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

Abstract

The kinetics of reaction of aquacobalamin (vitamin B_12a) 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.