Ultrafast polarized fluorescence measurements on tryptophan and a tryptophan-containing peptide

In this work polarized picosecond fluorescence measurements were performed on isolated tryptophan and tryptophan in a small 22-mer peptide using a streak camera coupled to a spectrograph as a detection system. In both cases the fluorescence decay was multiexponential with decay times of ∼500 ps and ∼4 ns. Suprisingly, also a short-lived (∼16 ps) isotropic fluorescence component was found for both tryptophan and the peptide which, to our knowledge, has never been observed before. Fluorescence anisotropy data showed rotational correlation times of 155 ps and 1.5 ns for the peptide, reflecting local and overall peptide dynamics, respectively. Recently it was argued [Lakowicz, J. R. Photochem. Photobiol. 2000, 72, 421] that the nonexponential fluorescence decay of tryptophan in proteins is mainly due to spectral relaxation, whereas for isolated tryptophan it is due to different rotamers. Our results do not support this view. In contrast, we conclude in both cases that the different fluorescence decay times should be ascribed to different rotameric states.