Copper: Hemocyanin/Tyrosinase ModelsBased in part on the article Copper: Hemocyanin/Tyrosinase Models by Nobumasa Kitajima which appeared in the Encyclopedia of Inorganic Chemistry, First Edition .

Abstract

Coupled dinuclear copper(II) centers are found in metalloenzymes such as hemocyanin (Hc), tyrosinase (Tyr), and catechol oxidase (Co), and perform functions such as dioxygen transport, o ‐phenol aromatic hydroxylations, and dehydrogenation of o ‐catechols. These enzymes all contain two‐copper centers, each ligated by imidazole nitrogen ligands from histidine residues, and in their oxyforms are bridged by a μ‐η 2 :η 2 ‐peroxo moiety. Synthetic model compounds have proven useful in elucidating a number of different aspects of the protein chemistry, such as spectroscopic and structural properties. Also, new coordination chemistry has been developed, such as a new and possibly important equilibrium form of the μ‐η 2 :η 2 ‐peroxo‐ $\hbox{Cu}^{{\rm II}}_{2}$ species, namely, the $\hbox{Cu}^{{\rm III}}_{2}$ ‐bis‐μ‐oxo moiety. In conjunction with stopped‐flow kinetic studies, new fundamental reactions involving Cu I complexes and dioxygen have also been elucidated, such as the demonstration that an initial Cu II ‐superoxo species is formed prior to $\hbox{Cu}^{{\rm II}}_{2}$ ‐peroxo formation. Both species are enthalpically favored at low temperatures, but an unfavorable entropy term precludes the isolation of many of these complexes near room temperature. Considerable efforts are directed towards elucidation of detailed mechanistic pictures for substrate oxidations with copper–dioxygen adducts. Ortho ‐phenol hydroxylations proceed via an electrophilic aromatic substitution reaction. The mechanism of Co activity is still widely debated with evidence in model systems pointing towards both proton‐coupled electron transfer pathways, as well as direct hydrogen atom transfer reaction. Such mechanistic inquiries, and determining the differential reactivity of μ‐η 2 :η 2 ‐peroxo‐ $\hbox{Cu}^{{\rm II}}_{2}$ and $\hbox{Cu}^{{\rm III}}_{2}$ ‐bis‐μ‐oxo complexes, are of continuing and future interest.