ELECTRONIC-STRUCTURE, SPECTRA AND EXCITED-STATE PROPERTIES OF N,N-DIMETHLYLAMINOGUANINE

An experimental and ah initio theoretical study has been performed on the electronic spectra and structure of N.N-dimethylaminoguanine (DMAG ). The experimental work included study of absorption and fluorescence spectra at different pH in aqueous solution while the theoretical stu dy included SCF-CIS ab initio gas phase computations of ground and exc ited state geometries and electronic transitions of the molecule. Vert ical transitions from the optimised ground state of the molecule were also studied theoretically under solvation in water using the polarize d continuum model (PCM). It has been shown that at the neutral and aci dic pH (similar to 3); the molecule would exist in the ground state in the keto-N9H form while at alkaline pH (similar to 11), it would main ly occur in the anionic form deprotonated at the N, site. Further, the molecule seems to get deprotonated in the lowest singlet pi-pi excit ed state even at the neutral pH. Molecular electrostatic potential map ping was performed in the ground and lowest two singlet excited states of the keto-N9H form of the molecule in order to study the mechanism of its phototautomerism to the keto-N7H form which appears to he impor tant from the point of view of fluorescence. (C) 1998 Elsevier Science B.V. All rights reserved.