MOLECULAR ASPECTS OF LIGHT‐INDUCED UPTAKE AND RELEASE OF PROTONS BY PURPLE MEMBRANES

Abstract— The precise molecular description of the time dependent steps in the uptake, translocation and release of protons by bacteriorhodopsin following photon absorption requires information on the time resolved changes in protonation of the side chains of specific amino acid residues and the correlation of these changes with photocycle kinetics. Thus far, the use of chemical modification to probe the role of amino acid side chains in this process has proven of value in demonstrating a role for tyrosine residues in release and uptake of protons associated with early and later stages (before and after M₄₁₂ formation) of the photocycle. In addition, it has demonstrated the essential role of ionic interactions between negatively charged carboxyl groups and positively charged guanidinium groups of arginine, and amino groups of lysine. The transmembrane regulatory effect of Δμ_H+ on the M₄₁₂ species of the photocycle provides additional evidence for the participation of reversible protonation of amino acid side chains at the surfaces of the purple membrane in the mechanism of proton translocation. Thus, our studies relate molecular events of proton translocation to the bioenergetics of the purple membrane.