Time-dependent Functional Theory Study On The Excited States Of The Purine Bases

The nucleic acid bases widely present in the biological, and arethe most fundamental building blocks of life. The nucleic baseshave very short lifetimes at their electronically excited state, andthey may undergo various types of photoinduced reaction, that leadto mutagenic and carcinogenic consequences. And a strikingfeature of the DNA and RNA bases is the efficient deactivationthrough very fast radiationless decay processes back to theelectronic ground state after absorption of ultraviolet (UV) light.Therefore, the understanding of the nucleic acid bases’photochemical and photophysical properties is essential forrationalization of the photostability as well as photodamagemechanisms of DNA and RNA at the current research work. In thispaper, we theoretically investigated the photochemical andphotophysical properties of the nucleic acid bases using densityfunctional theory (DFT) and time-dependent density functionaltheory (TDDFT). The main results are outlined as follows:1. Using the DFT and TDDFT methods, we performed simulationsof the adiabatic energies of four purine bases molecules for whichthe experimental0-0energies in gas phase are available. We useeight different functionals (BLYP, PBEPBE, B3LYP, PBE1PBE, M06, M062X, CAM-B3LYP and wB97XD) and fifteen basis sets to performthe calculation of the structural optimization and vibrationfrequency for9H-adenine,7H-xanthine,9H-guanine and7Hguanine,and to theoretically predict the corresponding0-0energies. Ourstudies have shown that:(1) Basis set effects are mainly related to vertical contributions,and both polarization and diffuse functions is important tocalculation of transition energy. In the following discussions, wehave selected6-311++G(d,p), which can produce reliable predictiveresults.(2) For simulating0-0energies for the purine bases, the PBE1PBEfunctional provides the smallest mean absolute error (MAE), but itgives the qualitatively incorrect excited-state geometries. However,both CAM-B3LYP and wB97XD provide very similar MAE, and theyavoid the obvious qualitative failures for excited-state geometryoptimization. B3LYP provides a slightly larger MAE, but it canprovide a very smaller standard deviation (SD) and has less costthan the CAM-B3LYP and wB97XD methods.2. Theoretical investigations were performed to study thephenomena of ground and electronic excited state proton transferin the isolated and monohydrated forms of xanthine. The ground andexcited state geometries of four tautomers of xanthine, theirmonohydrated forms, were studied computationally at the B3LYPand TD-B3LYP level using6-311++G(d,p) basis set, respectively. Wediscussed intramolecular proton transfer from two reaction pathswhich is from the keto form to the enol form. We comparised thebarrier of the ground and first singlet excited state. The effect of the bulk aqueous environment was considered using the polarizablecontinuum model (PCM). The studies have revealed:(1) The TD-B3LYP computed energies were found to be generally ingood agreement with the experimental data and the CASPT2results.(2) In gas, the ground state proton transfer reaction is characterizedby a very high barrier. The solvent effect obtained by PCM does notdecrease the barrier height. However, the inclusion of a watermolecule in the proton transfer reaction path significantlydecreases the barrier height.(3)In gas and aqueous, the barrier height of excited state issignificantly decrease compared to the corresponding values of theground state, namely, the proton-transfer barrier decrease in thelowest singlet excited state. Howere, the inclusion of the bulkaqueous salvation using the continuum model also does notdecrease the barrier height of the excited state.