Excited state proton transfer in guanine in the gas phase and in water solution: a theoretical study

Theoretical investigations were performed to study the phenomena of ground and electronic excited state proton transfer in the isolated and monohydrated forms of guanine. Ground and transition state geometries were optimized at both the B3LYP/6-311++G(d,p) and HF/6-311G(d,p) levels. The geometries of tautomers including those of transition states corresponding to the proton transfer from the keto to the enol form of guanine were also optimized in the lowest singlet pipi* excited state using the configuration interaction singles (CIS) method and the 6-311G(d,p) basis set. The time-dependent density function theory method augmented with the B3LYP functional (TD-B3LYP) and the 6-311++G(d,p) basis set was used to compute vertical transition energies using the B3LYP/6-311++G(d,p) geometries. The TD-B3LYP/6-311++G(d,p) calculations were also performed using the CIS/6-311G(d,p) geometries to predict the adiabatic transition energies of different tautomers and the excited state proton transfer barrier heights of guanine tautomerization. The effect of the bulk aqueous environment was considered using the polarizable continuum model (PCM). The harmonic vibrational frequency calculations were performed to ascertain the nature of potential energy surfaces. The excited state geometries including that of transition states were found to be largely nonplanar. The nonplanar fragment was mostly localized in the six-membered ring. Geometries of the hydrated transition states in the ground and lowest singlet pipi* excited states were found to be zwitterionic in which the water molecule is in the form of hydronium cation (H3O(+)) and guanine is in the anionic form, except for the N9H form in the excited state where water molecule is in the hydroxyl anionic form (OH(-)) and the guanine is in the cationic form. It was found that proton transfer is characterized by a high barrier height both in the gas phase and in the bulk water solution. The explicit inclusion of a water molecule in the proton transfer reaction path reduces the barrier height drastically. The excited state barrier height was generally found to be increased as compared to that in the ground state. On the basis of the current theoretical calculation it appears that the singlet electronic excitation of guanine may not facilitate the excited state proton transfer corresponding to the tautomerization of the keto to the enol form.     

Excited-state proton transfer through water bridges and structure of hydrogen-bonded complexes in 1H-pyrrolo[3,2-h]quinoline: adiabatic time-dependent density functional theory study

Proton transfer reaction is studied for 1H-pyrrolo[3,2-h]quinoline-water complexes (PQ-(H(2)O)(n), n = 0-2) in the ground and the lowest excited singlet states at the density functional theory (DFT) level. Cyclic hydrogen-bonded complexes are considered, in which water molecules form a bridge connecting the proton donor (pyrrole NH group) and acceptor (quinoline nitrogen) atoms. To understand the effect of the structure and length of water bridges on the excited-state tautomerization in PQ, the potential energy profile of the lowest excited singlet state is calculated adiabatically by the time-dependent DFT (TDDFT) method. The S(0) --> S(1) excitation of PQ is accompanied by significant intramolecular transfer of electron density from the pyrrole ring to the quinoline fragment, so that the acidity of the N-H group and the basicity of the nitrogen atom of the quinoline moiety are increased. These excited-state acid-base changes introduce a driving force for the proton transfer reaction. The adiabatic TDDFT calculations demonstrate, however, that the phototautomerization requires a large activation energy in the isolated PQ molecule due to a high energy barrier separating the normal form and the tautomer. In the 1:1 cyclic PQ-H(2)O complex, the energy barrier is dramatically reduced, so that upon excitation of this complex the tautomerization can occur rapidly in one step as concerted asynchronous movements of the two protons assisted by the water molecule. In the PQ-(H(2)O)(2) solvate two water molecules form a cyclic bridge with sterically strained and unfavorable hydrogen bonds. As a result, some extra activation energy is needed for initiating the proton dislocation along the longer hydrogen-bond network. The full tautomerization in this complex is still possible; however, the cooperative proton transfer is found to be highly asynchronous. Large relaxation and reorganization of the hydrogen-bonded water bridge in PQ-(H(2)O)(2) are required during the proton translocation from the pyrrole NH group to the quinoline nitrogen; this may block the complete tautomerization in this type of solvate.     

