Anomalous fragmentation of hydrated clusters of DNA base adenine in UV photoionization

A multiphoton ionization study was carried out on hydrated clusters of DNA base adenine in supersonic molecular beams. Resonant two-photon ionization at 266 nm showed that the relative ion intensity between the hydrated clusters A(m)(H2O)(n) and the unhydrated ones was anomalously small, particularly for m = 1, in contrast to the case of electron impact ionization. The ratio was of the order of 10(-2) for m = 1, 10(-1) for m = 2, but about 1 for m greater than or equal to 3. One-photon excitation to the first electronically excited state was found to be responsible for the extensive fragmentation of adenine monomer hydrates A(1)(H2O)(n). The water acts as a proton-donating solvent whose hydrogen bonding with the solute becomes weakened in the n pi* excited state, thereby giving the excited state its repulsive nature. Hydrates of adenine complex A(m)(H2O), (m greater than or equal to 2) tend to better survive the rupture of the water cage at higher rn, probably because the energy transfer between adenine molecules is not sufficiently fast. The fragmentation was found to be less extensive at higher levels of excitation with a much weaker n pi* character. Change of solvent to those of less proton-donating or even proton-accepting character systematically reduced the tendency of fragmentation.