Nature of the S2 state 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.