Low-Temperature Raman Spectroscopy Reveals Small Chromophore Distortion in Primary Photointermediate of Proteorhodopsin.

Proteorhodopsin (PR) is a microbial rhodopsin functioning as a light-driven proton pump in aquatic bacteria. We performed low-temperature Raman measurements of PR to obtain the structure of the primary photoproduct, the K intermediate (PRK). PRK showed the hydrogen-out-of-plane modes that are much less intense than those of bacteriorhodopsin as the prototypical light-driven proton pump from haloarchaea. The present results reveal the significantly relaxed chromophore structure in PRK, which can be coupled to the slow kinetics of the K intermediate. This structure suggests that PR transports protons using the small energy storage within the chromophore at the start of its photocycle.