High-light induced singlet oxygen formation in cytochrome b6f complex from Bryopsis corticulans as detected by EPR spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy was used to detect the light-induced formation of singlet oxygen (1O2*) in the intact and the Rieske-depleted cytochrome b6f complexes (Cyt b6f) from Bryopsis corticulans, as well as in the isolated Rieske Fe-S protein. It is shown that, under white-light illumination and aerobic conditions, chlorophyll a (Chl a) bound in the intact Cyt b6f can be bleached by light-induced 1O2*, and that the 1O2* production can be promoted by D2O or scavenged by extraneous antioxidants such as l-histidine, ascorbate, β-carotene and glutathione. Under similar experimental conditions, 1O2* was also detected in the Rieske-depleted Cyt b6f complex, but not in the isolated Rieske Fe-S protein. The results prove that Chl a cofactor, rather than Rieske Fe-S protein, is the specific site of 1O2* formation, a conclusion which draws further support from the generation of 1O2* with selective excitation of Chl a using monocolor red light.