Picosecond and Nanosecond Resonance Raman Evidence for Structural Relaxation in Bacteriorhodopsin’s Primary Photoproduct

Abstract

The protein bacteriorhodopsin (bR) pumps protons across the cell membrane of Halobacterium halobium, producing an electrochemical potential gradient that drives ATP synthesis [1]. Visible light absorbed by bacteriorhodopsin’s protonated retinal Schiff base chromophore initiates the proton-pumping photocycle (Fig. 1). Formation of the primary photoproduct, K, involves chromophore isomerization from all-trans to 13-cis [2] and the storage of ∼16 kcal/mole of potential energy [3]. The Schiff base nitrogen becomes deprotonated and reprotonated during subsequent events in the photocycle; presumably the Schiff base proton is “pumped.” Charge-separation (displacement of the Schiff base nitrogen away from a protein counterion) is thought to be responsible for differences in the photointermediates’ absorption maxima and probably plays a role in energy-storage [4]. To understand the mechanism of proton pumping in more detail, we are using time-resolved Raman spectroscopy to study the changes in chromophore structure that occur in picoseconds and nanoseconds.