Ultraviolet spectroscopy and tautomerism of the DNA base guanine and its hydrate formed in a supersonic jet

The electronic spectrum of laser-desorbed, jet-cooled guanine has been obtained by resonance enhanced two-photon ionization, laser induced fluorescence and hole-burning spectroscopy. Three tautomers of the molecule have been found, characterized by well-defined vibronic spectra and different excited state dynamics (fluorescence decay times of 12, 22 and 360 ns). For one of the tautomers the origin transition of the Lowest electronically excited state, proposed to be of n pi (*)-type, was found to be located at 32 803 cm(-1), very near to the origin of the first intense pi pi (*) transition located at 32 864 cm(-1). The pi pi (*)-type origin transitions of the other tautomers lie at 33 269 and 33 491 cm(-1). Three different forms have also been found for the guanine-water complex whose pi pi (*)-type origin transitions are shifted to higher energy and are located at 185, 352 and 435 cm(-1) above the origin of the lowest energy tautomer of guanine. (C) 2001 Elsevier Science B.V. All rights reserved.