Imidazolate, bridging

Imidazoles are of interest as bridging ligands particularly with regard to mimics of the active site of Cu-Zn superoxide dismutase (SOD). Structures with imidazolate bridges have been... 

The imidazolate bridged Cu/Zn bimetallic complex of the cryptand (13) was structurally characterized and shown to have a Cu-Zn distance of 5.93 A (native Cu, Zn-SOD 6.2 A).146 The complex shows some activity in the dismutation of superoxide at biological pH that is retained in the presence of bovine serum albumin. 

TN was increased by the presence of the general acid. These observations suggested that H2O serves to donate the protons required to form product H2O2. Values of Km and TN for the zinc-deficient enzyme were found to be approximately a factor of two less than those obtained for the holoenzyme under identical experimental conditions, whereas TN/Xm was largely unchanged. The authors concluded that the imidazolate bridge is thus not essential for catalytically competent extraction of a proton from the solvent by CuZnSOD. 

In addition, heterodinuclear Fe(III)Ni(II) compounds have been prepared starting from high spin [FeCl(salen)] and NiL with L being the di-anion of the N30 ligand depicted in Fig. 16 [191]. The imidazole-bridged [FeCl (salen)NiL] [191] and also [FeCl(5-OCH3-salen)NiL] both exhibit Fe(III) spin crossover [192]. 

The imidazole-bridge dimetallic centre in copper-zinc superoxide dismutase (EC 1.15.1.1) was a novel structural feature that had not previously been encountered in coordination chemistry [151], The Cu(II) ion is co-ordinated by four histidine side chains, His44, His46, His 118 and His61, and there is evidence for a fifth axial water ligand. 

The structure and enzyme kinetics of bovine erythrocyte superoxide dismutase are reviewed. The protein has a novel imidazolate-bridged copper(II)-zinc(II) catalytic center in each of two identical subunits. Since a C /Cu1 redox couple is responsible for the dismutase activity of the enzyme, the role of zinc is of interest. Both 220-MHz NMR measurements of the exchangeable histidine protons and chemical modifications using diethylpyrocarbonate demonstrate that zinc alone can fold the protein chain in the region of the active site into a conformation resembling that of the native enzyme. Other possible roles for zinc are discussed. Synthetic, magnetic, and structural studies of soluble, imidazolate-bridged copper complexes of relevance to the 4 Cu(II) form of the enzyme have been made. 

X-ray structural studies of [Cu2(bpim)(im)]2(N03)4 4H20 establish the presence of imidazolate bridges in III, Figure 6. Solutions of this compound reversibly take up four protons between the end points at PH 9.75 and 4.25, consistent with Reaction 4. Magnetic susceptibility studies show that the copper (II) centers in III are antiferromagnetically... 

Kinetic and equilibrium studies of the interaction of imidazole with a Mn111 porphyrin in aqueous solution have been reported.675 Imidazole bonds to the Mn111 porphyrin in a step-wise fashion to yield initially an imidazole-bridged dimeric species followed by a monomeric bis(imidazole)-MnIU porphyrin adduct. 

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