Nature of the S2state electron paramagnetic resonance signals from the oxygen-evolving complex of photosystem II: Q-band and oriented X-band studies

Abstract

The multiline and g= 4.1 EPR signals from the manganese-containing water oxidation site of plant photosystem II have been studied at Q-band (35 GHz). Comparisons with X-band spectra show a significant g anisotropy in the multiline signal, which is inequivalent for the plus and minus alcohol forms. Provisional values for the plus alcohol form are g∥= 1.970, g⊥= 1.984. The Q-band 4.1 spectrum indicates that the signal arises from a quasi axial, probably spin- system, with a slight splitting of the g⊥ components into g⊥x= 4.35 and g⊥y= 4.14. Each component has a (peak-to-peak) width of ca. 30 mT, similar to that of the (unresolved) signal at X-band. The 4.1 signal from one dimensionally ordered photosystem II samples has also been studied at X-band. This shows a variation of the apparent g⊥ value with sample orientation in the magnetic field, consistent with the above limits from the powder-pattern Q-band data. Assuming the transitions around g= 4 arise from the ⊥ components of a quasi-axial spin- system, the Q- and X-band results indicate that |D| > 5 cm–1 and |E/D|≈ 0.017 for the zero field terms of the state. The oriented X-band data then show that the D(∥) axis is nearly parallel to the thylakoid membrane plane. Further, Mn hyperfine structure is resolved on the oriented X-band 4.1 signals, the first such detection in unmodified enzyme. The spacing (ca. 4 mT) is similar to that reported recently for structure on the 4.1 signal of NH3 inhibited enzyme (Kim et al., J. Am. Chem. Soc., 1990, 112, 9389), but the lines are less distinct.