NATURE OF THE S2 STATE ELECTRON-PARAMAGNETIC-RESONANCE SIGNALS FROM THE OXYGEN-EVOLVING COMPLEX OF PHOTOSYSTEM-II - Q-BAND AND ORIENTED X-BAND STUDIES

The multiline and g = 4.1 EPR signals from the manganese-containing wa ter oxidation site of plant photosystem II have been studied at 0-band (35 GHz). Comparisons with X-band spectra show a significant g anisot ropy in the multiline signal, which is inequivalent for the plus and m inus alcohol forms. Provisional values for the plus alcohol form are g (parallel-to) = 1.970, g(perpendicular-to) = 1.984. The 0-band 4.1 spe ctrum indicates that the signal arises from a quasi-axial, probably sp in-3/2 system, with a slight splitting of the g(perpendicular-to) comp onents into g(perpendicular-to x) = 4.35 and g(perpendicular-to y) = 4 .14. Each component has a (peak-to-peak) width of ca. 30 mT, similar t o that of the (unresolved) signal at X-band. The 4.1 signal from one d imensionally ordered photosystem II samples has also been studied at X -band. This shows a variation of the apparent g(perpendicular-to) valu e with sample orientation in the magnetic field, consistent with the a bove limits from the powder-pattern 0-band data. Assuming the transiti ons around g = 4 arise from the perpendicular-to components of a quasi -axial spin-3/2 system, the 0- and X-band results indicate that Absolu te value of D > 5 cm-1 and \E/D\ almost-equal-to 0.017 for the zero-fi eld terms of the 3/2 state. The oriented X-band data then show that th e D(parallel-to) axis is nearly parallel to the thylakold membrane pla ne. Further, Mn hyperfine structure is resolved on the oriented X-band 4.1 signals, the first such detection in unmodified enzyme. The spaci ng (ca. 4 mT) is similar to that reported recently for structure on th e 4.1 signal of NH3 inhibited enzyme (Kim et al., J. Am. Chem. Soc., 1 990, 112, 9389), but the lines are less distinct.