculated vibrational frequencies are real.

The absorption quantum-chemical calculations of laurdan in gase phase for the S0 molecule geometry optimized by TDDFT method has been obtained (Table 2). Band in the range within 25,000–35,000 cm1 is formed by three electronic transitions (Table 2). Lowest state is pp* type. Table 2 represents the energies of transitions (in cm1), transitions oscillator strengths is calculated by the TDDFT method for geometric structure of the laurdan ground state. As for the method INDO we obtain three electronic transitions, but the order of the lower energy level is changed. The dipole moment of the laurdan ground state determined by the method TDDFT is 6.29 D. Fluorescence (experimental and theoretical data)

Absorption (experimental and theoretical data) Let us discuss the absorption spectrum (25,000–45,000 cm1) of laurdan (Fig. 2a). The long-wavelength absorption band occupies a considerable interval (10,000 cm1) and has a multiple form. S0 ? S1, S0 ? S2, and S0 ? S3 transitions are laid within the absorption band. In the shortwave range the second derivative method gives five electronic transitions (Fig. 2b). Quantum-chemical calculations of laurdan molecule (energy, transitions oscillator strengths, dipole moments) by INDO method are shown in Table 1. These data were obtained for planar and non-planar laurdan structures with turn of methyl groups on 60° (Table 1). According to data in Table 1 and Fig. 2, the long-wavelength absorption range (25,000–35,000 cm1) is formed by three electronic transitions: first is np* type, second and third is pp* type. The calculated results by INDO method for planar structure do not give the allowed transition within 37,000–38,380 cm1, obtained from the experimental spectrum. Calculation of non-planar structure is reveals this transition. Absorption range within 35,000– 45,000 cm1 is formed by eight electronic transitions, some of which are nearly spaced at energy and some is forbidden (Table 1). These data are in fairly good agreement with experiment. Table 1 collects the calculated by INDO method transition energy (cm1), oscillator strengths, electric dipole moments (D) for two conformations of laurdan. The dipole moment of the ground state for planar laurdan structure is 6.3 D. At rotation of methyl groups on corner to 60° the dipole moment is 5.55 D. The greatest change in the dipole moment of the molecule is observed in the state S3 (pp* type). This state with intramolecular charge transfer (requirement (c)).

0,8

220

260

0,6

340

280

330

D

0,4 0,2

a

S2 370

S3

S1

The calculations by INDO method (Table 1) were performed for the inert solvent. The next step in our investigation was analysis of laurdan fluorescence spectrum in n-hexane (Fig. 3). Analyze of this spectrum shows that the complicated band structure was observed at 3800 Å excitation wavelengths (kex). The maximum at 3950 Å (25,250 cm1) and an obvious pronounced shoulder at 4120 Å (24,270 cm1) are observed. The distance between maximum and shoulder is 170 Å (1080 cm1). In the fluorescence spectrum, a sharp band asymmetry on the long-wavelength side is observed. The Stokes shift for the laurdan molecule in n-hexane is 4550 cm1. The mirror symmetry rule in n-hexane is not observed (Fig. 4). This allows us to assume the existence in solute more than one radiation center which in our view corresponds to different conformations of the molecule. There are no data about laurdan fluorescent state geometry, molecule rate constants of photoprocesses, and probe specific salvation centers in our existing scientific literature. The S1 laurdan geometry was calculated by TDDFT method (package Gamess) (procedure 1) and on the basis of the linear dependence between the length and the population of bond in the ground and excited states (procedure 2) [28]. The laurdan S1 geometric structure obtained by TDDFT method is planar. Similarly the geometry of the ground state obtained by the same method, changes are observed in bond lengths C–C no more than 0.03 Å and 0.04 Å for aromatic ring and for dimethylamino group (C1–N25), respectively. Changes the length of C–C bonds of the hydrocarbon chain is negligible. The difference of valence angles at the transition from state S0 in state S1 in the aromatic ring is 1–2°. The geometry of the excited state of the laurdan molecule calculated as procedure 1 and procedure 2 differs insignificantly. Calculation of laurdan S1 geometry by the procedure 1 was carried out using TDDFT method, and by the procedure 2 – INDO method (Table 3) for the planar structure. Table 3 collects the energies of transitions (in cm1), transitions oscillator strengths is calculated by the TDDFT and INDO methods. Lowest laurdan state is pp* type. The transitions oscillator strengths of S1 state defined by the TDDFT and INDO methods differ by 2.5 times. Photoprocesses in the molecule of laurdan

