Imidazolate-Stabilized Cu(III): Dioxygen to Oxides at Type 3 Copper Sites
Tao A G Large1, William Keown1, J Brannon Gary1,2
1Department of Chemistry, Stanford University Stanford, California, 94305, USA.
Angewandte Chemie (International Ed. in English)
|October 18, 2024
Summary
The deprotonation of imidazole ligands in copper enzymes facilitates a novel redox isomerization, forming unprecedented copper(III) species. This finding suggests a new mechanism for oxygen reduction and phenol hydroxylation in biological systems.
Area of Science:
- Bioinorganic Chemistry
- Metalloprotein Chemistry
- Enzyme Mechanisms
Background:
- Imidazole ligation via histidine is common in metalloproteins.
- The role of the imidazolate conjugate base in metal ligation is often overlooked.
- Oxidized metal centers can potentially make imidazolate accessible.
Purpose of the Study:
- To investigate the role of imidazolate in copper enzyme mechanisms.
- To explore the redox properties of imidazole-ligated copper complexes.
- To propose alternative mechanisms for oxygen activation and substrate hydroxylation.
Main Methods:
- Synthesis of model compounds mimicking oxygenated tyrosinase enzymes.
- Spectrophotometric titrations (UV/Visible/near-IR).
- Copper K-edge X-ray absorption spectroscopy.
Main Results:
- Deprotonation of a μ2-η2:η2-peroxidodicopper(II) species induced redox isomerization to a bis(μ2-oxido)dicopper(III) species.
- This transformation involves cleavage of the peroxide O-O bond and formal two-electron oxidation to Cu(III).
- The isomerization was successfully extended to a protected histidine environment.
Conclusions:
- Imidazole deprotonation is a viable mechanism for facilitating redox transformations in copper centers.
- This pathway offers a potential new mechanism for the four-electron reduction of O2 and phenol hydroxylation at Type 3 copper sites.
- Imidazolate ligation to copper may be more prevalent in biological systems than previously recognized.
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