Dispersed fluorescence of jet-cooled tryptophan: Excited state conformers and intramolecular exciplex formation

Abstract

The dispersed fluorescence of the amino acid tryptophan has been measured in the environment of a cold, supersonic free jet. Analysis of the region of the spectrum near the electronic origin indicates that the electronic excitation spectrum contains features which arise from various ground state conformers of tryptophan, confirming our previous assignment of these features. Under the conditions of our experiment the conformers do not interconvert in the excited state during the fluorescence liftime. Analysis of the dispersed emission spectrum of one conformer reveals broad red-shifted fluorescence which exists even when the electronic origin transition is excited. This broad red-shifted fluorescence is produced by the formation of an intramolecular exciplex involving excited state proton transfer to form a zwitterion. Molecules which do not have the ability to form a zwitterion do not exhibit this behavior, and deuterated trytophan shows broad fluorescence in an amount consistent with a slower proton transfer rate. The significance of these results for understanding the excited state photophysics of tryptophan in solution is discussed.