Metal-Radical Arrays

Protein radicals, defined here as free radicals situated on amino acid or modified amino acid groups linked to a protein covalently, are involved in many enzymatic processes. The chemistry of such species has been surveyed in several comprehensive review articles.^1–5 Metal-containing cofactors are situated in close proximity to, and invariably are responsible for the generation of, protein radicals. In many instances, the radicals are transient species that react with substrate essentially independently of the metal site (e.g., cobalamins, see Chapter 8.25). In other cases, the metal site both generates and stabilizes the protein radical, and is intimately involved in the reactions it performs. These reactions often entail mechanistically novel multielectron redox processes that result from the close interplay between metal center and organic radical. The metal–radical interactions also result in interesting electronic structures, as revealed by physical/ spectroscopic studies of the proteins and of related synthetic complexes. A central issue in this regard concerns ligand ‘‘innocence,’’ namely, whether the electron ‘‘hole’’ is localized solely on the ligand or the metal, or is delocalized between them. This issue has been explored extensively in many contexts, including complexes of hemes (see Chapters 8.11 and 8.12), dioxolenes (catecholate/semiquinone/quinone),^6,7 and related ligands with imino and/or thiolate donors located at the 1 and/or 2-positions on aromatic rings.^8–15 The focus of this chapter is on those few examples of protein metal–radical arrays that have been reported in which the radical is stabilized by the metal, the metal and radical components function synergistically,....