A DFT and TDDFT investigation of interactions between 1-hydroxypyrene and aromatic amino acids

Computational and Theoretical Chemistry 1073 (2015) 9–19

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Computational and Theoretical Chemistry journal homepage: www.elsevier.com/locate/comptc

A DFT and TDDFT investigation of interactions between 1-hydroxypyrene and aromatic amino acids Tülay Kaya a, Cenk Selçuki b, Nursel Acar a,⇑ a b

_ Department of Chemistry, Faculty of Science, Ege University, 35100 Bornova, Izmir, Turkey _ Department of Biochemistry, Faculty of Science, Ege University, 35100 Bornova, Izmir, Turkey

a r t i c l e

i n f o

Article history: Received 3 August 2015 Received in revised form 12 September 2015 Accepted 14 September 2015 Available online 25 September 2015 Keywords: 1-Hydroxypyrene Aromatic amino acid Intermolecular charge transfer Density functional theory Time-dependent density functional theory Polarizable Continuum Model

a b s t r a c t This study presents a computational investigation of formation photoinduced charge transfer complexes between 1-hydroxypyrene (PyOH) and aromatic amino acids (Phenylalanine: Phe, Tyrosine: Tyr, Tryptophan: Trp) in gas phase and in water. Geometry optimizations were performed by density functional theory (DFT) at xB97XD/6-311++G(d,p) level. Time-dependent density functional theory (TDDFT) was used to calculate the electronic transitions of molecules at B3LYP/6-311++G(d,p) and CAM-B3LYP/6-311++G(d,p) levels using the ground-state geometries from xB97XD/6-311++G(d,p). Polarizable Continuum Model (PCM) is used for calculations in water. Total electronic energies, complexation energies, free energy differences, solvation energies, excitation wavelengths, and HOMO–LUMO energy gaps of complexes have been analyzed and compared in gas phase and in solution. The intermolecular distances between PyOH and amino acids increased in water compared to the gas phase. The optimized complexes display an increasing complex stability in the order Trp > Tyr > Phe. Analyses of first excited singlet states have revealed that there are charge transfers between PyOH and amino acids through p–p stacking except PyOH–Phe complexes in both media. The charge distributions increased in water. Among all studied systems, PyOH–Trp systems have the most significant charge transfer between HOMO
1 and LUMO (full CT, 59%). However, dipole moment and oscillator strength of this transition (S0–S1) are weaker compared to the other studied systems. PyOH–Trp systems are determined to be the best model to investigate and design bioorganic photosensitive materials with its charge transfer character. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Photoinduced electron transfer (PET) systems are of interest in the fundamental research of the molecular design for light emitting materials. The photonic energy harvested by an antenna is transformed into ground state chemistry utilizing photoinduced electron transfer or photochemical bond reorganization. Pyrene and its substituted derivatives attract considerable attention in photoinduced charge transfer studies because of their conjugated macrocyclic p-structures [1–8]. They also have strong UV–Vis absorption and emission spectra [9–11]. Amino acids often possess particular properties, such as weak van der Waals and hydrogen bonds, wide transparency ranges in the visible regions and zwitterionic nature of the molecules which are very important in materials science [12,13]. Many aromatic hydrocarbons may form charge transfer complexes in the excited states with amino acids which resemble aro⇑ Corresponding author. Tel.: +90 232 3112387; fax: +90 232 3888264. E-mail address: [email protected] (N. Acar). http://dx.doi.org/10.1016/j.comptc.2015.09.009 2210-271X/Ó 2015 Elsevier B.V. All rights reserved.

matic amines upon excitation of their p-electronic systems [14–19]. Therefore, it is very important to know their interactions with biologically important molecules. They can form intra- and intermolecular charge transfer complexes in solution [20]. Ground state interactions between pyrene and some amine systems have also been observed formerly [21]. Pyrene derivatives have also been studied to investigate proteins and nucleic acids [22–24], to develop emitting and charge transporting materials [1,25–27]. Pyrene strongly emits blue light; therefore, Pyrene and its derivatives have been frequently used as emitting materials in organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs) and organic photovoltaic cells (OPVs) [28–36]. They are also used as oxygen sensors, DNA intercalators and solar cell components [37–39]. As mentioned earlier, functionalized Pyrene derivatives may have mechanical, optical or electrical properties that are different from those of Pyrene [17]. Our former studies on Pyrene and its derivatives have revealed that geometrical structures and molecular orbital energies are affected by the substituents and substitution positions in the Pyrene ring [21,40,41]. The design of these new structures

