The binding of several primary amines to the O2-evolving center (OEC) of photosystem II (PSII) has been studied by using low-temperature electron paramagnetic resonance (EPR) spectroscopy of the S2 state. Spinach PSII membranes treated with NH4Cl at pH 7.5 produce a novel S2-state multiline EPR spectrum with a 67.5-G hyperfine line spacing when the S2 state is produced by illumination at 0 °C [Beck, W. F., de Paula, J. C., & Brudvig, G. W. (1986) J. Am. Chem. Soc. 108, 4018–4022]. The altered hyperfine line spacing and temperature dependence of the S2-state multiline EPR signal observed in the presence of NH4C1 are direct spectroscopic evidence for coordination of one or more NH3 molecules to the Mn site in the OEC. In contrast, the hyperfine line pattern and temperature dependence of the S2-state multiline EPR spectrum in the presence of tris(hydroxymethyl)aminomethane, 2-amino-2-ethyl-1,3-propanediol, or CH3NH2 at pH 7.5 were the same as those observed in untreated PSII membranes. We conclude that amines other than NH3 do not readily bind to the Mn site in the S2 state because of steric factors. Further, NH3 binds to an additional site on the OEC, not necessarily located on Mn, and alters the stability of the S2-state g = 4.1 EPR signal species. The effects on the intensities of the g =4.1 and multiline EPR signals as the NH3 concentration was varied indicate that both EPR signals arise from the same paramagnetic site and that binding of NH3 to the OEC affects an equilibrium between two configurations exhibiting the different EPR signals. The results of this paper support the proposal that a single Mn site functions on the electron donor side of PSII in the mechanism of photosynthetic O2 evolution; the Mn site functions both in the storage of oxidizing equivalents and, considering the steric selectivity of the Mn site for the coordination of small Lewis bases, in binding and oxidation of substrate H2O molecules.
ASJC Scopus subject areas