Subpicosecond fluorescence spectra of tryptophan in water

A femtosecond ultraviolet fluorescence spectrophotofluorometer has been exploited for the study of tryptophan and analogues. Using a time resolution better than 300 fs, time-resolved emission spectra have been constructed for TRP in water. They reveal an initial spectrum shifted 4900 cm(-1) from the putative 0-0 band at 295 nm, along with a picosecond relaxation to a spectrum 412 cm(-1) lower and 322 cm(-1) wider. The time constant of this "solvent relaxation" of tryptophan in water was similar to1.2 ps, as expected for forced diffusive reorientation of nearby solvent molecules upon excitation. Emission anisotropy measurements on the same time scale, obtained at a wide range of excitation and emission wavelengths, yielded "r(0)" (apparent initial anisotropies) compatible with classical measurements in glassy: solvents and single-exponential anisotropy decay. This implies that any state conversion must occur well within the impulse response of our instrument; eg., tau (IC) < 300 fs. Even upon "red edge" (301 nm) excitation, the initial anisotropy of tryptophan was still well below the theoretical. limit, 0.4. Either L-1(b) overlap of L-1(a) persists there, or the orientation of the emission transition dipole is not strictly parallel with that of L-1(a) absorption.