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T. Kaya et al. / Computational and Theoretical Chemistry 1073 (2015) 9–19

needs accurate knowledge of the effects induced by the substituents on the electronic properties of the Pyrene moiety. Quantum chemical studies can give useful information about the correlation between the structural and electronic properties [17,42]. Excitation of chromophoric systems and subsequent photochemical processes are still difficult to predict and to treat theoretically, even in some liquid phase. Most of the studies in the literature focused on intramolecular photoinduced charge transfer [43–45] and the number of studies on intermolecular photoinduced charge transfer is limited [21,40,43,46]. Therefore, the outcome of the work is important as it is expected to gain detailed insight about donor–acceptor systems composed of organic–biological hybrid components. This information will provide useful information for many related fields including health, electronics, materials science etc. In this study, 1-hydroxypyrene (PyOH) is chosen as the acceptor and aromatic amino acids (Phenylalanine – Phe, Tyrosine – Tyr and Tryptophan – Trp) that have nitrogen atoms and conjugated p-systems are chosen as donors. It is the aim of this study to investigate all possible intermolecular interactions between acceptor and donors using density functional theory (DFT) and time dependent density functional (TDDFT). Understanding the properties of the systems that are modified with electron donating part (hydroxyl group) and increasing conjugation of benzene ring, addition of nitrogen atom (in aromatic amino acids) will enhance the use of these compounds in forementioned fields and applications. This information may also help to design and to develop of new organic–biological hybrid components for the photosensitized materials.

in the present study. Conformational analyses have been performed to determine the initial structures for the monomers using Spartan 08. The ground state geometries of the molecules in gas phase were optimized using density functional theory (DFT) [50]. To improve the description of the van der Waals interactions, we employed the empirical van der Waals correction proposed by Grimme as implemented in xB97XD functional in conjunction with 6-311++G(d,p) basis set [51]. The initial complex structures were obtained by using different orientations of the donor and acceptor molecules with variable donor–acceptor distances and they were optimized in gas phase. The calculations were repeated in water. All optimized geometries have been verified as minima by frequency analysis. The hybrid DFT Becke’s three-parameter nonlocal exchange functional [52,53], with a correlation function similar to Lee– Yang–Parr (B3LYP) [54] and B3LYP [52–54] with Coulombattenuating Method (CAM-B3LYP) [55] were used for excited state calculations with time-dependent (TD) formalism in conjunction with the 6-311++G(d,p) basis set. 10 lowest singlet excited states were calculated for each molecule. Molecular orbital energies and the UV–Vis spectra of the studied molecules were illustrated with the same method using the Gaussview program using the ground state geometries. The total electron density surface of PyOH and its complexes with aromatic amino acids are mapped with the electrostatic potential values in gas phase and in water for the ground state equilibrium geometry. The Polarizable Continuum Model (PCM) [56,57] has been applied for all gas phase optimized structures to evaluate the solvation effect on the electronic transitions of the investigated systems in water.

2. Computational details

3. Results and discussion

The quantum chemical calculations were performed with Gaussian09 [47], Gaussview5.0 [48] and Spartan08 [49] programs

Pyrene is a planar symmetrical polycyclic aromatic hydrocarbon with its relatively high electron affinity values, better thermal

1-Hydroxypyrene (PyOH)

Tyrosine (Tyr)

Phenylalanine (Phe)

Tryptophan (Trp)

Fig. 1. Optimized molecular structures of 1-hydroxypyrene (PyOH) and aromatic amino acids Phenylalanine (Phe), Tyrosine (Tyr) and Trytophan (Trp) in gas phase at xB97XD/6-311++G(d,p) level.

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T. Kaya et al. / Computational and Theoretical Chemistry 1073 (2015) 9–19

in gas phase

in water

PyOH-Phe

PyOH-Tyr

PyOH-Trp

Fig. 2. Optimized molecular structures of the 1-hydroxypyrene (PyOH)–aromatic amino acid complexes in gas phase and in water at xB97XD/6-311++G(d,p) level.

stability and no absorption above 400 nm [23]. Pyrene has been used extensively to probe intermolecular interactions in bulk solutions and selected environments. Pyrene is used as a probe as its optical response is sensitive to the polarity of the local environment [58].

