Martinus W.G. De Bolster

Binding of metal ions to polysaccharides. II. The binding of metal ions to chondroitin sulphate in alkaline media

Abstract At high pH, polyelectrolytes like chondroitin sulphate form soluble complexes in water with a number of di- and trivalent cations probably through binding at the carboxylate groups. This interaction was investigated by ligand-field and ESR spectroscopy, H + and Cu 2+ potentiometry, and viscometry.

A 13C-N.m.r. study of the binding of ytterbium(III) to chondroitin sulphate and chondroitin☆

13C-N.m.r. spectra of chondroitin 4- and 6-sulphates, chondroitin, beta-D-glucuronate, and beta-D-glucose 6-sulphate were measured in the presence of ytterbium(III) in deuterium oxide. The structure of the ytterbium-polysaccharide compounds in solution was found to be similar to that reported for calcium chondroitin 4-sulphate in a stretched film. In the glucuronate complex, Yb(III) coordinates to the carboxylate group. For beta-D-glucose 6-sulphate, the ytterbium-induced shifts are too small to allow the structure to be determined.

Binding of metal ions to polysaccharides. VI: Spectroscopic and potentiometric studies of the binding of copper(II) and nickel(II) to some monosaccharide units in acidic, neutral and alkaline aqueous media

Binding of Cu2+ and Ni2+ to glucosamine, N-acetyl- glucosamine and other derivatives of glucose was investigated in acidic, neutral and alkaline aqueous media using H+ and Cu2+ potentiometry and ligand- field and ESR spectroscopy. In neutral medium, site binding with copper(II) and nickel(II) occurs when the monosaccharide possesses a potentially coordinating amine or charged group not attached to C-1. At high pH, a coordination entity is only formed if the C-1 hydroxyl group can be deprotonated and other stabilizing groups are present. The role of groups attached to C-1 reflects the different behaviour of monosaccharides compared with polysaccharides.

Model compounds for co-enzyme B12: kinetic data for cobalt-alkyl bond dissociation reactions

Abstract The effects of pressure and temperature on the rates of the thermal homolytic fission of the cobalt-carbon bond in some organocobalt model compounds for co-enzyme B 12 has been determined. The investigated model systems comprise six-coordinated benzyl complexes of dimethylglyoxime with nitrogen and phosphorus coordinated axial ligands and five- and six-coordinated butyl complexes of the Schiff base bis(salicylidene)ethylenediamine. The observed activation volumes, reported for the first time for this type of compound, are discussed in terms of the cage model and solvent dependence. The observed activation enthalpies, a measure for the cobalt-carbon bond dissociation energies, are discussed in terms of steric and electronic factors of the trans -ligand. Finally, an isokinetic relationship and the infiuence of the coordination number are mentioned.

Intramolecular alkoxycobaltation: a novel route to the cobalt–carbon bond in a coenzyme B12 model

CoII(salen) derivatives whose ethanediyl moiety carries an ω-alkenyl side chain react with oxygen and methanol or ethanol to give, via interaction between intermediate CoIII species and the pendant olefin, alkylated products with a β-alkoxy substituted carbon bridge between cobalt and the equatorial ligand; the corresponding intermolecular reaction between CoII(salen) and hex-1-ene does not take place.

A study of the cage mechanism for the homolytic cleavage of the cobalt-carbon bond in coenzyme B12 by varying the solvent viscosity

Abstract The effect of viscosity on the rates of anaerobic thermal and photolytical scission of the cobalt-carbon bond in coenzyme B 12 (AdoB 12 ) has been determined in different water/glycerol mixtures. The photolysis of the coenzyme is compared with [adenosylcobinamide] + OH − . Using a simple diffusion model, the fractional cage efficiency and the rate of recombination of the radicals trapped in the solvent cage could be determined from the viscosity dependence of the reaction rates. The activation parameters for the cage recombination were derived from the temperature dependence of the recombination reaction giving Δ H ‡ = 17.9±0.8 kJ mol −1 and Δ S ‡ = 21±2 J K −1 mol −1 . The results obtained are shown to be relevant to the discussion about the CoC bond dissociation energy of AdoB 12 , for which a combination of our results with published data gives 119±8 kJ mol −1 .

Effects of solvent and ionic medium on the kinetics of axial ligand substitution in vitamin B12. Part VII.The reaction between aquanitrocobaloxime and thiourea in dioxane-water mixtures

Abstract Rate constants for the reaction of aquanitrocobaloxime with thiourea were measured as a function of pH, solvent composition, pressure and temperature, in dioxane-water mixtures. With the aid of solubility measurements a complete quantitative analysis of solvent effects on the initial state and transition state transfer parameters could be made. It was found that the activation enthalpy and entropy vary strongly with solvent composition. This is in contrast to the variations found for vitamin B12, for which this cobaloxime is a model compound. The rate constants increase strongly after 50 vol.% dioxane in the dioxane-water mixtures, another difference with vitamin B12. The volumes of activation are small and positive, in accordance with a dissociative mode of activation.

A study of the cage mechanism for the homolytic cleavage of the cobalt-carbon bond in alkylcobalamins using pressure effects

The effect of pressure on the rate of the anaerobic thermal fission of the CoC bond in alkylcobalamins (RB12, R = adenosyl, methyl, isopropyl and neopentyl) and [adenosylcobinamide]+OH− has been determined in water and 1-propanol/water mixtures. The volumes of activation for homolysis are discussed in terms of the cage model and solvent dependence. It is shown that homolysis at high temperatures (370–390 K) can be described by a negligible cage efficiency and a large volume of activation. At lower temperatures the cage efficiency increases. The type of group R has hardly any influence on the size of the volume of activation for homolysis, which is constant at about 20 cm3 mol−1. The results are compared with those obtained earlier for organocobalt models of coenzyme B12. It is shown that pressure effects can be used as mechanistic probes to identify reactions in which the radicals have the opportunity to escape from the cage.

MODEL COMPOUNDS FOR CO-ENZYME B12 : THE EFFECT OF PRESSURE AND SOLVENT COMPOSITION ON A COBALT-ALKYL BOND DISSOCIATION REACTION

Abstract The effect of pressure on the rate of the thermal homolytic fission of the cobalt-carbon bond in the co-enzyme B 12 : model [C 6 H 5 CH 2 Co(Hdmg) 2 P(C 6 H 11 ) 3 ] has been determined in 1-propanol/water solvent mixtures. The observed volumes of activation are discussed in terms of the cage model and solvent dependence. Homolytic cobaltcarbon bond fission is compared to homolytic carbon-carbon bond fission. It is shown that there is a complete parallel and that the cage efficiency is large.

Bridged (alkoxo)CoIII(salen) complexes: synthesis and structure

Abstract The preparation and oxygenation of CoII(salen) complexes 2a–d whose ethanediyl moieties carry an ω-hydroxyalkyl pendant arm is reported. Air oxidation of 2a gives bridged (alkoxo)CoIII(salen) complex 1a with a planar salen ligand. Upon aeration of 2b in MeOH, monomeric bridged complex 1b with a planar salen ligand precipitates, whereas in CH2Cl2 a dimeric bridged complex 1b′ with twisted-salen ligands is obtained and characterized by X-ray crystallography. Formation of a bridged (alkoxo)CoIII(salen) complex upon aeration of 2c is probably followed by β-elimination. The tert-butyl substituted oxidation product of 2d is paramagnetic in non-coordinating and diamagnetic in coordinating solvents, supporting the concept that the salen moiety of (alkoxo)CoIII(salen) complexes can have either a planar or a twisted geometry depending on the solvent and the solid state properties.