title: The S0 state EPR signal from the Mn cluster in photosystem II arises from an isolated S = 1/2 ground state.
abstract: During oxygen evolution, the Mn cluster in Photosystem II cycles through five oxidation states, S0-S4. S0 and S2 are paramagnetic, and can be monitored by electron paramagnetic resonance (EPR). Recently a new EPR signal from the S0 state was discovered [Ahrling et al. (1997) Biochemistry 36, 13148-13152, Messinger et al. (1997) J. Am. Chem. Soc. 119, 11349-11350]. Here, we present a well-resolved S0 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 S0 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 S0 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.
authors.affiliation: Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, Sweden.
authors.forename: K A
authors.initials: KA
authors.lastname: Ahrling
authors.forename: S
authors.initials: S
authors.lastname: Peterson
authors.forename: S
authors.initials: S
authors.lastname: Styring
mesh_terms: D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D008345:Manganese / Q000737:chemistry* / Q000378:metabolism; D000432:Methanol / Q000737:chemistry; D010084:Oxidation-Reduction; D045322:Photosynthetic Reaction Center Complex Proteins / Q000737:chemistry* / Q000378:metabolism; D045332:Photosystem II Protein Complex; D018724:Spinacia oleracea; D013696:Temperature
chemical_list: D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D008345:Manganese; D000432:Methanol