0,0 -0,2

b

-0,4 200

250

300

350

Wavelength (nm) Fig. 2. Absorption spectrum of laurdan in n-hexane (a) and the second derivative of the absorption spectrum of laurdan in n-hexane (b).

Note that for the evaluation of the rate constants of photoprocesses, we used only one software package, based on the INDO method. Taking into account the geometry of excited state of laurdan for the planar and non-planar structures the energy level diagrams were constructed (Fig. 5). Rate constants of radioactive and non-radioactive processes were calculated (Fig. 5). In both diagrams the lowest singlet states is pp* type. The positions of the lowest singlet states differ by 1000 cm1. Probably both states are formed laurdan fluorescence spectrum. Laurdan

67

T.Yu. Titova et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 64–69 Table 1 Spectral characteristics of conformations by the INDO method. Laurdan (D = 6.30), optimized geometry

S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11

Laurdan (D = 5.55), methyl group rotation at 60°

Nature of state

E

f

l

Nature of state

E

f

l

np⁄

29,580 30,780 33,290 38,250 40,580 40,910 41,880 41,960 43,570 43,840 44,620

.0000 .0345 .4099 .0000 .4710 .0019 .0649 .4563 .6550 .0036 .0004

2.29 8.30 11.92 8.41 10.98 6.15 9.67 10.09 7.94 7.54 8.90

np⁄

29,310 31,490 33,540 38,540 39,350 40,830 42,060 43,230 43,780 44,520 45,030

.0000 .0058 .3019 .1264 .0059 .2595 .0074 .0201 .7199 .0007 .0675

1.49 7.13 11.85 11.80 6.83 11.89 8.41 7.22 6.97 6.45 8.89

pp⁄ pp⁄ pp⁄ pr⁄ pp⁄ pp⁄ pp⁄ pr⁄ pr⁄ pp⁄ pr⁄ pr⁄

Table 2 Absorption characteristics for plane geometric structure.

pp⁄ pp⁄ pr⁄ pp⁄ pp⁄ pr⁄ pr⁄ pp⁄ pp⁄ pr⁄ pr⁄ pp⁄

Table 3 Fluorescence characteristics of laurdan molecule by the TDDFT and INDO methods.

Si

E (cm1)

f

1 2 3

28,230 30,340 30,560

0.308 0.001 0.051

E (cm1) (procedure 1)

f (procedure 1)

E (cm1) (procedure 2)

f (procedure 2)

25,880 29,330 29,510

0.241 0.000 0.036

25,850 28,190 29,960

0.097 0.008 0.204

25000

2

Ifl , rel.un

20000

1

15000

3950

10000

4120

3980

5000

0 3600

4000

4400

4800

constants of the radiative transition laurdan molecule for states S1, S2 and S3 are the seventh, sixth, and eighth order, respectively. Comparing these results with the laurdan fluorescence spectrum in n-hexane is showed, the best agreement between experimental and theoretical data on the position of the S1 level is observed for the planar structure of the molecule. Laurdan maximum phosphorescence in ethanol is 17,000 cm1 [18] which corresponds to calculated by us level of T1 (Fig. 5). Information on experimental data about location of the higher triplets we have not. The calculation of the fluorescence quantum yield (u) demonstrated the following: uS1 !S0 are 0.031 and 0.129 for the planar and the non-planar structure, respectively.