To investigate the geometric structure and electronic properties of 1-hydroxypyrene–aromatic amino acid complexes, we applied density functional theory for molecules and compared the results in gas phase and in water (H2O). 3.1. Geometry optimization

Table 1 Dipole moments (l, Debye), sum of electronic energies and zero point energies (Eelec + ZPE, Hartree), sum of electronic energies and free energies (Eelec + DG, Hartree) of monomers in gas phase and in water, e = 78.4, calculated at xB97XD/6-311++G(d, p) level.

l (D)

Eelec + ZPE (Hartree)

Eelec + DG (Hartree)

In gas phase PyOH Phe Tyr Trp

1.64 5.19 3.81 3.71


690.701632
554.586441
629.807706
686.120497


690.738377
554.623697
629.846355
686.160318

In water PyOH Phe Tyr Trp

2.29 7.13 5.29 4.81


690.711778
554.601244
629.826349
686.138732


690.748478
554.638345
629.865150
686.178833

In the first part, full geometry optimizations were performed in the ground state at xB97XD/6-311++G(d,p) level using Gaussian 09. Secondly, absorption properties were calculated at the B3LYP/6-311++G(d,p) and CAM-B3LYP/6-311++G(d,p) levels by TDDFT using the optimized ground state geometries from xB97XD/6-311++G(d,p) level. The optimized molecular structures of the monomers and complexes are given in Figs. 1 and 2, respectively. Optimized geometries revealed that the aromatic substituents on the amino acids were oriented toward the pyrene ring but planar structure of pyrene remained same without any perturbations because of these interactions. The calculated interatomic distances (Fig. 2) indicate that the most stable complexes were formed by van der Waals interactions. Intermolecular distances between complex pairs increase in water compared to the gas phase except

Table 2 Dipole moments (l, Debye), sum of electronic energies and zero point energies (Eelec + ZPE, Hartree), sum of electronic energies and free energies (Eelec + DG, Hartree), complexation energies (DEC) and free energy differences (DDG) of complexes in gas phase and in water, e = 78.4, calculated at xB97XD/6-311++G(d,p) level.

a

l (D)

Eelec + ZPE (Hartree)

Eelec + DG (Hartree)

DECa (kcal/mol)

DDG (kcal/mol)

In gas phase PyOH–Phe PyOH–Tyr PyOH–Trp

6.58 4.34 4.22


1245.305195
1320.528852
1376.851066


1245.361425
1320.583944
1376.904782


10.75
12.25
18.16

0.41 0.49
3.82

In water PyOH–Phe PyOH–Tyr PyOH–Trp

9.30 6.77 3.97


1245.327373
1320.553010
1376.869204


1245.382513
1320.610430
1376.924977


9.00
9.34
11.73

2.71 2.01 1.47

DEC = EComplex
(EPyOH + EAmino

Acid).

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at xB97XD/6-311++G(d,p) level. Table 2 shows dipole moments (l, Debye), sum of electronic energies and zero point correction energies (Eelec + ZPE, Hartree), sum of electronic energies and free energy changes (Eelec + DG, Hartree), complexation energies (DEC) and free energy differences (DDG) of complexes in gas phase and in water, e = 78.4, calculated at xB97XD/6-311++G(d,p) level. Negative complexation energies for PyOH–amino acid systems show stable complexes in gas phase and in water. PyOH–Trp pairs

the Tyr complex which decreases slightly. The hydroxyl hydrogen of PyOH is oriented away from the amino acid terminal groups except PyOH–Trp in gas phase which eliminates a possible hydrogen bonding between two molecules. Table 1 summarizes the calculated dipole moments (l, Debye), total electronic energies including zero point correction energies (Eelec + ZPE, Hartree) and free energy changes (Eelec + DG, Hartree) for PyOH, Phe, Tyr and Trp in gas phase and in water calculated

Table 3 Calculated solvation energies (ESOLV) and gas phase-solution free energy differences (DGGS) of studied hydroxypyrene–amino acid complexes. Molecules

lG (D)

lS (D)

ESOLV = (Eelec + ZPE)S
(Eelec + ZPE)G (kcal/mol)

DGGS = (DG + ZPE)S
(DG + ZPE)G (kcal/mol)

