Species dependent differences in the secondaryelectron-donation reactions of photosystem II

Abstract

Under conditions when the primary electron-donation pathway from the O2-evolving complex in photosystem II (PSII) is inhibited, several alternate electron donors can be photooxidized. These include a monomeric chlorophyll (ChlZ), b-carotene (Car) and cytochrome b559. The involvement of these alternate electron donors and the redox role of a carotenoid in PSII are unique among photosynthetic reaction centers and stem from the very highly oxidizing intermediates that form in the process of water oxidation. ChlZ and Car photooxidation have been characterized by near-infrared absorbance, shifted-excitation Raman difference (SERDS) and EPR spectroscopies over a range of cryogenic temperatures from 30 to 120 K in both cyanobacterial PSII core complexes and spinach PSII membranes. The EPR signals of the individual species, previously not resolved at X-band frequency (9 GHz), are resolved at higher D-band frequency (130 GHz) in deuterated Synechococcus lividus PSII (Lakshmi, K.V., Reifler, M.J., Brudvig, G.W., Poluektov, O.G., Wagner, A.M. and Thurnauer, M.C. (2000) J. Phys. Chem. B 104, 10445). Optical and SERDS spectroscopies enable selective study of the formation and decay of the ChlZ and Car cation radicals (Tracewell, C.A., Cua, A., Stewart, D.H., Bocian, D.F. and Brudvig, G.W. (2001) Biochemistry 40, 193). These results provide insight into the alternate electron-donation pathways in PSII and will be discussed in light of the emerging structural information on PSII. Supported by the NIH through grants GM32715 (GWB), GM39781(DFB) and predoctoral training grant GM08283 (MJR, DHS and CAT). Work at ANL was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, under contract W-31-109-Eng-38.