The Two Alternative Oxidation State Assignments of Manganese Ions: What S2 CW-EPR Multiline (ML) Signal Simulations Reveal?

Characterizing the photosystem II (PSII) sample, continuous wave electron paramagnetic resonance (CW-EPR) simulations of the S-2 ML signal at X-band frequencies was our focus. This can help increase our understanding of how the manganese (Mn) atoms in the catalytic site of the PSII magnetically interact using ML signals. It can also be used to further the understanding of possible water-splitting mechanisms in the oxygen-evolving complex (OEC). The question that remains is how much does each manganese (Mn) ion contribute to the ML signal through its hyperfine interactions in the OEC? Currently, there are two proposals for the average oxidation states of the Mn ions, denoted the 'high' oxidation paradigm (HOP) and the 'low' oxidation paradigm (LOP). Majority of PSII researchers favour the HOP. Various experiments have been conducted to investigate the two alternative oxidation states, including EPR (Jin et al. in Phys Chem Chem Phys 16(17):7799-7812, 10.1039/c3cp55189j, 2014; Baituti in Hyperfine Interact 238(1), 10.1007/s10751-017-1440-8, 2017; Ioannidis et al. in Biochemistry, 10.1021/bi060520s, 2006). The S-2 ML EPR signal simulation using the Mn-55 hyperfine coupling constants, with one very large, one medium, and two small hyperfine values, fits the experimental data. The Mn1 has a large hyperfine coupling, which agrees well with earlier data by Jin et al. [19]. Three large fractional anisotropy observed on three Mn centers (Mn1,3,4), suggests the presence of three Mn-III ions, and Mn2 center is likely to be Mn-IV ion, hence favouring the LOP (Mn-III Mn-IV Mn-III Mn-III).