Time-resolved Raman spectroscopy of picosecond conformational intermediates in the bacteriorhodopsin photocycle

Abstract

The biochemical activity of the bacterium halobacterium halobrium is initiated through light-driven chemical changes occurring in its purple membrane. The purple membrane is comprised of the protein bacteriorhodopsin (BR) which contains a single retinal chromophore. The structural and conformational changes in this protein-bound retina caused by light abosrption are thought to initiate the proton and ion transport across the membrane that is essential for biochemical activity. The value of BR as a model for the molecular dynamics associated with the retinal chromophore in visual pigments and for proton and ion transport across membranes is well established. Picosecond time-resolved resonance Raman (PTR3) spectroscopy has been used to record the vibrational spectra of conformational intermediates in BR formed within a few picoseconds of optical excitation. A two-laser pump-probe configuration using an actively mode-locked cw Nd:YAG laser to synchronously pump two cavity dumped dye lasers is used to obtain PTR3 spectra. Several molecular motions within the retinal chromophore, formulated in terms of a normal coordinate approximation, have been monitored by PTR3 spectroscopy over the initial 40-ps interval. These molecular motions characterize the overall conformational and structural changes in retinal chromophore occurring on the picosecond time scale.