Electron donation by the high-potential haems in Rhodopseudomonas viridis reaction centres at low temperatures

AbstractEPR signals due to c-type cytochromes in Rhodopseudomonas viridis reaction centres are reported. Haems with Em = 380 mV (1), 310 mV (1) and about 0 mV (2) were identified. At redox potentials where the low-potential cytochromes are oxidised, but the high-potential cytochromes are reduced, photooxidation of the high-potential cytochromes is observed as QA is photoreduced by low-temperature (15 K) illumination. Cyt c-556→cyt c-559→reaction centre→QA However, the signal attributed to QA is only 30–40% of the intensity and is narrower than that observed when QA photoreduced with all the haems reduced. It is suggested that reduction of the low-potential haems causes conformational changes in the reaction centre, altering the iron-quinone interaction and the electron-transfer process in the cytochrome chain