Color-discriminating retinal configurations of sensory rhodopsin I by photo-irradiation solid-state NMR spectroscopy

SRI (sensory rhodopsin I) can discriminate multiple colors for the attractant and repellent phototaxis. Studies aimed at revealing the color-dependent mechanism show that SRI is a challenging system not only in photobiology but also in photochemistry. During the photoreaction of SRI, an M-intermediate (attractant) transforms into a P-intermediate (repellent) by absorbing blue light. Consequently, SRI then cycles back to the G-state. The photoreactions were monitored with the ¹³C NMR signals of [20- ¹³C]retnal-SrSRI using in situ photo-irradiation solid-state NMR spectroscopy. The M-intermediate was trapped at -40 C by illumination at 520 nm. It was transformed into the P-intermediate by subsequent illumination at 365 nm. These results reveal that the G-state could be directly transformed to the P-intermediate by illumination at 365 nm. Thus, the stationary trapped M- and P-intermediates are responsible for positive and negative phototaxis, respectively. The photocycle of a sensory rhodopsin I (SrSRI) was examined using in situ photo-irradiation solid-state NMR spectroscopy. The M-intermediate was trapped by illumination (520 or 595 nm) and is responsible for the positive phototaxis. The M-intermediate was transferred to the P-intermediate (negative phototaxis) by irradiation at 365 nm. These results provide insights into the mechanism of the color-discriminating functional switch of SrSRI.