EPR–ENDOR Characterization of (17O, 1H, 2H) Water in Manganese Catalase and Its Relevance to the Oxygen-Evolving Complex of Photosystem II
2012
McConnell, Iain L. | Grigoryants, Vladimir M. | Scholes, Charles P. | Myers, William K. | Chen, Ping-Yu | Whittaker, James W. | Brudvig, Gary W.
The synthesis of efficient water-oxidation catalysts demands insight into the only known, naturally occurring water-oxidation catalyst, the oxygen-evolving complex (OEC) of photosystem II (PSII). Understanding the water oxidation mechanism requires knowledge of where and when substrate water binds to the OEC. Mn catalase in its Mn(III)–Mn(IV) state is a protein model of the OEC’s S₂ state. From ¹⁷O-labeled water exchanged into the di-μ-oxo di-Mn(III,IV) coordination sphere of Mn catalase, CW Q-band ENDOR spectroscopy revealed two distinctly different ¹⁷O signals incorporated in distinctly different time regimes. First, a signal appearing after 2 h of ¹⁷O exchange was detected with a 13.0 MHz hyperfine coupling. From similarity in the time scale of isotope incorporation and in the ¹⁷O μ-oxo hyperfine coupling of the di-μ-oxo di-Mn(III,IV) bipyridine model (Usov, O. M.; Grigoryants, V. M.; Tagore, R.; Brudvig, G. W.; Scholes, C. P.J. Am. Chem. Soc. 2007,, 129, 11886−11887), this signal was assigned to μ-oxo oxygen. EPR line broadening was obvious from this ¹⁷O μ-oxo species. Earlier exchange proceeded on the minute or faster time scale into a non-μ-oxo position, from which ¹⁷O ENDOR showed a smaller 3.8 MHz hyperfine coupling and possible quadrupole splittings, indicating a terminal water of Mn(III). Exchangeable proton/deuteron hyperfine couplings, consistent with terminal water ligation to Mn(III), also appeared. Q-band CW ENDOR from the S₂ state of the OEC was obtained following multihour ¹⁷O exchange, which showed a ¹⁷O hyperfine signal with a 11 MHz hyperfine coupling, tentatively assigned as μ-oxo-¹⁷O by resemblance to the μ-oxo signals from Mn catalase and the di-μ-oxo di-Mn(III,IV) bipyridine model.
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