Detection of an electron paramagnetic resonance signal in the S-0 state ofthe manganese complex of photosystem II from Synechococcus elongatus

The Mn-4-cluster of photosystem II (PSII) from Synechococcus elongatus was studied by electron paramagnetic resonance (EPR) spectroscopy after a serie s of saturating laser flashes given in the presence of either methanol or e thanol. Results were compared to those obtained in similar experiments done on PSII isolated from plants. The flash-dependent changes in amplitude of the EPR multiline signals were virtually identical in all samples. In agree ment with earlier work [Messinger, J., Nugent, J, H. A., and Evans, M. C. W . (1997) Biochemistry, 36, 11055-11060; Ahrling, K. A., Peterson, S., and S tyring, S, (1997) Biochemistry 36, 13148-13152], detection of an EPR multil ine signal from the So state in PSII from plants was only possible with met hanol present. In PSII from S. elongatus, it is shown that the S-0 state ex hibits an EPR multiline signal in the absence of methanol (however, ethanol was present as a solvent for the artificial electron acceptor). The hyperf ine lines are better resolved when methanol is present. The S-0 multiline s ignals detected in plant PSII and in S, elongatus were similar but not iden tical. Unlike the situation seen in plant PSII, the S-2 state in S. elongat us is not affected by the addition of methanol in that (i) the S-2 multilin e EPR signal is not modified by methanol and (ii) the spin state of the S-2 state is affected by infrared light when methanol is present. It is also s hown that the magnetic relaxation properties of an oxidized low-spin heme, attributed to cytochrome c(550), vary with the S states. This heme then is in the magnetic environment of the Mn-4 cluster.