SUBPICOSECOND FLUORESCENCE ANISOTROPY STUDIES OF TRYPTOPHAN IN WATER

Ultraviolet fluorescence upconversion optical gating has been used to measure the excitation and emission wavelength dependence of the fluorescence depolarization of tryptophan in water with subpicosecond resolution. An initial anisotropy of 0.4 is observed for the first time. The short time decay behavior is found to be complicated by non-rotational contributions to the anisotropy decay arising from the interaction of the two low-lying 1La and 1Lb excited states. The time constant for 1Lb to 1La internal conversion is found to be 1.6 ± 0.2 ps. A generalized version of the level kinetics model of Cross et al. 29 is found to adequately describe the fluorescence anisotropy decay dependence on excitation and emission wavelength. The anisotropy data are interpreted in terms of a model for tryptophan fluorescence depolarization that includes the effects of vibronic coupling and vibrational relaxation. Our results indicate that the fluorescence anisotropy will be an ambiguous method of studying protein motion for times less than 5-10 ps. © 1990, American Chemical Society. All rights reserved.