Douhal A, Kim S K, Zewail A H
Arthur Amos Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena 91125, USA.
Nature. 1995 Nov 16;378(6554):260-3. doi: 10.1038/378260a0.
Hydrogen bonds commonly lend robustness and directionality to molecular recognition processes and supramolecular structures. In particular, the two or three hydrogen bonds in Watson-Crick base pairs bind the double-stranded DNA helix and determine the complementarity of the pairing. Watson and Crick pointed out, however, that the possible tautomers of base pairs, in which hydrogen atoms become attached to the donor atom of the hydrogen bond, might disturb the genetic code, as the tautomer is capable of pairing with different partners. But the dynamics of hydrogen bonds in general, and of this tautomerization process in particular, are not well understood. Here we report observations of the femtosecond dynamics of tautomerization in model base pairs (7-azaindole dimers) containing two hydrogen bonds. Because of the femtosecond resolution of proton motions, we are able to examine the cooperativity of formation of the tautomer (in which the protons on each base are shifted sequentially to the other base), and to determine the characteristic timescales of the motions in a solvent-free environment. We find that the first step occurs on a timescale of a few hundred femtoseconds, whereas the second step, to form the full tautomer, is much slower, taking place within several picoseconds; the timescales are changed significantly by replacing hydrogen with deuterium. These results establish the molecular basis of the dynamics and the role of quantum tunnelling.
氢键通常赋予分子识别过程和超分子结构稳定性和方向性。特别是，沃森-克里克碱基对中的两个或三个氢键维系着双链DNA螺旋结构，并决定了碱基配对的互补性。然而，沃森和克里克指出，碱基对的可能互变异构体（其中氢原子与氢键的供体原子相连）可能会扰乱遗传密码，因为这种互变异构体能够与不同的配对伙伴结合。但是，一般氢键的动力学，尤其是这种互变异构过程的动力学，目前还没有得到很好的理解。在此，我们报告了含有两个氢键的模型碱基对（7-氮杂吲哚二聚体）中互变异构的飞秒动力学观测结果。由于质子运动的飞秒分辨率，我们能够研究互变异构体形成的协同性（其中每个碱基上的质子依次转移到另一个碱基上），并确定在无溶剂环境中运动的特征时间尺度。我们发现，第一步发生在几百飞秒的时间尺度上，而形成完整互变异构体的第二步则要慢得多，发生在几皮秒内；用氘取代氢会显著改变时间尺度。这些结果确立了动力学的分子基础以及量子隧穿的作用。
