VIBRATIONALLY EXCITED RETINAL IN THE BACTERIORHODOPSIN PHOTOCYCLE - PICOSECOND TIME-RESOLVED ANTI-STOKES RESONANCE RAMAN-SCATTERING

Time-resolved (6 ps) spontaneous resonance Raman (RR) scattering from the retinal chromophore in the purple membrane of Halobacterium halobium is recorded as Stokes and anti-Stokes spectra. Anti-Stokes RR scattering is used to identify and characterize vibrationally excited, ground-state populations in retinal formed during the optical initiation of the bacteriorhodopsin photocycle. The frequencies and intensities of anti-Stokes RR bands, relative both to the corresponding Stokes RR features and to each other, are shown to be functions of the energy used in single and pump-probe, pulsed laser experiments. Time-dependent intensities of anti-Stokes RR bands demonstrate that relaxation of vibrationally excited BR-570 occurs with a rate of almost-equal-to 7 ps. The vibrational relaxation properties characterizing retinal as a chromophore embedded within a protein environment are discussed relative to those found in room temperature solutions of polyatomic molecules.