S-band EPR studies of the S2-state multiline signal from the photosynthetic oxygen-evolving complex

The S₂-state multiline EPR signal observed in photosynthetic membrane preparations has been previously well characterized at X-band frequencies (9.1-9.5 GHz). These studies have indicated that the signal, centered at g = 2, arises from a multinuclear mixed-valence Mn center of the O₂-evolving complex. In the present study, the multiline EPR signal from spinach photosystem II enriched membranes is characterized at an S-band frequency (3.9 GHz). At this lower frequency, the resolution and complexity of the signal increase markedly compared with its appearance in the X-band. While the multiline signal covers similar magnetic field ranges at the two frequencies, the S-band signal has a greater number of lines narrower line widths, and a different overall appearance. Replacement of Cl^- with Br^- or ¹H₂O with ²H₂O in the buffer shows that neither exchangeable Cl^- nor protons cause superhyperfine structure in the S-band multiline signal. Membrane preparations oriented on mylar sheets show dependence of the S-band signal on the angle between the mylar sheet normal and the magnetic field direction, indicationg that the multiplicity of lines is in part due to signal anisotropy. The results, combined with previous work at X-band, indicate that a minimal working model for the species responsible for the multiline signal is a mixed-valence binuclear Mn complex with an anisotropic hyperfine interaction that includes second-order contributions. (author)