The S0 state EPR signal from the Mn cluster in photosystem II arises from an isolated S=1/2 ground state

During oxygen evolution, the Mn cluster in Photosystem II cycles through five oxidation states, S-0-S-4. S-0 and S-2 are paramagnetic, and can be monitored by electron paramagnetic resonance (EPR), Recently a new EPR signal from the S-0 state was discovered [Ahrling et al. (1997) Biochemistry 36, 13148-13152, Messinger et al, (1997), J Ant. Chem. Sec. 119, 11349-11350]. Here, we present a well-resolved So spectrum, taken at high power and low temperature. The spectrum is wider and more resolved than previously thought, with structure over more than 2500 G, and appears to have at least 20 reproducible peaks on each side of g = 2. We also present the temperature dependence of the unsaturated S-0 signal amplitude. A linear relationship was found between signal intensity and reciprocal temperature (1/T) in the region 5-25 K, clearly extrapolating to 0. This obeys the Curie law, indicating that the S-0 state is a ground S = 1/2 state with no thermally accessible excited state. The data are consistent with a minimum energy gap of 30 cm(-1) between the ground and first excited states.