Femtosecond cluster studies of the solvated 7-azaindole excited state double-proton transfer

Presented here are femtosecond pump-probe studies on the water-solvated 7-a zaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers a nd also provide new insights establishing that the excited state double-pro ton transfer in the dimer occurs in a stepwise rather than a concerted mann er. A major question addressed is whether the incorporation of a water mole cule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent stud y reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K ., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Ph ys. Chem. 101, 392-398]. In contrast to this earlier work, our present find ings reveal that both reactive and nonreactive dimers can coexist in the mo lecular beam under the same experimental conditions and definitively show t hat the clustering of water does not induce the formation of the nonreactiv e dimer. Rather, when present with a species already determined to be a non reactive dimer, the addition of water can actually facilitate the occurrenc e of the proton transfer reaction. Furthermore, on attaining a critical hyd ration number, the data for the nonreactive dimer suggest a solvation-induc ed conformational structure change leading to proton transfer on the photoe xcited half of the 7-azaindole dimer.