PyOH Phe Tyr Trp PyOH
Phe PyOH
Tyr PyOH–Trp

1.64 5.19 3.81 3.71 6.58 4.34 4.22

2.29 7.13 5.29 4.81 9.30 6.77 3.97


6.37
9.29
11.70
11.44
13.92
15.16
11.38


6.34
9.19
11.79
11.62
13.23
16.62
12.67

in gas phase HOMO (eV)

in water LUMO (eV)

ΔEH-L (eV)

HOMO (eV)

LUMO (eV)

ΔEH-L (eV)

PyOH B3LYP

3.76 -5.42

-1.66

CAMB3LYP

3.75 -5.57

-1.82

5.99 -6.59

-0.60

5.98 -6.74

- 0.76

Phe 6.25

B3LYP -7.09

-0.85

CAMB3LYP

6.29 -7.04 eV

-0.75

8.51 -8.55

-0.044

8.85 -8.54

0.31

Tyr 5.62

B3LYP -6.48

-0.86

CAMB3LYP

5.61 -6.42

-0.81

7.87 -7.89

-0.017

8.17 -7.83

0.34

Trp 5.06

B3LYP -5.95

-0.89

CAMB3LYP

5.05 -5.89

-0.84

7.37 -7.30

0.066

7.55 -7.24

0.31

Fig. 3. Frontier molecular orbitals, their energies and energy differences for studied monomers calculated in gas phase and in water at B3LYP/6-311++G(d,p) and CAM-B3LYP/ 6-311++G(d,p) levels.

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form the most stable complexes in gas phase and in water. Negative DG values observed in gas phase indicate that PyOH-aa complexes form more easily compared to the complexes in water. Table 3 summarizes the calculated solvation energies which are simply the energy differences between the gas phase energies and solution energies. Dipole moment of molecules in water increases and solvation energy decreases for each system as expected. The changes are more significant for Tyr and PyOH–Tyr as they are affected more by the polarity of the medium. This difference is most probably caused by the presence of the OH group on Tyr. As seen from Table 3, the changes have similar trends for all studied compounds; thus, all molecules are stabilized in water. Solvation energies and free energy differences are very similar with slight differences. The largest difference between them is about 1 kcal/mol for PyOH–Trp system. The negative solvation energies indicate that all monomers and complexes are stabilized in solution. 3.2. Electronic transitions Ground state electronic structures (molecular orbitals and their energies) of electron donor and acceptor molecules have been calculated with Gaussian09 software using Time-Dependent Density Functional Theory (TDDFT) at B3LYP/6-311++G(d,p) level. It is known that B3LYP may fail to produce excitation properties [59–63]; therefore, TDDFT calculations have been repeated using

CAM-B3LYP, a specific version of B3LYP designed for excitedstate calculations [55]. To assess the solvent effect, TDDFT/ B3LYP/6-311++G(d,p) and TDDFT/CAM-B3LYP/6-311++G(d,p) calculations coupled with the Polarizable Continuum Model (PCM) have been performed in water with a dielectric constant of 78.4. The energy of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) transitions are illustrated for the monomers in Fig. 3. Four frontier orbitals for the complexes in gas phase and in water are displayed in Figs. 4 and 5, respectively, with HOMO–LUMO energy levels and energy differences, DEH–L. As it is seen in Figs. 3–5, the B3LYP and CAM-B3LYP results are similar in gas phase, although some significant differences are observed for aromatic amino acids (Fig. 3). However, these discrepancies disappeared when the medium is changed from gas phase to water. The DEH–L values are significantly higher (over 2 eV, Fig. 3) for CAM-B3LYP compared to B3LYP for each of the studied systems which make the charge transfer almost impossible. Therefore, B3LYP results are used to discuss the characteristics of the excited state properties. Ionization potential is related to HOMO while electron affinity is related to LUMO. The energies of both HOMO and LUMO for all studied molecules are very low with negative values in gas phase and in water. As energy gap decreases, intermolecular charge transfer becomes easier and its probability increases. The energy gap (DEH–L) of complexes increased in water. HOMO and LUMO

