Recent Advances in One‐Electron‐Oxidized Cu^II–Diphenoxide Complexes as Models of Galactose Oxidase: Importance of the Structural Flexibility in the Active Site

Abstract

The phenoxyl radical plays important roles in biological systems as cofactors in some metalloenzymes, such as galactose oxidase (GO) catalyzing oxidation of primary alcohols to give the corresponding aldehydes. Many metal(II)–phenoxyl radical complexes have hitherto been studied for understanding the detailed properties and reactivities of GO, and thus the nature of GO has gradually become clearer. However, the effects of the subtle geometric and electronic structural changes at the active site of GO, especially the structural change in the catalytic cycle and the effect of the second coordination sphere, have not been fully discussed yet. In this Review, we focus on further details of the model studies of GO and discuss the importance of the structural change at the active site of GO.

Solid State Characterization of One‐ and Two‐Electron Oxidized CuII‐salen Complexes with para‐Substituents: Geometric Structure‐Magnetic Property Relationship

“…This may serve as further evidence for the weak intramolecular interaction between the phenoxyl radical moiety and the side chain aromatic ring even in frozen solution. One of the peculiar properties of methylthiophenoxyl radicals is considered to be the preference for the π‐π stacking interaction shown in the active form of GO, [2–8] and the π‐π stacking of the phenoxyl radical moieties has been recently demonstrated as a favorable and effective interaction [19–21,30,50] …”

“…The present findings suggest that the intermolecular π‐π stacking interaction between the phenoxyl radical and phenolate moieties is effective for attaining the ferromagnetism. The π‐π stacking interaction is well known to be seen in the active site of GO, [2–8] and recent model studies have established that the properties of the metal phenoxyl radical complexes are affected by the π‐π stacking interaction [19–21,50] . Fine‐tuning with respect to the location and configuration of the radical sites by the intermolecular π‐π stacking interaction in the metal‐radical framework may lead to the strong ferromagnetism and thus provide an alternative idea for the molecular designs of the metal‐radical‐based magnetic materials.…”

“…Further development of these magnetic materials would be achieved by aligning these magnetic molecules. The Cu II ‐phenoxyl radical species is considered to favor the π‐π stacking interaction based on our recent results, [19–21,30] and as seen in the active form of GO [4–8] . Introduction of such a weak interaction into the phenoxyl radical moiety may lead to the development of the functional magnetic materials.…”

“…This phenoxyl radical moiety is involved in the π‐π stacking interaction with the proximal indole ring for stabilization of the phenoxyl radical [2–8] . In order to understand the detailed properties of Cu II ‐phenoxyl radical complexes, various one‐electron oxidized Cu II ‐phenolate complexes have been synthesized and characterized [9–22] . We have demonstrated that the one‐electron oxidized Cu II ‐salen type complexes vary their electronic structures depending on the small differences of the ligands [13,17,18] …”

π–π Stacking Interaction of Metal Phenoxyl Radical Complexes

“…In order to find the way to the functionalization of metal complexes by π–π stacking interaction, we focused on model studies of the single copper enzyme, galactose oxidase (GO), which catalyzes primary alcohol oxidation to give the corresponding aldehyde [ 32 , 33 , 34 , 35 , 36 , 37 ]. The catalytic alcohol oxidation mechanism involves the hydrogen atom abstraction from the π-position of primary alcohol to the phenoxyl radical bound to the copper(II) ion in the active form of GO [ 35 ].…”