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 ∼1.2 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., τ 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 1 L b overlap of 1 L a persists there, or the orientation of the emission transition dipole is not strictly parallel with that of 1 L a absorption.