Evaluation of the role of polyisoprenyl functional groups in biological electron transfer. Transition metal models

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Abstract
Cu(I) coordination to olefin bonds in pyridine compounds containg di- and triisoprenyl substituent groups was investigated. Results from Raman and optical spectroscopic studies in aqueous ethanolic solutions indicate formation of .pi. complexes of 1:1 stoichiometry, with K .simeq. 104 M-1. Despite there being several potential Cu(I) ligation sites on the alkyl side chain, only a single olefin bond is coordinated. The data are consistent with a model comprising extensive folding of the isoprenyl groups in the polar medium, with Cu(I) binding occurring at the exposed olefin group on the terminal unit. Ligand-bridged binuclear ions were formed by simultaneous coordination of an oxidant metal ion, (NH3)5RuIII, to the pyridine ring N atoms and Cu(I) to side-chain olefin bonds. Electron-transfer pathways were determined by kinetic analysis; both rate laws and comparative redox rates for complexes containing a variety of 4-alkylpyridine ligands indicate reaction predominantly by intermolecular processes. No evidence for intramolecular electron transfer, i.e., from Cu(I) through the bridging ligand to the bound Ru(III) center, could be found. This result is discussed both in terms of its implications toward the existence of very similar pathways proposed for electron transfer between heme and Cu redox sites in cytochrome oxidase and within the wider context of apparent differences in the fundamental mechanisms of electron transfer in biological particles and transition metal ions.