Wavelength (Å) Fig. 3. Room temperature fluorescence spectra of the laurdan in n-hexane for excitation at 250 (curve 1) and 380 nm (curve 2).

1,0

1

2

0,8

0,6

0,6

0,4

0,4

0,2

0,2

0,0 300

350

400

450

Fluorescence

Absorption

0,8

1,0

0,0 500

Wavelength (nm) Fig. 4. Room temperature normalized absorption (curve 1) and fluorescence spectra (curve 2) in n-hexane for laurdan.

fluorescence spectrum of the solution is defined by several conformations of the molecule. The fluorescence spectrum of each conformation of laurdan formed one electronic transition. The rate

Estimation of the laurdan centers specific solvation To confirm the requirement (d) (see the introduction) is necessary to know the centers of specific solvation of probe. Table 4 shows the data obtained by MEP and effective charges on the atoms, calculated by INDO method. Analysis of the distribution charges on atoms showed that in the ground state planar structure of the molecule is the strongest negative charge is localized on the oxygen and nitrogen atoms. Slight negative charge localized on the carbon atoms of the hydrocarbon chain and the aromatic ring (QC2 = .070, QC6 = .041, QC7 = .030, QC10 = .027). The charges on the oxygen atom in the S0 and S2 states are almost identical. In state S1 a negative charge of the oxygen atom is distributed between the nitrogen atom (Q = .293) and the carbon atoms of the aromatic ring (QC10 = .157, QC4 = .042, QC5 = .108, QC6 = .101, QC7 = .106, QC2 = .140). In the excited state S2 (pp* type) the negative charge of the nitrogen atom is distributed between the oxygen atom (Q = .620) and the carbon atoms of the aromatic ring. The distribution of effective charges in the molecule for the non-planar structure of laurdan is similar. The MEP calculations show that molecule of laurdan at the ground state has two proton-acceptor centers. The strongest one is related to the oxygen atom of the carbonyl group and localized in the molecule plane. At S0 state there are two planar minimum (multicenter probe (d)) near the oxygen atom of CO-group for both

68

T.Yu. Titova et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 64–69

Fig. 6 represents the experimental fluorescence spectrum of laurdan in solvents of different polarity. Maximum fluorescence of laurdan in n-hexane is 25,800 cm1, in triton X-100 is 21,740 cm1 and in ethanol is 20,830 cm1. Fluorescence band for solvents used in this study occupies a region about 9000 cm1. For the characteristic of polarity solvent we used an empirical parameter ENT . ENT value for n-hexane and ethanol has a value of 0.009 and 0.654, respectively [23]. ENT parameter for triton X-100 was obtained by us on a scale of polarity for prodan and laurdan using the maxima of the fluorescence bands in different solvents [23]. ENT parameter for triton X-100 is 0.38. Stokes shift of laurdan in n-hexane is 4550 cm1, triton X-100 is 6040 cm1, in ethanol is 6600 cm1. Stokes shift is dependent on parameter ENT [18,23,31]. Based on the data (method MEP) is suggested for the interaction of molecule of laurdan with solvents. Ethanol and triton X-100 can interact with laurdan on the OH– group and hydrocarbon chain. For non-planar and planar structure interaction with ethanol in the first place will be on the oxygen atom of the dodecanoyl group. For a non-planar structure, there are additional minima of aromatic rings. As a result, the maximum shift of the fluorescence band to the red region for both geometric structures in this case, probably should be about the same. The calculations showed that the energy levels location is little change for the considered conformations of laurdan. Thus in the

105000 90000

structures are localized. For a planar structure the minima relating to the nitrogen atom lower the molecule plane by 1.5 Å is observed. This minimum is 3 times less than the minima on the oxygen atom (in the molecule plane). Furthermore, in this case there are additional minima near 9th and 10th carbon atoms of aromatic ring are localized (distance of molecule plane at 1.6 Å). In state S2 pp* interaction between a molecule of laurdan with proton-donor solvent for both structures possible by the oxygen atom, aromatic ring and carbon of hydrocarbon chain. Interaction in the hydrocarbon chain is confirmed ability to use laurdan as a fluorescent probe in the study of lipid membranes [16].