Theoretical analysis of excited states and energy transfer mechanism in conjugated dendrimers

The excited states of the phenylene ethynylene dendrimer are investigated comprehensively by various electronic‐structure methods. Several computational methods, including SCS‐ADC(2), TDHF, TDDFT with different functionals (B3LYP, BH&HLYP, CAM‐B3LYP), and DFT/MRCI, are applied in systematic calculations. The theoretical approach based on the one‐electron transition density matrix is used to understand the electronic characters of excited states, particularly the contributions of local excitations and charge‐transfer excitations within all interacting conjugated branches. Furthermore, the potential energy curves of low‐lying electronic states as the functions of ethynylene bonds are constructed at different theoretical levels. This work provides us theoretical insights on the intramolecular excited‐state energy transfer mechanism of the dendrimers at the state‐of‐the‐art electronic‐structure theories. © 2014 Wiley Periodicals, Inc.     

Conformational equilibria and photoinduced tautomerization in 2-(2′-pyridyl)pyrrole

Depending on the polarity and protic abilities of the solvent, 2-(2′-pyridyl)pyrrole can exist in either syn or anti rotameric forms. In nonpolar solvents, intramolecular excited state single proton transfer is observed, manifested by the appearance of low-energy tautomeric emission. The solvent-assisted excited state double proton transfer reaction is also detected. DFT calculations confirm low barriers for both single and double proton transfer processes in the lowest excited singlet state and show different character of the tautomerization in both cases: in the intramolecular reaction, mutual approach of two nitrogen atoms plays an important role.     

Theoretical investigation of the photophysics of methyl salicylate isomers

The photophysics of methyl salicylate (MS) isomers has been studied using time-dependent density functional theory and large basis sets. First electronic singlet and triplet excited states energies, structure, and vibrational analysis were calculated for the ketoB, enol, and ketoA isomers. It is demonstrated that the photochemical pathway involving excited state intramolecular proton transfer (ESIPT) from the ketoB to the enol tautomer agrees well with the dual fluorescence in near-UV (from ketoB) and blue (from enol) wavelengths obtained from experiments. Our calculation confirms the existence of a double minimum in the excited state pathway along the O-H-O coordinate corresponding to two preferred energy regions: (1) the hydrogen belongs to the OH moiety and the structure of methyl salicylate is ketoB; (2) the hydrogen flips to the closest carboxyl entailing electronic rearrangement and tautomerization to the enol structure. This double well in the excited state is highly asymmetric. The Franck-Condon vibrational overlap is calculated and accounts for the broadening of the two bands. It is suggested that forward and backward ESIPT through the barrier separating the two minima is temperature-dependent and affects the intensity of the fluorescence as seen in experiments. When the enol fluoresces and returns to its ground state, a barrier-less back proton transfer repopulates the ground state of methyl salicylate ketoB. It is also demonstrated that the rotamer ketoA is not stable in an excited state close to the desired emission wavelength. This observation eliminates the conjecture that the near-UV emission of the dual fluorescence originates from the ketoA rotamer. New experimental results for pure MS in the liquid state are reported and theoretical results compared to them.     

DFT based computational study on the excited state intramolecular proton transfer processes in o-hydroxybenzaldehyde

Potential energy (PE) curves for the intramolecular proton transfer in the ground (GSIPT) and excited (ESIPT) states of o-hydroxybenzaldehyde (OHBA) were studied using DFT-B3LYP/6-31G(d) and TD-DFT-B3LYP/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer in this compound. Excited states PE calculations support the ESIPT process in OHBA. The contour PE diagram and the variation of oscillator strength along the proton transfer co-ordinate support the dual emission in OHBA. Our calculations also support the experimental observations of Nagaoka et al. [S. Nagaoka, U. Nagashima, N. Ohta, M. Fujita, T. Takemura, J. Phys. Chem. 92 (1988) 166], i.e. normal emission of the title compound comes from S(2) state and the red-shifted proton transfer band appears from the S(1) state. ESIPT process has also been explained in terms of HOMO and LUMO electron density of the enol and keto tautomer of OHBA and from the potential energy surfaces.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

Photoreaction in BLUF receptors: proton-coupled electron transfer in the flavin-Gln-Tyr system

Photoinduced electron transfer from tyrosine to the flavin chromophore is involved in activation of BLUF (sensor of blue light using FAD) photoreceptors. We studied the electron transfer (ET) coupled with proton-transfer (PT) reactions, by means of XMCQDPT2//CASSCF calculations on a molecular cluster model. By defining a minimum active space in the CASSCF calculations, we could compute the entire photoreaction pathway. We find that the crossing of the locally excited and ET states is located along the flavin bond-stretching coordinate. The ET state is stabilized by a proton transfer from the electron donor to the electron acceptor. We mapped two different PT pathways from tyrosine to flavin via the conserved glutamine. These reactions generate a tautomeric form of glutamine. Along the PT coordinates, we find geometries where the ET and the electronic ground states degenerate. At the state crossing structures, either formation of the ground state biradical intermediate or a relaxation back to the Franck-Condon minimum takes places. The computed relaxation pathways reveal that the hydrogen bonds involving glutamine in the chromophore-binding pocket control BLUF photoefficiency.     

