Subpicosecond fluorescence spectra of tryptophan in water

A femtosecond ultraviolet fluorescence spectrophotofluorometer has been exp loited for the study of tryptophan and analogues. Using a time resolution b etter 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 spect rum 412 cm(-1) lower and 322 cm(-1) wider. The time constant of this "solve nt 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 wi de range of excitation and emission wavelengths, yielded "r(0)" (apparent i nitial anisotropies) compatible with classical measurements in glassy: solv ents and single-exponential anisotropy decay. This implies that any state c onversion 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) abs orption.