Kinetics of reaction of aquacobalamin with azide ion and hydrazoic acid: pH profile and activation parameters

Abstract

The kinetics of reaction of aquacobalamin (vitamin B12a) with azide anion and hydrazoic acid, producing azidocobalamin, has been investigated as a function of ligand concentration, pH, temperature, and pressure, by stoped-flow spectrophotometry. The pH-independent rate constants (25 °C, Ionic strength I= 1.0 mol dm–3), and activation parameters are k1= 525 ± 14 dm3 mol–1 s–1, ΔH1= 65 ± 1 kJ mol–1, ΔS1= 26 ± 2 J K–1 mol–1, and ΔV1= 6.4 ± 0.1 cm3 mol–1 for the reaction with N3–, and k2= 103 ± 13 dm3 mol–1 s–1, ΔH2= 65 ± 3 kJ mol–1, ΔS2= 13 ± 8 J K–1 mol–1, and ΔV2= 5.5 ± 0.3 cm3 mol–1 for the reaction with HN3. The experimental data are considered in terms of both a limiting dissociative and a dissociative interchange mechanism and it is concluded that they cannot be used to distinguish between the two possible mechanistic pathways.