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Fluorescence lifetimes of jet-cooled tryptophan: elimination of complications from 1La emission

Fluorescence lifetime measurements can be made on the decay of individual 'frozen' conformers of tryptophan analogs in supersonic gas expansions. These measurements have shown differences in single exponential lifetimes among conformers of a given analog, results which are strongly consistent with the presence of conformation-dependent charge transfer quenching. For jet cooled tryptophan, however, one of the conformers emits from 1La, thereby complicating any interpretation of the results. To remove this problem we considered tryptophan analogs with C-5 substituents, in which all the jet cooled conformers emit from 1Lb. We observed differences in conformer lifetimes similar to cases considered earlier. In the course of this work we discovered that, in contrast to tryptophan, 5- methoxytryptophan shows single exponential decay in solution. Based on additional data for jet cooled samples, we propose the most likely explanation to be substituent induced changes in conformer geometries and populations.