High-field EPR study of carotenoid and chlorophyll cation radicals in photosystem II

In photosystem II (PS II), chlorophyll, p-carotene, and cytochrome b(559) a n alternate electron donors that may be involved in a photoprotection mecha nism. The present study describes the use of high-field EPR spectroscopy to characterize the low-temperature photooxidation of Chit and Car cofactors in PS II. The EPR signals of the individual species, previously not resolve d at X-band frequency (9 GHz), are resolved at higher D-band frequency (130 GHz) in deuterated Synechococcus lividus PS II. Deuteration of PS II resul ts in significant narrowing of the EPR lines, yielding well-resolved EPR sp ectra of the Car(+) and Chl(Z)(+) radicals at 130 GHz. The g tensors of the individual species were determined by EPR spectral simulations. The g tens or determined for the Car(+) radical (g(xx) = 2.00335, g(yy) = 2.00251, g(z z) = 2.00227) is similar to that previously observed for a canthaxanthin ca tion radical but with a slightly rhombic tensor. The Chl(Z)(+) g tensor (g( xx) = 2.00312, g(yy) = 2.00263, g(zz) = 2.00202) is similar to that of a ch lorophyll a cation radical. This study shows that both the carotenoid and c hlorophyll radicals are generated in PS II by illumination at temperatures from 6 to 190 K and that there is no interconversion of Car(+) and Chl(Z)() radicals upon dark annealing at temperatures up to 160 K. This study also establishes the feasibility of using deuteration and high-field EPR to res olve previously unresolvable cofactor signals in PS II.