55Mn ESE-ENDOR of a Mixed Valence Mn(III)Mn(IV) Complex: Comparison with the Mn Cluster of the Photosynthetic Oxygen-Evolving Complex

Analysis of 55Mn electron spin echo—electron nuclear double resonance (ESE—ENDOR) spectra obtained on a dinuclear mixed valence Mn(III)Mn(IV) complex [di-μ-oxotetrakis(2,2’-bipyridine)dimanganese(III,IV)] (1) reveals the hyperfine and nuclear quadrupolar parameters for the spin I = 5/2 55Mn nucleus of both Mn(III) and Mn(IV) ions. The following parameters are obtained: for Mn(IV), A⊥ = +212 MHz, A∥ = +231 MHz, P∥ = 3e2Qq/40 = +2.0 MHz, and η = 0.3, and for Mn(III), A⊥ = –480 MHz, A∥ = –360 MHz, P∥ = –4.5 MHz, and η = 0.1. The 55Mn ESE–ENDOR data obtained on the g = 2 Mn multiline EPR signal of the S2 state of the photosystem II oxygen-evolving complex demonstrate that this EPR signal cannot arise from a dinuclear Mn(III)-Mn(IV) center. The ENDOR spectra are consistent with a tetranuclear Mn cluster origin for the photosystem II multiline EPR signal. © 1995, American Chemical Society. All rights reserved.