Low-temperature optical and resonance Raman spectra of a carotenoid cationradical in photosystem II

Low-temperature (77 K) illumination of manganese-depleted Synechocystis PCC 6803 photosystem II core complexes caused the reversible photooxidation of a carotenoid, forming a carotenoid cation radical with an absorbance maxim um at 984 nm. Resonance FT-Raman spectra obtained with 1064 nm excitation g ave a spectrum characteristic of carotenoid cation radicals in solution. Th is is the first example of a resonance Raman spectrum of a carotenoid catio n radical in a protein. The carotenoid photooxidation requires prior chemic al oxidation of cytochrome b(559) which implicates the carotenoid in the se condary electron-transfer pathway of photosystem II that may play a role in photoprotection, The possible nature of the pathway and the structure of t he carotenoid cation radical are discussed.