Characterization of Polynuclear Mixed-Valence Manganese Complexes as Models of the Water Oxidation Site in Photosystem II
In order to understand the electronic structure of the manganese water oxidation site of the photosystem II, a number of polynuclear mixed-valence manganese complexes are characterized by means of various physical methods such as electrochemistry, EPR spectroscopy and magnetic susceptibility measurements. Distributions in the weak antiferromagnetic interactions in two mixed-valence Mn$\sp{\rm II}$Mn$\sp{\rm III}$ complexes (LMn$\sb2$Cl$\sb2$Br) $\cdot$H$\sb2$O ($\sp\sim -2$cm$\sp{-1}$) and (LMn$\sb2$Br$\sb3$) $\cdot$1/2CH$\sb2$Cl$\sb2$ ($\sp\sim -1$cm$\sp{-1}$) together with distributions in single-ion zero-field interactions for these two Mn$\sp{\rm II}$Mn$\sp{\rm III}$ complexes lead to g- and A-strains which give broad signal in the EPR spectra. The antiferromagnetic interactions in (Mn$\sb2$(bpmp) ($\mu$-OAc)$\sb2$) (ClO$\sb4)\sb2$ ($-6$cm$\sp{-1}$) and (Mn$\sb2$(bcmp)($\mu$-OAc)$\sb2$) (ClO$\sb4)\sb2\cdot$1/2CH$\sb2$Cl$\sb2$ ($-7.7$cm$\sp{-1}$) result in well-resolved 29-line Mn hyperfine patterns in the g = 2 region at liquid-helium temperatures. Experimental and simulated X- and Q-band EPR spectra for these 29-line patterns indicate that they are due to only the z-component of the g tensor. The x- and y-components of the g-tensor are broadened to the point of not contributing to the observed EPR spectra. The differential broadening could be a reflection of g- and A-strain effects and increased nitrogen superhyperfine interactions in the g$\sb{\rm x}$ and g$\sb{\rm y}$ directions.
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