An ab initio study of electronic structure and spectra of 8-bromoguanine: a comparative study with guanine

Ground state geometries of the four tautomeric forms keto-N9H, keto-N7H, en ol-N9H and enol-N7H of 8-bromoguanine (8BG) were optimized using the ab ini tio RHF procedure employing a mixed basis set consisting of the 6-311 + G* basis set for the nitrogen atom of the amino group and the bromine atom, an d the 4-31G basis set for all other atoms. These calculations were followed by correlation correction of the total energy at the MP2 level using the s ame basis set. The different tautomeric forms of 8BG in the ground state we re solvated using the isodensity surface polarized continuum model (IPCM) o f the SCRF theory both at the RHF and MP2 levels. Excited states were gener ated employing configuration interaction among singly excited configuration s (CIS) obtained using a limited window of filled and empty molecular orbit als. Formation of hydrogen-bonded complexes between 8BG and three water mol ecules in the ground and excited states was considered in order to account for solvent effects approximately. Excited state geometry was optimized in each case for the lowest singlet excited state which was found to be of pi- pi* type. Vibrational frequency analysis was performed in order to ensure t hat the stationary points located on the potential energy surfaces by geome try optimization were minima. It is found that 8BG would occur in the groun d state dominantly in the keto-N7H form both at the aqueous solution-air in terface and inside the bulk liquid. The observed absorption and fluorescenc e spectra of 8BG can be explained satisfactorily considering only the keto- N7H form of the molecule. The enol tautomers of 8BG do not appear to be imp ortant from the point of view of ground state properties or electronic spec tra. The observed differences between the behaviors of guanine and 8BG can be easily explained on the basis of the results obtained. (C) 2001 Elsevier Science B.V. All rights reserved.