Dennis G. Brown

A cyclic voltammetric study of vitamin B12 derivatives

Summary The cyclic voltammograms of vitamin B12 and a number of derivatives were investigated. Variation of the cathodic peak potentials with axial ligand indicated the same trend that has been reported earlier from polarography. The interconversion between the cobalt(II) and cobalt(I) oxidation states was studied in some detail. For the cobalamins, this interconversion is not reversible unless the lower benzimidazole nucleotide base is protonated. On the other hand, for the cobinamides in which there is no strong axial ligand present, the cobalt(II)-cobalt(I) interconversion is a reversible process

Electronic and X-ray photoelectron spectra of copper catecholate complexes

Abstract The electronic and X-ray photoelectron spectra (XPS) of a number of copper(II) catecholato complexes are reported. The electronic spectra exhibit a rather intense visible absorption. Comparison of trends in the position of this intense absorption with the copper 2p binding energies supports the view that the visible 3h absorption is due to a copper → catecholato (π*) transition.

The interaction between vitamin B-12 and micelles in aqueous solution

The apparent pK for benzimidazole displacement of a number of cobalamins is markedly affected by the presence of sodium lauryl sulfate micelles. However, micelles of cetyltrimethylammonium bromide or Triton X have little or no effect on the pK. By measuring the apparent pK as a function of sodium lauryl sulfate concentration, the association constants between the micelles and both base on and base off methylcobalamin were calculated. This calculation indicates that the base off form is strongly associated with the micelle while the base on form is not.

Thermodynamic measurements of the cis and trans effects in alkylcobalt(III) chelates

Abstract Thermodynamic parameters for bonding a sixth ligand to a number of five coordinate alkylcobalt(III) complexes have been measured. The enthalpies of adduct formation demonstrate the magnitudes of the ground state trans and cis effects in these systems. Substantial differences in the enthalpies of axial bond formation are caused by changes in both the trans and cis ligands. Unusual behavior is observed when tetrahydrofuran and tetrahydrothiophene are the entering ligands. Comparison of the equilibrium constants for these two bases indicates that the cobalt center prefers the more polarizable sulfur donor. On the other hand, the enthalpy measurements show that the metal ligand bond formed is stronger with the oxygen donor. This unusual behavior has been previously observed for the binding of these two bases to methylcobaloxime.

On the reversibility of the B1 2r−B1 2s couple in cobalamins

Abstract The cobalt(II)—cobalt(I) interconversion in a number of vitamin B1 2 derivatives was investigated by cyclic voltammetry. Particular attention was focused on the factors determining whether the interconversion is reversible. When the lower axial coordination position is occupied by a strong ligand, such as the benzimidazole nucleotide in “base on” cobalamins, the cobalt(II)—cobalt(I) interconversion is irreversible due to a slow reduction of the cobalt(II). However, when the lower axial coordination position is free of a strong ligand, as in most cobinamides or in “base off” cobalamins, the cobalt(II)—cobalt(I) interconversion is nearly perfectly reversible. Possible implications of the observations to B1 2-dependent enzymes are briefly discussed.

Preparation and Properties of Two Tris o-Semiquinone Complexes

Abstract Tris o-benzosemiquinone complexes of iron and chromium are reported. These complexes contain three 3,5-di-t-butyl-o-semiquinato ligands and a trivalent metal ion. The iron complex can be obtained from either the more highly oxidized o-quinone or reduced catechol dianion precursor. The coordinated o-semiquinone ligand is surprisingly stable. A simple infrared criterion is suggested for detecting the presence of a coordinated o-semiquinone.

XPS Spectra of Copper and Nickel Biuret Complexes — Observations of Intense Satellite Structure in the 2P Spectrum of a Copper(III) System

Abstract A series of copper and nickel biuret complexes has been studied by X-ray photoelectron spectroscopy (XPS). The results include one of the first reports of the XPS spectrum of a copper(III) complex. Significantly, this diamagnetic complex exhibits intense satellite structure in both the Cu 2P3/2 and Cu 2Pi/2 regions. The isoelectronic nickel(II) complex shows no comparable satellite structure.

Intradiol Aromatic Ring Cleavage in a Dioxygenase Model System

Abstract Dioxygenase Model, Copper, Catechol The complex (3-n-nonylcatecholato)(1,10-phenanthroline)copper(II) reacts with molecular oxygen to give an isomeric mixture of 2-n-nonylmuconic acid and several secondary oxidation products. The catecholato ligand is oxidatively cleaved in an intradiol fashion reminescent of the reactions catalyzed by intradiol aromatic dioxygenases. The 1,10-phenanthroline ligand can be quantitatively recovered and is not oxidized. In addition to characterization of oxidation products, the kinetics of oxygen consumption have been analyzed. The rate law for oxygen uptake is -d[O2]/dt = k[O2] [copper]2 .