title: Formation of split electron paramagnetic resonance signals in photosystem II suggests that tyrosine(Z) can be photooxidized at 5 K in the S0 and S1 states of the oxygen-evolving complex.
abstract: The effect of illumination at 5 K of photosystem II in different S-states was investigated with EPR spectroscopy. Two split radical EPR signals around g approximately 2.0 were observed from samples given 0 and 3 flashes, respectively. The signal from the 0-flash sample was narrow, with a width of approximately 80 G, in which the low-field peak can be distinguished. This signal oscillated with the S(1) state in the sample. The signal from the 3-flash sample was broad, with a symmetric shape of approximately 160 G width from peak to trough. This signal varied with the concentration of the S(0) state in the sample. Both signals are assigned to arise from the donor side of PSII. Both signals relaxed fast, were formed within 10 ms after a flash, and decayed with half-times at 5 K of 3-4 min. The signal in the S(0) state closely resembles split radical signals, originating from magnetic interaction between Y(Z)(*) and the S(2) state, that were first observed in Ca(2+)-depleted photosystem II samples. Therefore, we assign this signal to Y(Z)(*) in magnetic interaction with the S(0) state, Y(Z)(*)S(0). The other signal is assigned to the magnetic interaction between Y(Z)(*) and the S(1) state, Y(Z)(*)S(1). An important implication is that Y(Z) can be oxidized at 5 K in the S(0) and S(1) states. Oxidation of Y(Z) involves deprotonation of the tyrosine. This is restricted at 5 K, and we therefore suggest that the phenolic proton of Y(Z) is involved in a low-barrier hydrogen bond. This is an unusually short hydrogen bond in which proton movement at very low temperatures can occur.
authors.affiliation: Department of Biochemistry, Center for Chemistry and Chemical Engineering, P.O. Box 124, Lund University, S-221 00 Lund, Sweden.
authors.forename: Chunxi
authors.initials: C
authors.lastname: Zhang
authors.forename: Stenbjörn
authors.initials: S
authors.lastname: Styring
mesh_terms: D002734:Chlorophyll / Q000378:metabolism; D004578:Electron Spin Resonance Spectroscopy; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes; D008280:Magnetics; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism*; D045322:Photosynthetic Reaction Center Complex Proteins / Q000528:radiation effects*; D045332:Photosystem II Protein Complex; D018724:Spinacia oleracea / Q000378:metabolism; D014443:Tyrosine / Q000378:metabolism*
chemical_list: D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D002734:Chlorophyll; D014443:Tyrosine; D010100:Oxygen