Sijbe Balt

Effects of solvent and ionic medium on the kinetics of axial ligand substitution in vitamin B12, Part I. The reaction between aquocobalamin and the thiosulphate ion in dioxane-water mixtures

SummaryThe rate constants for the reaction of aquocobalamin with thiosulphate were measured as a function of ionic strength and solvent composition in dioxane-water mixtures.The solvent effects are rather small and of the same order as the ionic strength effects. A linear free-energy relationship was found between the aquation rate constants and the dissociation constants of the thiosulphatocobalamin in dioxane-water mixtures. The reaction of aquocobalamin with thiosulphate has a dissociative character.

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.

Effects of solvent and ionic medium on the kinetics of axial ligand substitution in vitamin B12. Part VI.Partial molar volumes of some cobalamin derivates

Abstract Apparent molar volumes φ v , were measured at 25 °C in 0.1 molal NaClO 4 for aquocobalamin chloride, methylcobalamin, 5′-deoxyadenosylcobalamin and aquanitrocobaloxime. For the organocobalamins the pH dependence of φ v was studied, and for aquocobalamin and aquanitrocobaloxime the dependence on solvent composition was studied. The base-off forms of the organocobalamins have the same volumes as the base-on forms. The apparent molar volume of aquocobalamin chloride is almost independent of solvent composition in dioxane-water mixtures, but increases dramatically in acetonitrile-water mixtures.

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.

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

Abstract The rate constants for the reaction of aquocobalamin with thiourea were measured as a function of ionic strength, pH and solvent composition in dioxane-water and acetonitrile-water mixtures. With the help of solubility measurements a complete quantitative analysis of solvent effects on the reaction profile could be made. The transfer Gibbs energy of the initial state strongly depends on solvent composition. Because the transition state and the final state closely follow the initial state, this is not reflected in the rate constants. For the acetonitrile-water mixtures the transfer enthalpy and transfer entropy were determined and were found to exhibit the familiar compensation effect. It is concluded that, when the solvent changes, vitamin B12 creates its own micro-environment around the active metal site, so that the reactivity is effectively solvent independent. The mechanism of activation is dissociative.

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.

Base hydrolysis of mono-alkylsubstituted chloropentaamminecobalt(III) complexes

SummaryThe preparation of the series ofcis- andtrans-[Co(NH3)4(RNH2)Cl]2+ complexes (withcis, R = Me orn-Pr andtrans, R = Me, Et,n-Pr,n-Bu ori-Bu) is described. The u.v-visible spectra indicate a decrease of the ligand field on increasing chain length. Infrared spectra show an enhanced Co-Cl bond strength compared to the pentaammine. Partial molar volumes of the complex cations do not reveal steric compression. From proton exchange studies in D2O it follows that [Co(NH3)5Cl]2+ and thecis- andtrans-[Co(NH3)4-(CH3NH2)C1]2+ complexes exchange the amine protons on the grouptrans to the chloro faster than those on thecis. A coordinated methylamine group exchanges its amine protons slower than a corresponding NH3 group in the parent pentaammine, but the methyl introduction accelerates the exchange of the other NH3 groups. The aquation of thetrans-alkylamine complexes (studied at 52° C) is acceleratedca. 10 times compared to the parent pentaammine, irrespective of the nature of the alkyl group. Thecis complexes do not show this acceleration of aquation. In base hydrolysis (studied at 25° C) thecis complexes are the most reactive (a factor 20 over the parent ion). Thecis/trans product ratio in base hydrolysis and the competition ratio in the presence of azide ions were calculated from the 500 MHz1H n.m.r. spectra, which display distinctly different alkyl resonances for each individual complex. Thecis ions react under stereochemical retention of configuration; thetrans compounds give 10±1%trans tocis rearrangement. The ionic strength (⩽4 mol dm−3) and the pH do not affect this result. The same product ratio is obtained in methanol-water and DMSO-water mixtures. Ammoniation in liquid ammonia gives the same ratios as in base hydrolysis, base-catalyzed solvolysis in neat methylamine gives stereochemical retention for both thecis- andtrans-methylamine ion. The product competition ratio (Co-N3)/(Co-OH2) for thecis compounds and the bulkier amines (R =n- andi-Bu), 15–25% at 1 mol dm−3N3−, isca. twice that of thetrans compounds and the pentaammine. The results are interpreted in the classical conjugate base mechanism, and discussed in the context of current ideas about stereochemistry of base hydrolysis.