Hyperfine Coupling to the Bridging ~(17)O in the Di-μ-oxo Core of a Mn~Ⅲ-Mn~Ⅳ Model Significant to the Core Electronic Structure of the O_2-Evolving Complex in Photosystem Ⅱ

摘要

It is generally accepted that Mn in the S_2 state of the oxygen-evolving center (OEC) is in the +3 and +4 oxidation states with μ-oxo linkages. X-ray crystallography has provided the resolution to model the OEC as a Mn_3CaO_4 cube connected to a fourth Mn via a μ-oxo bridge. Mn electron nuclear double resonance (ENDOR) is highly consistent with the cuboidal structure.rnDi-μ-oxo-bridged Mn~Ⅲ—Mn~Ⅳ compounds are widely studied models for this S_2 state. Such models present a manganese multiline EPR pattern from S = 2 Mn~Ⅲ and 5 = 3/2 Mn~Ⅳ, antiferrromagnetically coupled to each other to yield a net spin S = 1/2. Electron spin echo envelope modulation (ESEEM) and ENDOR of such Mn~Ⅲ—Mn~Ⅳ models have resolved hyperfine couplings for Mn~Ⅳ and Mn~Ⅲ, protons, and liganding nitrogen. However, the ubiquitous μ-oxygens which physically couple the Mn~Ⅲ and Mn~Ⅳ have yet to have their electronic structure and electron-spin density elucidated.