HOMO-1

in gas phase HOMO

LUMO

LUMO+1

-6.66 eV

-5.68 eV

-1.91 eV

-1.28 eV

-7.99 eV

-6.84 eV

-0.85 eV

-1.18 eV

-6.14 eV

-5.49 eV

-1.76 eV

-1.17 eV

-7.54 eV

-6.65 eV

-0.70 eV

-0.12 eV

-5.74 eV

-5.42 eV

-1.72 eV

-1.09 eV

-7.07 eV

-6.58 eV

-0.67 eV

-0.11 eV

PyOH-Phe B3LYP ΔEH-L = 3.77 eV

CAMB3LYP ΔEH-L = 5.99 eV PyOH-Tyr B3LYP ΔEH-L = 3.73 eV CAMB3LYP ΔEH-L = 5.95 eV PyOH-Trp B3LYP ΔEH-L = 3.70 eV CAMB3LYP ΔEH-L = 5.91 eV

Fig. 4. Frontier molecular orbitals, their energies and energy differences for complexes calculated in gas phase at B3LYP/6-311++G(d,p) and CAM-B3LYP/6-311++G(d,p) levels.

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do not display any significant changes except Trp. It can be seen that HOMO in Trp can be found completely on the ring with a small extension to the amine group. LUMO is found completely on the Trp ring alone (Fig. 3). As a result, a weak intramolecular charge transfer (ICT) is observed. PyOH–Trp has the smallest HOMO– LUMO energy gap in gas phase (3.70 eV) (Fig. 4) and in water (3.71 eV) (Fig. 5). The spatial distributions of electron density of HOMO and LUMO orbitals are mostly dispersed in PyOH with local excitation (LE) character except PyOH–Trp in gas phase and in water where electron density moves from the substituent toward the pyrene ring indicating an increase in the charge transfer (CT) character of the transition (Figs. 4 and 5). Gas phase LUMO energies indicate that complexes have lower LUMO energies compared to PyOH (
1.66 eV). However, the LUMO energies are similar to the energy of PyOH in water (
1.81 eV). Excitation energies and the maximum absorption wavelengths of all molecules from S0 to S10 states have been calculated by using the time-dependent density functional theory (TDDFT) at the same level using the optimized ground state geometries. For simplicity, only the first lowest energy transition, namely S0 ? S1 is discussed in the text and S0 ? Sn (n = 1–6) electronic transitions with the highest four occupied and the lowest four unoccupied molecular orbitals except those for PyOH–Trp system are given in Supplementary information.

The S0 ? S1 electronic transition properties of studied molecules in gas phase and in water are given in Table 4. In Table 4, the most dominant transition is given where there are more than one transition of the same character. As in the case of DEH–L values B3LYP results are better than CAM-B3LYP results for S0 ? S1 electronic transition properties. In order to justify our choice of method for discussion, computed results are compared with the available experimental data in the literature (Table 4). The B3LYP kex values are much closer to experimental values, whereas the CAM-B3LYP results significantly deviates from experiments. The oscillator strengths (f) and transition dipole moments (ltr) for monomers are very similar in both methods except Trp which are significantly lower in B3LYP. The monomer orbitals corresponding to S0 ? S1 transitions are different in both methods. The extinction coefficient (e) is mostly overestimated by both methods but B3LYP results are much more close to the experimental values. As a result, it is clear that for the studied systems B3LYP give better results for excited state properties provided by comparison with experiment. Therefore, the following discussion on monomers and complexes is based on B3LYP results but the CAM-B3LYP results are displayed, too. Although, numerical values differ from each other, the observed complex formation and charge transfer is supported by both B3LYP and CAM-B3LYP. The values observed for S0 ? S1 transitions for PyOH and PyOH– Phe systems are similar. This result indicates that PyOH–Phe

HOMO-1

in water HOMO

LUMO

LUMO+1

-6.67 eV

-5.55 eV

-1.81 eV

-1.17 eV

-7.97 eV

-6.73 eV

-0.75 eV

-0.08 eV

-6.32 eV

-5.56 eV

-1.82 eV

-1.20 eV

-7.73 eV

-6.73 eV

-0.77 eV

-0.10 eV

-5.79 eV

-5.52 eV

-1.81 eV

-1.16 eV

-7.13 eV

-6.69 eV

-0.76 eV

-0.06 eV

PyOH-Phe B3LYP ΔEH-L = 3.77 eV

CAMB3LYP ΔEH-L = 5.98 eV PyOH-Tyr B3LYP ΔEH-L = 3.73 eV CAMB3LYP ΔEH-L = 5.96 eV PyOH-Trp B3LYP ΔEH-L = 3.71 eV CAMB3LYP ΔEH-L = 5.93 eV Fig. 5. Frontier molecular orbitals, their energies and energy differences for complexes calculated in water at B3LYP/6-311++G(d,p) and CAM-B3LYP/6-311++G(d,p) levels.