Ring-hydrogen participation in the keto--enol isomerization of the acetophenone radical cation

Molecular ions obtained from acetophenone have been observed to undergo proton transfer reactions in competition with unimolecular blackbody dissociation in a Fourier transform ion cyclotron resonance spectrometer provided with an in situ high temperature blackbody source. The ionizing energy dependence of these two processes and generation of the enol molecular ion by fragmentation of butyrophenone reveal that the keto ion undergoes blackbody dissociation exclusively while the enol ion promotes fast proton transfer reactions and undergoes very slow blackbody induced dissociation. Experiments with labeled acetophenone either on the methyl group or on the ring reveal that the enol ions can transfer both H+ and D+ suggesting that the mechanism responsible for the tautomerization process of these radical cations may involve scrambling of the methyl and ring hydrogens, or more than one mechanism. Theoretical calculations at the B3LYP level predict that the most favorable pathway for unimolecular isomerization of the keto ion involves initial migration of an ortho hydrogen to the carbonyl. The subsequent rearrangement to the enol form is calculated to require enough internal energy that would allow hydrogen walk around the benzene ring in agreement with the experimental results. The possibility that isomerization may also occur by a direct 1,3-hydrogen migration is also explored in terms of possible excited electronic states of the ion.     

Spectral and acid-base features of 3,7-dihydroxy-2,8-diphenyl-4H,6H-pyrano[3,2-g]chromene-4,6-dione (diflavonol)--a potential probe for monitoring the properties of liquid phases

Diflavonol is a molecule that can exist in neutral or anionic form and in several tautomeric forms in ground and excited states. Absorption and emission spectroscopy combined with theoretical calculations have shown that only one tautomer of neutral diflavonol exists in the ground state, but two exist in the excited state. In the latter case, one is the tautomer originating from the ground state tautomer, which exists in strongly protic solvents, the other is the phototautomer occurring in weakly protic or aprotic solvents as a result of the intramolecular transfer of one proton. The OH groups present in diflavonol and involved in weak intramolecular hydrogen bonds exhibit a proton-donating ability reflected by the experimental values of acidity constants or theoretical enthalpies and free energies of proton detachment. The electronically excited molecule is a relatively strong acid when it loses one proton. With increasing basicity of the medium, monoanionic and dianionic forms occur which exhibit spectral characteristics and an emission ability different from those of neutral diflavonol. These interesting features of diflavonol open up possibilities for the analytical use of the compound and its application as a spectral probe sensitive to the properties of liquid phases.     

硫代乙酰胺光化学反应机理的理论研究

用B3LYP, MP2和CASSCF方法, 采用cc-pVDZ和6-31++G**基组, 研究了硫代乙酰胺在基态和最低三态上消除硫化氢以及其它光解离反应, 并考虑了单个溶剂分子参与反应对质子迁移反应的影响, 得到了消除硫化氢反应的反应机理, 计算结果可以很好地解释实验结果. 进而用CASSCF方法计算了第一激发单态上的各驻点, 以及各交叉点. 计算结果表明, 在S1和T1态上发生除分子内转动以外的化学反应的可能性比较小, 当分子被激发到S2态上时, 将通过S2/S1交叉点到S1态, 在S1态上的分子有两条途径去活化, 通过S1/S0交叉点到热基态, 通过S1/T1交叉点系间窜越到T1态. 因而得出CH3CSNH2发生光解离反应的可能性不大. 基于此, 可将硫代酰胺结构引入蛋白或多肽中, 有望在不破坏分子整体结构的情况下对其进行光化学研究.     

