Spectroscopic ruler for measuring active-site distortions based on Raman optical activity of a hydrogen out-of-plane vibration

Significance Many biological cofactors, including light-absorbing chromophores in photoreceptors, are modulated upon insertion into a protein. The steric contribution can cause structural distortions in the cofactor, and such effects are considered to be crucial for biological function. For example, the out-of-plane distortion of chromophores is a key factor in controlling their absorption spectra. In spite of the functional importance, such structural distortions are difficult to measure experimentally. In this study, we used a unique capability of Raman optical activity (ROA) to address the above-mentioned issue. This study applied ROA spectroscopy to a photoreceptor protein and indicates that a hydrogen out-of-plane ROA band provides a spectroscopic ruler for the out-of-plane distortion of the chromophore that is embedded in a protein environment.