Solid-state NMR of the retinal protonated Schiff base in microbial rhodopsins

Rhodopsin is a seven-helical transmembrane protein with a retinal chromophore covalently bound to a conserved lysine in helix G via a retinal protonated Schiff base (RPSB). Microbial rhodopsins absorb light through chromophore and play a fundamental role in optogenetics. Numerous microbial rhodopsins have been discovered, contributing to diverse functions and colors. Solid-state NMR spectroscopy has been instrumental in elucidating the conformation of chromophores and the three-dimensional structure of microbial rhodopsins. This review focuses on the ¹⁵N chemical shift values of RPSB and summarizes recent progress in the field. We displayed the correlation between the ¹⁵N isotropic chemical shift values of RPSB and the maximum absorption wavelength of rhodopsin using solid-state NMR spectroscopy.