TWO PHOTON STUDIES OF AMINO ACIDS AND NUCLEIC ACIDS

长期目标是实现对较低层的精确描述， 色氨酸和核苷酸的激发态 准确的意义上说，有用的结构和动力学信息， 由紫外吸收、圆二色性、可见双光子 吸收和时间分辨发射。这三年的重点是 色氨酸的两个最低激发态1 La和1 Lb， 相对能量对分子环境极其敏感。两 近年来， 新奇的实验我们建议继续进行旨在追踪 色氨酸及其模拟物在环境中的1 La和1 Lb态 包括分离的和复合的分子，各种冷冻溶剂， 液氦温度和蛋白质，使用中等分辨率（0.1厘米- 1)激发和检测以选择特定位点中的分子。二-- 光子激发将占显著地位，因为它可以区分1 La 和1 Lb态，即使在喷射冷却的光束。了有力的理论 组件继续发展半经验量子方法的水平 预测单电子振动态的相对能量， 用分子力学和动力学方法预测 分子环境。这项工作的一个重要部分是继续 测试常见的分子动力学方法， 应用于生物聚合物。上述技术可以应用于 核苷酸，其中存在类似的理论困难。的计算 晶体环境的影响将继续下去，因为它们 发现是必不可少的光学实验的解释 核酸碱基的晶体。

The long range goals are to achieve accurate descriptions of the lower excited states of the amino acid tryptophan and of the nucleotides-- accurate in the sense that useful structural and dynamical information can be deduced from UV absorption, circular dichroism, visible two-photon absorption, and time resolved emission. This three year period focuses upon the two lowest excited states of tryptophan, 1La and 1Lb, whose relative energy is extremely sensitive to the molecular environment. Both states have been implicated as contributing to the fluorescence in recent novel experiments. We propose to continue experiments designed to track the 1La and 1Lb states of tryptophan and model analogues in environments including the isolated and complexed molecules, various frozen solvents at liquid helium temperature, and proteins, using medium resolution (0.1 cm- 1) excitation and detection to select molecules in specific sites. Two- photon excitation will figure prominently because it can distinguish 1La and 1Lb states, even in the jet-cooled beams. A Strong theoretical component continues to develop semiempirical quantum methods to the level of predicting single vibronic state relative energies and to marry these with molecular mechanics and dynamics methods for predicting the effect of the molecular surroundings. An important part of this work is to continue testing the common molecular dynamics methods which are being increasingly applied to biopolymers. The above techniques can be applied to the nucleotides, where similar theoretical difficulties exist. Computations of the effect of crystal environments will be continued because they are found to be essential in the interpretation of optical experiments on crystals of nucleic acid bases.