Complex-formation reactions of aquacobalamin revisited: effect of chloride on the rate and activation parameters
- 1 January 1995
- journal article
- research article
- Published by Royal Society of Chemistry (RSC) in J. Chem. Soc., Dalton Trans.
- No. 4,p. 685-688
- https://doi.org/10.1039/dt9950000685
Abstract
Complex-formation reactions of aquacobalamin were reinvestigated to clarify the role of high chloride concentrations (up to 1 mol dm–3). It was found that high chloride concentrations retard the reactions due to the formation of the less substitution-labile chloro complex, for which the formation constant was found to be 0.8 dm3 mol–1 at 25 °C and ionic strength 1.0 mol dm–3. The pH dependence of the complex formation with hydrazoic acid was also re-examined. Volumes of activation were determined for the reactions with HN3 and N3 – in NaClO4 medium and are discussed in comparison with those measured in KCl medium. Direct kinetic evidence for the occurrence of a reverse, acid-catalysed aquation reaction was found and this step is characterised by an activation volume of +8.3 ± 1.6 cm3 mol–1. The results are all in agreement with the concept of a dissociative interchange substitution mechanism.Keywords
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