Multiple Redox-Active Chlorophylls in the Secondary Electron-Transfer Pathways of Oxygen-Evolving Photosystem II

Photosystem II (PS II) is unique among photosynthetic reaction centers in having secondary electron donors that compete with the primary electron donors for reduction Of P-680(+). We have characterized the photooxidation and dark decay of the redox-active accessory chlorophylls (Chl) and P-carotenes (Car) in oxygen-evolving PS II core complexes by near-IR absorbance and EPR spectroscopies at cryogenic temperatures. In contrast to previous results for Mn-depleted PS II, multiple near-IR absorption bands are resolved in the light-minus-dark difference spectra of oxygen-evolving PS II core complexes including two fast-decaying bands at 793 and 814 nm and three slow-decaying bands at 810, 825, and 840 nm. We assign these bands to chlorophyll cation radicals (Chl(+)). The fast-decaying bands observed after illumination at 20 K could be generated again by reilluminating the sample. Quantization by EPR gives a yield of 0.85 radicals per PS II, and the yield of oxidized cytochrome b(559) by optical difference spectroscopy is 0.15 per PS II. Potential locations of Chl(+) and Car(+) species, and the pathways of secondary electron transfer based on the rates of their formation and decay, are discussed. This is the first evidence that Chls in the light-harvesting proteins CP43 and CP47 are oxidized by P-680(+) and may have a role in Chl fluorescence quenching. We also suggest that a possible role for negatively charged lipids (phosphatidyldiacylglycerol and sulfoquinovosyldiacylglycerol identified in the PS H structure) could be to decrease the redox potential of specific Chl and Car cofactors. These results provide new insight into the alternate electron-donation pathways to P-680(+).