Probing irradiation induced DNA damage mechanisms using excited state Car-Parrinello molecular dynamics

Photoinduced proton transfer in the Watson-Crick guanine (G)-cytosine (C) base pair has been studied using Car-Parrinello molecular dynamics (CP-MD). A flexible mechanical constraint acting on all three hydrogen bonds in an unbiased fashion has been devised to explore the free energy profile along the proton transfer coordinate. The lowest barrier has been found for proton transfer from G to C along the central hydrogen bond. The resulting charge transfer excited state lies energetically close to the electronic ground state suggesting the possibility of efficient radiationless decay. It is found that dynamic, finite temperature fluctuations significantly reduce the energy gap between the ground and excited states for this charge transfer product, promoting the internal conversion process. A detailed analysis of the internal degrees of freedom reveals that the energy gap is considerably reduced by out-of-plane molecular vibrations, in particular. Consequently, it appears that considering only the minimum energy path provides an upper-bound estimate of the associated energy gap compared to the full-dimension dynamical reaction coordinate. Furthermore, the first CP-MD simulations of the G-C base pair in liquid water are presented, and the effects of solvation on its electronic structure are analyzed.     

Photo-isomerization of Aromatic α-Hydroxy Hydrazone

Photo-isomerization of aromatic α-hydroxy hydrazone was reported. We investigated the structures of salicylaldehyde phenylhydrozone(SP) in the ground state using density functional theory(DFT) with the B3LYP functional and the 6-311+G(d) basis set. All nine possible isomers of SP in the ground state consist of seven phenol forms and two ketone forms. Intrisic reaction coordinate(IRC) analysis discloses the existence of a cycle driven by the two proton transfer processes in the ground and excited states of SP, which suggests that no ketonic form could exist in the ground state. Further theoretical studies of the potential energy surfaces support a trans-cis conversion followed by a relaxation to the stable form of SP in the excited states.     

Synthesis, characterization, and spectroscopic investigation of benzoxazole conjugated Schiff bases

Two Schiff bases were synthesized by reaction of 2-(4'-aminophenyl)benzoxazole derivatives with 4-N,N-diethylaminobenzaldehyde. UV-visible (UV-vis) and steady-state fluorescence in solution were applied in order to characterize its photophysical behavior. The Schiff bases present absorption in the UV region with fluorescence emission in the blue-green region, with a large Stokes' shift. The UV-vis data indicates that each dye behaves as two different chromophores in solution in the ground state. The fluorescence emission spectra of the dye 5a show that an intramolecular proton transfer (ESIPT) mechanism takes place in the excited state, whereas a twisted internal charge transfer (TICT) state is observed for the dye 5b. Theoretical calculations were performed in order to study the conformation and polarity of the molecules at their ground and excited electronic states. Using density functional theory (DFT) methods at theoretical levels BLYP/Aug-SV(P) for geometry optimizations and B3LYP/6-311++G(2d,p) for single-point energy evaluations, the calculations indicate that the lowest energy conformations are in all cases nonplanar and that the dipole moments of the excited state relaxed structures are much larger than those of the ground state structures, which corroborates the experimental UV-vis absorption results.     

Reconsideration on hydrogen bond strengthening or cleavage of photoexcited coumarin 102 in aqueous solvent: a DFT/TDDFT study

In this work, the excited-state hydrogen bonding dynamics of photoexcited coumarin 102 in aqueous solvent is reconsidered. The electronically excited states of the hydrogen bonded complexes formed by coumarin 102 (C102) chromophore and the hydrogen donating water solvent have been investigated using the time-dependent density functional theory method. Two intermolecular hydrogen bonds between C102 and water molecules are considered. The previous works (Wells et al., J Phys Chem A 2008, 112, 2511) have proposed that one intermolecular hydrogen bond would be strengthened and the other one would be cleaved upon photoexcitation to the electronically excited states. However, our theoretical calculations have demonstrated that both the two intermolecular hydrogen bonds between C102 solute and H(2)O solvent molecules are significantly strengthened in electronically excited states by comparison with those in ground state. Hence, we have confirmed again that intermolecular hydrogen bonds between C102 chromophore and aqueous solvents are strengthened not cleaved upon electronic excitation, which is in accordance with Zhao's works.     

基于异噁唑酮类份菁染料的激发态分子内质子转移的研究

运用量子化学理论计算方法研究了3-甲基-4-(1H-吲哚-3-次甲基)-异噁唑-5-酮(A)及其衍生物份菁染料的激发态分子内质子转移性质.研究表明:在基态3种染料AH(R=H),AO(R=—O(H3))和AP(R=—O(H2Ph))只存在酮式构型,在激发态AH与AP存在酮式和烯醇式2种构型,而AO存在酮式、烯醇式和仲胺式3种构型.红外光谱表明化合物从基态跃迁到激发态存在分子内的氢键增强作用,势能曲线显示激发态的质子转移为放热反应且能垒较低,通过分析电子光谱得到具有较大斯托克位移的激发态分子内质子转移的荧光发射峰,前线分子轨道理论计算进一步说明了其质子转移的发生过程.     

