Photo-Switching in Crystalline OCP*

Orange Carotenoid Protein (OCP) is a photoprotective protein in cyanobacteria. In solution under high UV intensity, OCP switches from its resting state, or orange state, OCP^O, peak absorbance at 495nm, to its photoprotective red state, OCP^R peak absorbance 515 nm. The transition involves a large scale structural reorganization that enables fluorescence quenching. Time resolved X-ray crystallography and anisotropic terahertz microspectroscopy (ATM) can measure the sequence of the structural and dynamical changes occurring with the transition, however these techniques require protein crystals, and to date photo switching has not been demonstrated in OCP crystals. Here we report the conditions to achieve crystal phase photoswitching. Using a homebuilt micro spectrometer, we found that in solution, photoswitching with 65 mW at 450 nm within 10 s results in a peak change in absorbance at 550 nm. Photoswitching for the crystals was found with the same power as the solution, but only after 2 min. of illumination. The red state appears to be irreversible after at least 2 days, and the peak in the change in the absorbance for the crystal phase shifts to 565 nm. The results provide the first demonstration that the early steps in photoprotective state transition are accessible in crystals.