Cytochrome c oxidase, the enzyme complex responsible for the four-electron reduction of O 2 to H 2O, contains an unusual histidine-tyrosine cross-link in its bimetallic heme a 3-Cu B active site. We have synthesised an unhindered, tripodal chelating ligand, BPAIP, containing the unusual ortho-imidazole-phenol linkage, which mimics the coordination environment of the Cu B center. The ligand was used to investigate the physicochemical (pK a, oxidation potential) and coordination properties of the imidazole-phenol linkage when bound to a dication. Zn(ii) coordination lowers the pK a of the phenol by ∼0.6 log units, and increases the potential of the phenolate/phenoxyl radical couple by approximately 50 mV. These results are consistent with inductive withdrawal of electron density from the phenolic ring. Spectroscopic data and theoretical calculations (DFT) were used to establish that the cationic complex [Zn(BPAIP)Br] + has an axially distorted trigonal bipyramidal structure, with three coordinating nitrogen ligands (two pyridine and one imidazole) occupying the equatorial plane and the bromide and the tertiary amine nitrogen of the tripod in the axial positions. Interestingly, the Zn-N amine bonding interaction is weak or absent in [Zn(BPAIP)Br] + and the complex gains stability in basic solutions, as indicated by 1H NMR spectroscopy. These observations are supported by theoretical calculations (DFT), which suggest that the electron-donating capacity of the equatorial imidazole ligand can be varied by modulation of the protonation and/or redox state of the cross-linked phenol. Deprotonation of the phenol makes the equatorial imidazole a stronger σ-donor, resulting in an increased Zn-N imd interaction and thereby leading to distortion of the axial ligand axis toward a more tetrahedral geometry.
ASJC Scopus subject areas
- Inorganic Chemistry