Manganese and vanadium complexes: Synthetic models for the oxygen evolving complex and vanadium-bromoperoxidases.

A manganese center is contained in the oxygen evolving complex (OEC) of photosystem II. Several models have been proposed for this active center based on direct biochemical and biophysical studies. In the course of this thesis work, an inorganic model approach to this enzyme has been initiated, and synthetic entry into mono-, bi-, and trinuclear manganese complexes has been achieved. The structure of these manganese complexes have been studied and most of these have been characterized by X-ray crystallography. The mononuclear Mn(IV) complexes give low field EPR signals somewhat similar to that observed in the S$\sb2$ state of the OEC as has been suggested by Hansson et al. The hyperfine structure is variable depending on the coordination geometry around Mn(IV). Electrochemical studies have shown that the Mn(IV) oxidation level is stabilized by alkoxide groups. The trinuclear complexes are the first reported multinuclear manganese species that have an S = 3/2 ground state and give rise to a wide range of EPR spectroscopic features including both low field and multiline type spectra. Small conformational changes and some group substitutions for these trinuclears have little or no effect on magnetic exchange and EPR spectral features. The cluster integrity is a function of the donor capacity of the solvent. A comparison of structural and mechanistic properties is made between the small molecules and the OEC to evaluate different structural proposals. Mn(III) EHPG (ethylene-bis-(o-hydroxyphenylgycine)) complexes have been obtained. The Mn(IV)EHPG are seen to promote the stepwise oxidative decarboxylation of the ligand. This process, which involves two electron reduction of manganese at each step, was monitored by paramagnetic NMR spectra and electrochemical methods. The mechanisms are proposed. These studies demonstrate that the reactivity patterns of mononuclear manganese complexes more closely mimic vanadium than iron under certain circumstances. The observation that manganese(IV) can participate in both one and two electron oxidation reactions has important implications for Mn enzymes. A full series of V(IV) and V(V) SHED (N-(salicylideneaminato)-N$\sp\prime$-(2-hydroxyethyl)ethylenediamine) complexes are presented, most of which have been characterized by X-ray crystallography. The acid-base and redox relationship of all these vanadium compounds are discussed. Their potential biological relevance to vanadium bromoperoxidases are put forth.