Project Summary/Abstract The proposed work will provide detailed understanding of geometries, electronic structures, bonding, and chemical reaction mechanisms for copper complexes relevant to key postulated intermediates in copper enzymes. In specific aim 1, the properties and bio-relevant reactivity of [CuO]+ and [CuOOR]2+ complexes will be characrterized in order to evaluate their feasibility as intermediates in oxidation catalysis by monocopper sites in enzymes. Specific emphasis will be placed on evaluating mechanistic hypotheses put forth for lytic polysaccharide monooxygenase (LPMO), which is of particular interest due to its use in biotechnology applications and the very strong (>95 kcal/mol) C-H bond of its substrate that is attacked. In specific aim 2, a novel synthetic route will be used to access the first examples of complexes with the [CuIIOCuIII]3+ core, which has been postulated on the basis of theory to be particularly reactive and thus an attractive structural candidate for the key intermediate in particulate methane monooxygenase. This route will involve O-O bond cleavage of [CuII(- OOR)CuIII]3+ complexes using continuous irradiation or time-resolved transient spectroscopy methods in a collaborative effort, and will provide experimental evidence pertinent to the potential involvement of such species in pMMO and other catalytic systems that attack recalcitrant C-H bonds. In specific aim 3, the preparation and exploration of the properties and reactivity of novel tricopper-peroxo [Cu3(O22-)]n+ (n = 2-4) and sulfide-containing [Cu3(-S)]n+ (n = 1-4) clusters supported by new multinucleating ligands are proposed. The studies of the former will test specific structural proposals for a key intermediate (?PI?) in O2 reduction to H2O by the tricopper active site in the multicopper oxidases (MCO's). The studies of the latter will aim to evaluate mechanistic hypotheses for the sulfide-bridged tetracopper CuZ site in the key global nitrogen cycle enzyme nitrous oxide reductase (N2OR), for which the targeted tricopper clusters will serve as a subunit model. Through these exploratory synthetic studies, thorough examinations of molecular properties, and detailed kinetic and mechanistic evaluation of biomimetic reactions, new insights into the fundamental chemistry of reactive species relevant to putative active site intermediates will be obtained. Ultimately, these synthetic studies will show what is possible for copper protein active sites in terms of structures, bonding, reactivity, and reaction pathways, thus providing a fundamental basis for understanding copper protein structure/function relationships. Such knowledge is critically important in view of the broad importance of copper-promoted biological reactions and, ultimately, will enable strategies for manipulation of enzyme function and development of new catalysts.