C

75000

Ifl , rel.un

Fig. 5. The calculated energy levels and rate constants (s1) for the photophysical processes of laurdan molecule (the geometry of the excited state) with the planar (a) and non-planar structures (b).

60000 45000

B

30000 15000

A

0 16000

18000

20000

22000

24000

26000

-1

Wavelength (cm ) Fig. 6. Fluorescence spectra of laurdan in solvents: n-hexane (A), triton (B), and ethanol (C).

Table 4 Charge distribution and values of MEP minima (kJ/mol) for centers of specific interaction of the laurdan molecule in the ground and excited states for planar (a = 0°) and nonplanar a = 60° structure.

a = 0°

a = 60°

Atom

S0 (charge)

S2 (pp⁄) (charge)

S0 (MEP)

S2 (pp⁄) (MEP)

S0 (charge)

S2 (pp⁄) (charge)

S0 (MEP)

S2 (pp⁄) (MEP)

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C12 O14 N25

0.135 0.070 0.030 0.001 0.016 0.041 0.030 0.007 0.017 0.027 0.488 0.574 0.281

0.146 0.015 0.021 0.024 0.032 0.061 0.016 0.050 0.012 0.078 0.375 0.620 0.096

– – – – – – – – – – – 410, 330, z = 0.0 120, z = 1.2

– – 24, z = 1.00 – 24, z = 1.00 – – – – – 22, z = 1.2 550, 472, z = 0.0 –

0.144 0.047 0.027 0.002 0.018 0.021 0.028 0.006 0.013 0.028 0.489 0.572 0.328

0.151 0.014 0.020 0.010 0.027 0.030 0.026 0.042 0.024 0.070 0.380 0.618 0.109

– – – – – – – – 71, z = 1.6 71, z = 1.6  413, 330, z = 0.0 –

– – – – – – – – – – 20, z = 0.0 550, 470, z = 0.0 –

Here z is the distance from the molecule plane, Å.

T.Yu. Titova et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 124 (2014) 64–69

fluorescence spectrum of laurdan in ethanol difficult to identify these structures separately. Triton X-100 has a lower polarity than ethanol. Vibrations of dimethylamino group of laurdan in triton X-100 are hindered due to its high viscosity [32]. Ethanol has both acidic (0.658 [33]) and basic (0.400 [33]) properties. Therefore, besides the hydrogen bond on nitrogen and oxygen atoms of laurdan, interaction will be realized with the positive charged carbon atoms of the molecule. Conclusion In this paper we have reported the theoretical calculations of laurdan ground and excited states by ab initio and semiempirical methods. The laurdan structure was optimized using MM2 and TDDFT/B3LYP methods. It was shown that the molecule of laurdan satisfies to all requirements [3] imposed on fluorescent probes. The absorption and fluorescence of laurdan spectra in a nonpolar solvent are interpreted by semiempirical and TDDFT/B3LYP methods. We have also shown that laurdan to have two possible conformations (planar and non-planar) that form absorption and fluorescence spectra of the molecule. The quantitative estimation of specific solvation centers of laurdan is given. Acknowledgements This work has been supported by the Russian Foundation for Basic Research (RFBR) (12-03-31,408 Mol_a_2012 Grant). References [1] G. Weber, F.J. Farris, Biochemistry 18 (1979) 3075–3078. [2] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, second ed., Kluwer Academic Plenum Publishers, New York, 1999. [3] G.E. Dobretsov, Fluorescence Probes during Investigation of Cells, Membranes and Lypoproteins, Nauka, Moscow, 1989. [4] S.V. Ivanova, L.N. Kirpichenok, Med. Novosti 12 (2008) 56–61. [5] K.A. Kozyra, J.R. Heldt, M. Engelke, H.A. Diehl, Spectrochim. Acta 61 (2005) 1153–1161.

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