The Blue-Light Quenching Effect of Proteorhodopsin

Abstract

From bacteriorhodopsin as well as proteorhodopsin (PR), it is known that excitation of the M intermediate which contains a 13-cis retinal with deprotonated Schiff base disrupts the photocycle significantly. This so called blue-light quenching effect (BLQ) results in an immediate halt of proton pumping activity and the protein returns to its ground state or a state spectroscopically very similar to the ground state. Thus, the BLQ effect offers a way for light control of microbial retinal proteins. However, almost 40 years after its discovery, still little is known about the molecular mechanism. The particular processes that are triggered by the BLQ effect and the time scale of which they occur are unclear. We investigate the BLQ effect on PR with time-resolved spectroscopy. Background illumination of the PR proton donor mutant E108Q leads to an increased accumulation of the M intermediate in the photostationary equilibrium. Subsequently, fs pump-probe experiments in the visible and mid-IR range are conducted and provide insight into the isomerization dynamics of the photoexcited M state together with the nanosecond reprotonation step of the Schiff base. Based on our findings of the BLQ effect in PR, we currently extend our experiments on other microbial rhodopsins, like channelrhodopsin.