TY - JOUR
T1 - Model studies of the Cu B site of cytochrome c oxidase utilizing a Zn(ii) complex containing an imidazole-phenol cross-linked ligand
AU - Pesavento, Russell P.
AU - Pratt, Derek A.
AU - Jeffers, Jerry
AU - Van Der Donk, Wilfred A.
PY - 2006
Y1 - 2006
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=33745597884&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=33745597884&partnerID=8YFLogxK
U2 - 10.1039/b516090a
DO - 10.1039/b516090a
M3 - Article
C2 - 16820845
AN - SCOPUS:33745597884
SN - 1477-9226
SP - 3326
EP - 3337
JO - Dalton Transactions
JF - Dalton Transactions
IS - 27
ER -