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Ionization of nucleobases is affected by their biological environment, which includes both the effect of adjacent nucleotides as well as the presence of water around it. Guanine and its nucleotide have the lowest ionization potentials among the various DNA bases. Therefore, the threshold of ionization is dependent on that of guanine and its characterization is crucial to the prediction of interaction of light with DNA. We investigate the effect of solvation on the vertical ionization energies (VIEs) of guanine and its nucleotide. In this work, we have used hybrid quantum mechanics/molecular mechanics (QM/MM) approach with effective fragment potential as the MM method of choice and equation-of-motion coupled-cluster for ionization potential with singles and doubles (EOM-IP-CCSD) as the QM method. The performance of the hybrid scheme with respect to the full QM method shows an accuracy of [ABSTRACT FROM AUTHOR]

Photo absorption properties of p-coumaric acid, the chromophore of photoactive yellow protein, in aqueous solution were investigated by means of reference interaction site model self-consistent field with spatial electron density distribution (RISM-SCF-SEDD) method. RISM-SCF-SEDD is a combination methodology of electronic structure theory and statistical mechanics for molecular liquids. Here, time-dependent density functional theory was coupled with RISM equation to study the electronic structure of p-coumaric acid in aqueous system. Excitation energies of the chromophore in its neutral, two monoanionic and dianionic forms were computed to elucidate the effect of the deprotonation and solvation on the spectroscopic properties. We found that solvation strongly affects the excitation character of the chromophore, especially for phenolate anion and dianion. The free energy difference among the four protonation states is also discussed. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Free energy profiles for the proton transfer reactions in hydrogen-bonded complex of phenol with trimethylamine in methyl chloride solvent are studied with the reference interaction site model self-consistent field method. The reactions in both the electronic ground and excited states are considered. The second-order Møller-Plesset perturbation (MP) theory or the second-order multireference MP theory is used to evaluate the effect of the dynamical electron correlation on the free energy profiles. The free energy surface in the ground state shows a discrepancy with the experimental results for the related hydrogen-bonded complexes. To resolve this discrepancy, the effects of chloro-substitutions in phenol are examined, and its importance in stabilizing the ionic form is discussed. The temperature effect is also studied. In contrast to the ground state, the ππ* excited state of phenol-trimethylamine complex exhibits the proton transfer reaction with a low barrier. The reaction is almost thermoneutral. This is attributed to the reduction of proton affinity of phenol by the ππ* electronic excitation. We further examine the possibility of the electron-proton-coupled transfer in the ππ* state through the surface crossing with the charge transfer type πσ* state. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010 [ABSTRACT FROM AUTHOR]