Photosystem II (PSII) contains a non-heme ferrous ion, located on the stromal side of the protein in close proximity to quinones A and B (Q A and QB). We used EPR spectroscopy to examine the temperature-dependent redox reactions of the iron-quinone site, using it as a probe of potentially physiologically relevant proton-coupled electron-transfer (PCET) reactions. Complete chemical oxidation of the non-heme iron at ambient temperatures was followed by cryogenic photoreduction, producing a temperature-dependent yield of Fe2+QA (or Fe 3+QA-)⋯Chl+/Car +/YD• charge separations. These charge separations were subsequently observed to partially recombine in the dark at cryogenic temperatures. We observed no double photochemical charge separations upon illumination at temperatures ≤30 K, demonstrating that QA and Fe3+ together act as a single electron-accepting moiety at very low temperatures. Our results indicate the existence of two populations of the iron-quinone site in PSII, one whose Fe3+ signal is abolished by illumination at liquid helium temperatures and one whose Fe3+ signal is abolished by illumination only above 75 K. The observation of non-heme iron photoreduction at cryogenic temperatures (possibly at liquid helium temperatures and certainly above 75 K) implies the existence of a low reorganization energy proton-transfer (ET) pathway within the protein to the non-heme iron environment, of possible relevance to the PCET reactions of QB and/or the non-heme iron itself. Furthermore, we observed the partial reoxidation of the non-heme iron by charge recombination with previously oxidized chlorophyll, carotenoid, and YD within PSII. This electron transfer might be important in the photoprotective transfer of oxidative power away from P 680+ and the oxygen-evolving complex in stressed PSII centers.
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