Department of Chemistry, University of Southern California, Los Angeles, California 90089-0482, USA.
J Phys Chem A. 2013 Aug 8;117(31):6789-97. doi: 10.1021/jp406029p. Epub 2013 Jul 25.
Water plays a central role in chemistry and biology by mediating the interactions between molecules, altering energy levels of solvated species, modifying potential energy profiles along reaction coordinates, and facilitating efficient proton transport through ion channels and interfaces. This study investigates proton transfer in a model system comprising dry and microhydrated clusters of nucleobases. With mass spectrometry and tunable vacuum ultraviolet synchrotron radiation, we show that water shuts down ionization-induced proton transfer between nucleobases, which is very efficient in dry clusters. Instead, a new pathway opens up in which protonated nucleobases are generated by proton transfer from the ionized water molecule and elimination of a hydroxyl radical. Electronic structure calculations reveal that the shape of the potential energy profile along the proton transfer coordinate depends strongly on the character of the molecular orbital from which the electron is removed; i.e., the proton transfer from water to nucleobases is barrierless when an ionized state localized on water is accessed. The computed energetics of proton transfer is in excellent agreement with the experimental appearance energies. Possible adiabatic passage on the ground electronic state of the ionized system, though energetically accessible at lower energies, is not efficient. Thus, proton transfer is controlled electronically, by the character of the ionized state, rather than statistically, by simple energy considerations.
水通过介导分子间的相互作用、改变溶剂化物种的能级、改变反应坐标上的势能分布以及促进质子通过离子通道和界面的有效传输，在化学和生物学中起着核心作用。本研究调查了由碱基的干燥和微水合簇组成的模型系统中的质子转移。通过质谱和可调谐真空紫外同步辐射，我们表明水会阻止碱基之间的离子化诱导质子转移，这种转移在干燥的簇中非常有效。相反，打开了一条新的途径，其中质子化的碱基通过从离子化水分子中转移质子和消除一个羟基自由基而产生。电子结构计算表明，质子转移坐标上的势能分布的形状强烈依赖于电子被移除的分子轨道的性质；即，当访问定域在水上的离子态时，质子从水转移到碱基是无势垒的。质子转移的计算能学与实验出现能非常吻合。虽然在较低能量下，离子化体系的基态上的绝热通道是能量上可及的，但效率不高。因此，质子转移是由离子态的性质控制的，而不是由简单的能量因素控制的。
