Photoswitching of Terahertz Structural Dynamics in the Photosynthesis Photo Protective Orange Carotenoid Protein*

Protein long-range structural vibrations enable efficient conformational transitions during biological function.  Conformational changes play a critical role in the regulation of activity of the photo protective protein, orange carotenoid protein (OCP).  Exposure to bright blue-green light results in conversion from the compact orange resting state (OCP^o) to the open active red state (OCP^R) which can then interact with the light harvesting antenna, the phycobilisome (PBS), to induce fluorescent quenching.  We examine how structural vibrations may actuate the transition using anisotropic terahertz microspectroscopy (ATM).   We find that the intramolecular vibrations switch with photo excitation, with reversible shifting of the anisotropic bands in the ca. 50 cm^-1 range.  Comparison with normal mode ensemble analysis calculations indicates that the measured changes coincide with the loss of the H-bonds between residues Y201 and W288 and the keto group of the carotenoid in the C terminal domain.  This change in the internal dynamics with the initial photoexcitation may provide the motional bias towards the subsequent reorganization of the N-terminal domain (NTD) and the eventual opening between the NTD and the C- terminal domain necessary for the interaction with the phycobilisome.