Yoshiteru Makino

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 (13)C NMR signals of [20-(13)C]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.

Photoirradiation and Microwave Irradiation NMR Spectroscopy

In situ photoirradiation solid-state nuclear magnetic resonance (NMR) spectroscopy is designed for optical irradiation from the top part of a zirconia rotor through a glass cap, which makes it possible to efficiently irradiate the inside of the rotor. This experimental method has made it possible to observe photo-intermediates of sensory rhodopsins, such as sensory rhodopsin I (SRI) and sensory rhodopsin II (SRII), and bacteriorhodopsin (bR) Y185F mutant. In SRI, green light generates M-intermediates, which exhibit positive phototaxis, while blue light generates P-intermediates, which exhibit negative phototaxis. In SRII, green light generates M-intermediates and blue light generates O-intermediates. In Y185F-bR, O-intermediates were first observed using solid-state NMR spectroscopy. The microwave irradiation NMR spectrometer was developed in-house by modification of a commercial NMR spectrometer. A flat long copper ribbon was used as a capacitor and a half turn of copper ribbon at the edge was used as an inductor for the microwave resonance circuit, which was coaxially inserted inside the radiofrequency induction coil and allowed NMR signals to be observed under microwave irradiation conditions. The temperature of N-(4-methoxybenzylidene)-4-butylaniline (MBBA) during microwave irradiation was estimated by measuring the temperature-dependent chemical shifts, whereby different protons were found to indicate significantly different temperatures in the molecule. Liquid crystalline-isotropic phase correlation 2D NMR spectra were observed using pulsed microwave irradiation for rapid temperature jump experiments.