Photoactive yellow protein (PYP), from the phototrophic bacterium Halorhodospira halophila, is a small water-soluble photoreceptor protein and contains p-coumaric acid (pCA) as a chromophore. PYP has been an attractive model for studying the physical chemistry of protein active sites. Here, we explore how Raman optical activity (ROA) can be used to extract quantitative information on distortions of the pCA chromophore at the active site in PYP. We use 13C8-pCA to assign an intense signal at 826 cm-1 in the ROA spectrum of PYP to a hydrogen out-of-plane vibration of the ethylenic moiety of the chromophore. Quantum-chemical calculations based on density functional theory demonstrate that the sign of this ROA band reports the direction of the distortion in the dihedral angle about the ethylenic C=C bond, while its amplitude is proportional to the dihedral angle. These results document the ability of ROA to quantify structural deformations of a cofactor molecule embedded in a protein moiety.
Keywords: chromophore; density functional theory; molecular strain; photoreceptor; vibrational spectroscopy.