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-azaindole dimer, a model DNA base pair. In particular, studies are presented that further elucidate the nature of the reactive and nonreactive dimers and also provide new insights establishing that the excited state double-proton transfer in the dimer occurs in a stepwise rather than a concerted manner. A major question addressed is whether the incorporation of a water molecule with the dimer results in the formation of species that are unable to undergo excited state double-proton transfer, as suggested by a recent study reported in the literature [Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392–398]. In contrast to this earlier work, our present findings reveal that both reactive and nonreactive dimers can coexist in the molecular beam under the same experimental conditions and definitively show that the clustering of water does not induce the formation of the nonreactive dimer. Rather, when present with a species already determined to be a nonreactive dimer, the addition of water can actually facilitate the occurrence of the proton transfer reaction. Furthermore, on attaining a critical hydration number, the data for the nonreactive dimer suggest a solvation-induced conformational structure change leading to proton transfer on the photoexcited half of the 7-azaindole dimer.

本文呈现了针对水溶剂化7-氮杂吲哚二聚体（7-azaindole dimer）的飞秒泵浦-探测（femtosecond pump-probe）研究，该二聚体是一类DNA碱基对模型体系。具体而言，本研究进一步阐明了反应性与非反应性二聚体的本质，并提供新的认识以证实：该二聚体的激发态双质子转移以分步而非协同的方式进行。本研究探讨的核心科学问题之一是：向二聚体中引入水分子是否会生成无法发生激发态双质子转移的物种——这一假设源自近期发表的一项文献研究[Nakajima, A., Hirano, M., Hasumi, R., Kaya, K., Watanabe, H., Carter, C. C., Williamson, J. M. & Miller, T. (1997) J. Phys. Chem. 101, 392–398]。与该项早期研究结论不同，本研究的实验结果表明：在相同实验条件下的分子束中，反应性与非反应性二聚体可稳定共存；并且本研究明确证实，水分子的团簇并不会诱导非反应性二聚体的生成。相反，当水分子与已被证实为非反应性二聚体的物种共存时，添加水分子反而会促进质子转移反应的发生。此外，当达到临界水化数时，针对非反应性二聚体的实验数据表明：溶剂化诱导的构象变化会使得7-氮杂吲哚二聚体的光激发半区发生质子转移。