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Table 4 Calculated (B3LYP-plain, CAM-B3LYP-italic) and experimental (given in paranthesis) S0 ? S1 electronic transition properties of all investigated molecules in gas phase and in water. S0 ? S1 Gas

PyOH

Phe

Tyr

Trp

PyOH–Phe

PyOH–Tyr

PyOH–Trp

Orbitals

348 322 12000 10200 0.25 0.21 2.82 2.19 H?L

235 229 100 45 0.0006 0.0005 0.005 0.004 H?L

253 244 1500 3000 0.026 0.030 0.22 0.24 H?L

273 253 3000 6000 0.051 0.084 0.46 0.70 H?L

Character

H?L LE1

H ? L+1 LE

H ? L+2 LE

H ? L+1 LE+ICT

348 323 12000 8000 0.20 0.16 2.27 1.67 H?L H
1 ? L+1 H?L LE1 LE1+CT1 LE1

353 326 11000 13000 0.21 0.19 2.48 1.98 H?L H
2 ? L+1 H?L LE1 LE1+CT1 LE1

366 329 2000 11900 0.034 0.21 0.41 2.27 H-1 ? L H?L H?L CT1 LE1 LE1+CT1

355 328

358 329

373 330

14000 17000

15000 18100

3500 16600

kex, nm

e, M


1


1

cm

f

ltr, D

H2O

LE1

LE

LE

LE

kex, nm

354 327 (383) [64]

e, M
1 cm
1

16200 22500 (10000) [64]

f

0.37 0.41

0.06 0.33

Orbitals

276 256 (280) [65] (280) [66] 4400 10000 (3200) [65] (5600) [66] 0.078 0.11 (0.03) [65] 0.70 0.93 H?L

0.32 0.37

4.33 4.40 H?L

254 244 (275) [65] (274) [66] 2500 3000 (1400) [65] (1400) [66] 0.033 0.040 (0.02) [65] 0.28 0.32 H?L

0.31 0.34

ltr, D

235 229 (260) [65] (258) [66] 100 60 (200) [65] (200) [66] 0.0006 0.0005 (0.001) [65] 0.005 0.004 H?L

3.65 3.66 H?L

Character

H?L LE1

H ? L+1 LE

H ? L+1 LE

H?L LE

H?L LE1

LE1

LE

LE

LE

LE1

3.81 3.96 H?L H-2 ? L+1 H?L LE1 LE1+CT1 LE1

0.74 3.53 H-1 ? L H?L H?L CT1 LE1+CT1 LE1+CT1

LE1: locally excited PyOH, CT1: intermolecular charge transfer, ICT: intramolecular charge transfer.

HOMO

LUMO

-5.42 eV

-1.72 eV

HOMO-1

LUMO+1

-5.74 eV

-1.09 eV

HOMO-2

LUMO+2

-6.23 eV

-0.98 eV

HOMO-3

LUMO+3

-6.57 eV

-0.68 eV

HOMO-4

LUMO+4

-7.13 eV

-0.59 eV

Fig. 6. Molecular orbitals (MOs) and their energies of electronically excited states of PyOH–Trp in gas phase.

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Table 5 Electronic transitions (kex) corresponding to vertical excitation energies (DE), transition dipole moments (ltr), oscillator strengths (f), excitation character, molecular orbitals and their % contributions of PyOH–Trp system in the gas phase.

DE (eV)

kex (nm)

ltr

S1

3.39

365.8

S2

3.48

S3

f

Charactera

Predominant transitions

%

0.4058

0.0337

356.4

1.7269

0.1472

3.64

340.3

0.1516

0.0135

S4

3.91

317.3

0.0453

0.0043

S5

3.94

314.6

0.1805

0.0174

S6

4.02

308.5

0.0536

0.0053

CT1 LE1+CT1 LE1+CT1 CT1 LE1+CT1 LE1 LE1+CT1 LE1+CT1 LE1 LE1 LE1+CT1 LE1 LE1 LE1+CT1 LE1+CT1 LE1 LE1+CT2 LE1+CT2 CT1 LE1+CT2 LE1+CT2

