Raman Optical Activity Reveals Carotenoid Photoactivation Events in the Orange Carotenoid Protein in Solution.

The orange carotenoid protein (OCP) plays an important role in photoprotection in cyanobacteria, which is achieved by the photoconversion from the orange dark state (OCPO) to the red active state (OCPR). Using Raman optical activity (ROA), we studied the conformations of the carotenoid chromophore in the active sites of OCPO and OCPR. This ROA measurement directly observed the chromophore conformation of native OCP in solution, and the measurement of OCPR first demonstrated the ROA spectroscopy for the transient species. For OCPO, the spectral features of ROA were mostly reproduced by the quantum chemical calculation based on the crystal structure of the OCP. Within the spatial resolution (∼2 Å), a slight modification of the polyene-chain distortion improved the agreement between the observed and calculated ROA spectra. While the crystal structure of OCPR is not available, the ROA spectrum of OCPR was reproduced by using the crystal structure of red carotenoid protein (RCP), an OCPR proxy. The present results showed that the chromophore conformations in the crystal structures of OCP and RCP hold true for OCPO and OCPR in solution. Particularly, ROA spectroscopy of the native OCPR provides a direct support for the 12 Å translocation of chromophore in the photoactivation, which was proposed by X-ray crystallography using RCP [R. L. Leverenz, M. Sutter, et al. Science 2015, 348, 1463-1466].