Unique Spectroscopic Features and Electronic Structures of Copper Proteins: Relation to Reactivity

Abstract

Copper active sites play a major role in a wide range of biological processes. These include long-range electron transfer, binding, activation, and two-/four-electron reduction of dioxygen, and two-electron reduction of nitrous oxide. Traditionally, copper sites have been classified into three types based on their EPR features: the type 1 “blue,” the type 2 “normal,” and the type 3 “coupled binuclear” sites. However, more recent discoveries of the mixed-valent binuclear CuA, the trinuclear Cu cluster in the multicopper oxidases, and the tetranuclear CuZ sites show that biological copper centers are even more diverse than previously believed. In this review, EPR and other spectral features of the different copper active sites are developed and compared. The origins of the unique spectroscopic features are discussed with respect to the novel geometric and electronic structures that are intimately coupled to their catalytic functions. High covalency is shown to activate specific pathways for long-range electron transfer and exchange interactions between copper centers to control the two vs. one electron activation of O2 for different chemistries and the four-electron reduction of O2 to H2O. In addition, electron delocalization between mixed-valent copper centers can lower reorganization energy and activate copper clusters for catalysis.