H
1 ? L H?L H
2 ? L+1 H
1 ? L H?L H ? L+1 H
3 ? L H
2 ? L H ? L+1 H ? L+2 H
2 ? L H ? L+1 H ? L+2 H
3 ? L H
2 ? L H ? L+2 H ? L+3 H ? L+4 H
1 ? L+1 H ? L+3 H ? L+4

59 38 11 37 55 12 26 28 52 25 25 38 51 20 58 28 10 14 58 26 27

State

(D)

a LE1 = local excitation of PyOH; CT1 = charge-transfer from amino acid to PyOH; CT2 = charge-transfer from PyOH to amino acid.

system does not form a complex in ground state in gas phase or in water. Additionally, S0 ? S1 electronic transitions corresponding to HOMO–LUMO orbitals indicate a locally excited PyOH (LE1) both in gas phase and in water which also supports this conclusion. The LE +CT1 character observed between H
1 ? L+1 orbitals for this system in gas phase is very low (15%). There is no CT1 observed for PyOH–Phe system in transitions up to S6 in water. There is only a CT2 (charge transfer from PyOH to Phe) observed in S0 ? S3 transition (Table S2). These observations indicate that Phe do not act as a donor in contrast to the other studied aromatic amino acids. In gas phase, PyOH–Tyr system has its maximum absorption for S0 ? S1 transition. Although this system has a low charge transfer character (17% in gas phase, 15% in water), locally excited PyOH (65% and 67%) is dominant (Tables S3 and S4). On the other hand, a full CT state between H
1 ? L orbitals for S0 ? S3 transition (70%) was observed both in gas phase and in water (Figs. S3 and S4). However, oscillator strength and transition dipole moment of this transition are very weak. In PyOH–Trp system, HOMO
1 is located on Trp; in contrast, LUMO is located completely on the PyOH (Fig. 6). In other words, donor (Trp) and acceptor (PyOH) are spatially well separated and almost no overlap between the HOMO and LUMO exists. It can be concluded that there is an intermolecular charge transfer (CT, 59% in gas phase (Table 5), 51% in water (Table S5)) from Trp to PyOH corresponding to the S0 ? S1 transition between H
1 ? L orbitals. This peak was observed at 366 nm in gas phase, and it is significantly different than the peak for PyOH observed at

in gas phase (au)

in water (au)

PyOH

±6.51x10-2

±7.64x10-2

±5.95x10-2

±7.32x10-2

±6.40x10-2

±7.16x10-2

±6.04x10-2

±7.12x10-2

Phe Fig. 7. Normalized UV/Vis absorption spectra of monomers in water.

Tyr

Trp

Fig. 8. Normalized UV–Vis absorption PyOH–amino acid complexes in gas phase.

spectra

of

1-hydroxypyrene

and

Fig. 9. Total electron density surfaces colorized with respect to the molecular electrostatic potential values for monomers in gas phase and in water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

T. Kaya et al. / Computational and Theoretical Chemistry 1073 (2015) 9–19

348 nm in gas phase. Additionally, it shifted to a longer wavelength with the effect of solvent (373 nm in water). This supports that the new peak belongs to a charge transfer. The S0 ? Sn (n = 1–6) electronic transitions (kex) corresponding to vertical excitation energies (DE), transition dipole moments (ltr), oscillator strengths (f), excitation character, molecular orbitals and their % contributions for PyOH–Trp system in gas phase are summarized in Table 5. In order to evaluate complex formation, the UV–Vis absorption spectra of monomers are also predicted by TDDFT/B3LYP/6-311+ +G(d,p) method in gas phase and in water. Normalized UV–Vis absorption spectra of monomers in water are presented in Fig. 7. The spectrum of PyOH shows two peaks with maxima at 275 nm and at 353 nm. These peaks belong to p–p⁄ transitions of Py ring and shifted 1 nm and 12 nm to longer wavelengths in water compared to unsubstituted Py (Fig. S7) due to the presence of OH substituent. These shifts are observed as 1 nm and 10 nm, respectively, in gas phase (Fig. S6). S0 ? S1 electronic transition of PyOH between HOMO and LUMO is affected from the solvent as there is a 7 nm red shift from gas phase to solution with an increase of the intensity (Table 4). HOMO has the contribution from the p orbitals delocalized over PyOH, also LUMO is the antibonding p⁄ orbitals delocalized over PyOH. So, this excitation for PyOH has main p–p⁄ transition property both in gas phase and in solution (Fig. 3). This red shift is observed as 1 nm and 3 nm for Tyr and Trp, respectively (Table 4). However, there is no significant change in this excitation wavelength with the increasing solvent polarity for Phe. There is a 7 nm, 5 nm and 7 nm red shift in S0 ? S1 transition from gas phase

in gas phase (au)

