Photoreaction pathways and photointermediates of retinal-binding photoreceptor proteins as revealed by in situ photoirradiation solid-state NMR spectroscopy

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Abstract
Photoirradiation solid-state NMR spectroscopy is a powerful means to study photoreceptor retinal-binding proteins by the detection of short-lived photointermediates to elucidate the photoreaction cycle and photoactivated structural changes. An in situ photoirradiation solid-state NMR apparatus has been developed for the irradiation of samples with extremely high efficiency to enable observation of photointermediates which are stationary trapped states. Such observation enables elucidation of the photoreaction processes of photoreceptor membrane proteins. Therefore, in situ photoirradiation is particularly useful study the photocycle of retinal-binding proteins such as sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII) because functional photointermediates have relatively longer half-lives than other photointermediates. As a result, several photointermediates have been trapped as stationary state and their detailed structures and photoreaction cycles have been revealed using photoirradiation solid-state NMR spectroscopy at low temperature. Photoreaction intermediates of bacteriorhodopsin, which functions to provide light-driven proton pump activity, were difficult to trap because the half-lives of the photointermediates were shorter than those of sensory rhodopsin. Therefore, these photointermediates are trapped in a freeze-trapped state at a very low temperature and the NMR signals were observed using a combination of photoirradiation and dynamic nuclear polarization (DNP) experiments.

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