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

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.