Solvent effect on the excited-state proton transfer of 7-hydroxyquinoline along a hydrogen-bonded ethanol dimer

We have studied the solvent effect on structures and potential energy surfaces along proton transfer in the ground and the excited states of 7-hydroxyquinoline interacting with an ethanol dimer using ab initio calculations. The proton transfer is forbidden in the ground state not only in vacuum but also in solvents of n-heptane, ethanol, and dimethyl sulfoxide. In the excited state, although the proton transfer is forbidden in vacuum, it is possible in solvent due to its greatly reduced barrier (～10 kcal mol(-1)) and highly stabilized product. It has also been found from the calculations that the proton-transfer barrier in the excited state decreases as the dielectric constant of a solvent increases. Our calculations are consistent with experimental results that the proton transfer does not take place in the ground state and that the excited-state proton-transfer rate increases as the solvent polarity increases. Our calculated absorption and emission properties are in excellent agreement with experimental results. Projection factors (reflecting geometrical change from the ground state to the excited state) and reorganization energies for several low frequency vibrations in connection with the excited-state proton transfer are discussed as well.     

The effect of hydrogen bonding on the excited-state proton transfer in 2-(2'-hydroxyphenyl)benzothiazole: a TDDFT molecular dynamics study

The dynamics of the excited-state proton transfer (ESPT) in a cluster of 2-(2'-hydroxyphenyl)benzothiazole (HBT) and hydrogen-bonded water molecules was investigated by means of quantum chemical simulations. Two different enol ground-state structures of HBT interacting with the water cluster were chosen as initial structures for the excited-state dynamics: (i) an intramolecular hydrogen-bonded structure of HBT and (ii) a cluster where the intramolecular hydrogen bond in HBT is broken by intermolecular interactions with water molecules. On-the-fly dynamics simulations using time-dependent density functional theory show that after photoexcitation to the S(1) state the ESPT pathway leading to the keto form strongly depends on the initial ground state structure of the HBT-water cluster. In the intramolecular hydrogen-bonded structures direct excited-state proton transfer is observed within 18 fs, which is a factor two faster than proton transfer in HBT computed for the gas phase. Intermolecular bonded HBT complexes show a complex pattern of excited-state proton transfer involving several distinct mechanisms. In the main process the tautomerization proceeds via a triple proton transfer through the water network with an average proton transfer time of approximately 120 fs. Due to the lack of the stabilizing hydrogen bond, intermolecular hydrogen-bonded structures have a significant degree of interring twisting already in the ground state. During the excited state dynamics, the twist tends to quickly increase indicating that internal conversion to the electronic ground state should take place at the sub-picosecond scale.     

The half projected Hartree-Fock function for determining singlet excited states. Application to cyclobutanone and 3-cyclopenten-1-one

The Half-Projected-Hartree-Fock procedure (HPHF) for determining singlet ground states is briefly described and extended to the direct determination of singlet excited states. The procedure is applied, using a [7s,3p/2s,1p] basis set, to determine the optimal geometry of two relatively large molecules, to which large CI calculations cannot be easily applied. These two molecules are cyclobutanone and 3-cyclopenten-1-one in their lowest singlet (n → π) excited state. Both molecules are found to exhibit in their excited state a pyramidal structure with the carbonyl oxygen atom pointing outward from the molecular plane. RHF calculations for the singlet ground state were also performed for comparison. The theoretical geometrical parameters compare well with the experimental data.     

Excited state intramolecular proton transfer in 5-hydroxy flavone: A DFT study

Potential energy surfaces (PES) for the ground and excited state intramolecular proton transfer (ESIPT) processes in 5-hydroxy-flavone (5HF) were studied using DFT-B3LYP/6-31G(d) and TD-DFT/6-31G(d) level of theory, respectively. Our calculations suggest the non-viability of ground state intramolecular proton transfer (GSIPT) in 5HF. Excited states PES calculations support the existence of ESIPT process in 5HF. ESIPT in 5HF has been explained in terms of HOMO, LUMO electron density of the enol and keto tautomer of 5HF. PES scan by phenyl group rotation suggests that the twisted form, i.e., phenyl group rotated by 18.7° out of benzo-γ-pyrone ring plane is the most stable conformer of 5HF.