Formation of isolated DNA bases pairs and their water clusters

我们开发了一种新的激光解吸方法与超音速喷射技术相结合来产生DNA碱基及其水合团簇的冷隔离团簇。这种方法被发现在产生水合的非挥发性分子簇方面特别有效，因此在对DNA水合结构的微观理解中很有用。结果表明，碱基对的激发态动力学强烈依赖于各自碱基的互变异构体。例如，嘌呤基鸟嘌呤类化合物的激发态寿命为亚皮秒量级，而其他非正则形式的互变异构体的寿命为纳秒量级。此外，腺嘌呤的激发态动力学在水合过程中发生了显著的变化。这可以用初始激发态到低激发态的超快弛豫和随后的解离来解释。这与2-氨基嘌呤的结果一致，在2-氨基嘌呤中没有发生超快动力学，因此已知是强荧光的。我们还利用这种解吸技术成功地制备了DNA核苷鸟苷的水合簇合物。这是首次在孤立条件下观察到核苷的冷水合团簇。鸟苷的电子光谱在水合时发生了显著的红移，这表明激发态动力学受溶剂的影响。这种激光解吸技术有望有效地汽化热脆的生物分子

We have developed a novel laser desorption method in combination with the supersonic-jet technique for producing cold isolated clusters of the DNA bases and their hydrated clusters. This method is found to be especially effective in producing hydrated clusters of the nonvolatile molecules and thus useful in the microscopic understanding of hydration structures of DNA. The results show that excited-state dynamics of the base pairs are strongly dependent on the tautomers of the respective bases. For example, the purine base guanine reveals a tautomers of the canonical form whose excited-state lifetime is on the order of subpicosecond while other tautomers of non-canonical forms have nanosecond lifetimes. Furthermore, excited-state dynamics of adenine are altered significantly upon hydration. This is explained by the ultrafast relaxation of the initially excited state to a low-lying excited state and subsequent dissociation. This is found to be consistent with the result for 2-aminopurine in which no ultrafast dynamics occurs and thus is known to be strongly fluorescent.We have also succeeded in producing hydrated clusters of the DNA nucleoside guanosines with high efficiency by employing this desorption technique. This is the first report of the observation of cold hydrated clusters of the nucleosides under isolated conditions. The electronic spectra of guanosines are found to be significantly red-shifted upon hydration, suggesting that the excited state-dynamics are affected by the solvation.This laser desorption technique is expected to be effective for vaporizing thermally fragile biomolecules