The electronic spectrum of the amino acid tryptophan in the gas phase

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Abstract
The electronic spectrum of the amino acid tryptophan has been measured in the environment of a cold, supersonic molecular beam. The resonantly enhanced two-photon ionization spectrum of tryptophan shows some features characteristic of more volatile indole derivitives, however the region of the spectrum near the origin shows distinctive low frequency structure absent from the simpler indole containing molecules. The power dependence of the spectrum reveals features which can be attributed to several conformers of tryptophan in the molecular beam. One of these conformers shows a nearly harmonic 26 cm−1 vibrational progression which does not appear in the spectra of other indole derivitives, and the intensity pattern of this progression indicates that this particular conformer undergoes a significant geometry change upon electronic excitation. The lack of many extensive vibrational progressions in the electronic spectrum indicates that the excited state conformers of tryptophan are similar to those in the ground electronic state. The identification of a small number of stable tryptophan conformers is important for understanding the photophysics of tryptophan in solution.