TWO PHOTON STUDIES OF AMINO ACIDS AND NUCLEIC ACIDS

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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.