The Spectroscopy and Photophysics of the Amino Acid Tryptophan in the Gas Phase
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The resonantly enhanced two photon ionization spectrum and the fluorescence spectrum of the amino acid tryptophan have been observed in the gas phase in a supersonic molecular beam. The gas phase spectra are well resolved as compared with solution spectra, and this resolution permits the assignment of features in the excitation spectrum to distinct conformers of the tryptophan molecule. The emission spectra of the various conformers contain both sharp and broad features, and the broad features have been assigned to emission from an intramolecular exciplex formed from an interaction between the indole chromophore and the amino acid sidechain. Excitation is initially to a non-exciplex excited electronic state that is similar in geometry to the ground electronic state. This excitation is followed by energy redistribution to the exciplex state that takes place on the timescale of the fluorescence lifetime. A prerequisite to intramolecular exciplex formation is the formation of a zwitterion in the excited electronic state. The zwitterion is formed by proton tunneling through the barrier that separates the initially excited state from the exciplex state.
KeywordsVibrational Level Ground Electronic State Excited Electronic State Fluorescence Excitation Spectrum Energy Redistribution
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