Pressure induced isomerization of retinal on bacteriorhodopsin as disclosed by Fast Magic Angle Spinning NMR

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Bacteriorhodopsin (bR), a retinal protein in purple membrane of H.salinarum, shows functions as a light-driven proton pump. We have detected pressure induced isomerization of retinal in bR by 15N cross polarization-magic angle spinning (CP-MAS) NMR spectra of [ε-15N]Lys-labeled bR. In the 15N NMR spectra, both all-trans and 13-cis retinal configurations have been observed at 148.0 and 155.0 ppm, respectively, at the MAS frequency of 4 kHz in the dark. When the MAS frequency was increased up to 12 kHz corresponding to the sample pressure of 80 atm, the 15N NMR signals of Schiff Base of retinal were broadened. The signal intensity of 13-cis retinal at 155.0 ppm was increased when the MAS frequency was decreased from 12 kHz to 4 kHz. These results showed that the equilibrium constant of [13-cis –bR] /[all-trans-bR] increased by the pressure of 80 atm. It was also revealed that the changes induced by the pressure were quite local. Therefore, microscopically, hydrogen-bond network around retinal would be disrupted or distorted by a constantly applied pressure. It is, therefore, clearly demonstrated that increased pressure induced by fast MAS frequencies generated 13-cis isomerization of retinal in the membrane protein bR.