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，结合超音速射流技术来产生DNA碱基的冷分离簇及其水合簇。这种方法被发现在产生非挥发性分子的水合簇方面特别有效，因此对DNA水合结构的微观理解是有用的。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，嘌呤基鸟嘌呤揭示了规范形式的互变异构体，其激发态寿命在亚皮秒量级，而其他非规范形式的互变异构体具有纳秒寿命。此外，在水合作用下，腺嘌呤的激发态动力学发生了显著改变。这可以用初始激发态的超快弛豫到低激发态和随后的解离来解释。这与2-氨基嘌呤的结果一致，其中没有超快动力学发生，因此已知具有强荧光性。我们还成功地利用这种解吸技术高效率地生产了水合的DNA核苷鸟苷簇。这是在孤立条件下观察核苷冷水合团簇的第一份报告。鸟苷的电子能谱在水化过程中发生了明显的红移，表明其激发态动力学受到了溶剂化的影响。这种激光解吸技术有望有效地汽化热脆性生物分子

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