17

to polar solvent for PyOH–Phe, PyOH–Tyr and PyOH–Trp, respectively. These peaks belong to locally excited PyOH for PyOH–Phe and PyOH–Tyr, but charge transfer (CT) character occurs from amino acid to PyOH for PyOH–Trp system. This shift can be confirmed easily in the normalized UV–Vis absorption spectra of systems in gas phase in Fig. 8 and in water in Fig. S8. Total electron densities are mapped with electrostatic potential surface for the excited state equilibrium geometry of the monomers in gas phase and in water (Fig. 9). Electron density may provide useful information in order to understand molecular properties like possible hydrogen bonding sites, dipole moments and electronegativity. Molecular Electrostatic Potential (MEP) gives charge distribution of the molecule. Determination of the excited state charge distribution can provide a better understanding of intramolecular charge transfer properties. The color sheme for the molecular electrostatic potential surface is as follows: red, electron rich, partially negative charge; blue, electron deficient, partially positive charge; light blue, slightly electron deficient; yellow, slightly electron rich region; green, neutral. The electron density was analyzed on the basis of the gross orbital population of Mulliken charge. The maximum and minimum limits of the total electron density colors for studied complexes are shown in Fig. 10. Due to the electron donating nature of OH group in PyOH, the charge is almost equally distributed over the Pyrene ring. On the other hand, it is mostly distributed around carboxyl terminals of amino acids on PyOH–Phe, PyOH–Tyr and PyOH–Trp systems where oxygen atoms are present. In general, the charge distribution increases in water.

in water (au)

PyOH-Phe

±7.26x10

-2

±9.10x10

±6.83x10

-2

±8.17x10

±5.41x10

-2

±7.23x10

-2

PyOH-Tyr

-2

PyOH-Trp

-2

Fig. 10. Total electron density surfaces colorized with respect to the molecular electrostatic potential values for PyOH–aromatic amino acid complexes in gas phase and in water. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

18

T. Kaya et al. / Computational and Theoretical Chemistry 1073 (2015) 9–19

4. Conclusion The aromatic amino acids interact with PyOH by using both the side chains and the terminal groups. This indicates that the interactions between the two components is not limited to p–p stacking interactions. The optimized geometries show that the p–p stacking interactions between aromatic rings increase in the order Trp > Tyr > Phe complexes. Donor–acceptor distances in complexes increase in water compared to the gas phase except the Tyr complex which in contrast, decreases slightly. The hydroxyl hydrogen of PyOH is directed away from the amino acid terminal groups except for gas-phase PyOH–Trp complex avoiding a possible hydrogen bonding between two molecules. Calculated negative complexation energies for studied complexes reveal that PyOH–Trp complex can form much more easily and thus, can make a charge transfer more easily in gas phase and in water. Solvation energies and free energy differences between gas phase and solution are calculated as negative; therefore, it is concluded that all complexes are stabilized in solution. HOMO and LUMO energy gaps may be used to explain charge transfer interactions taking place within the molecule. PyOH–Trp has the smallest HOMO–LUMO gap in gas phase and in water. UV–Vis absorption spectra exhibit red shifts of 7, 5 and 7 nm in S0 ? S1 transitions when the medium changes from gas to water for PyOH–Phe, PyOH–Tyr and PyOH–Trp, respectively. In general, S0 ? S1 transitions correspond to locally excited PyOH (LE1) in all studied complexed except PyOH–Trp where there is a charge transfer from Trp to PyOH (CT1). Total electron distribution shows that electrons are mostly localized on oxygen atoms on both donor and acceptor. The distribution is increased in water. These distributions in addition to the calculated results show that both p–p⁄ transitions and n–p⁄ transitions are possible, the former being dominant in studied systems. In conclusion, PyOH–Trp system seems to be the most suitable candidate for electron transfer systems in photosensitive products. Acknowledgements The authors gratefully acknowledge Ege University for financial support of this research work (BAP Project No: 2012 FEN 051).

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