KINETICS OF REACTION OF AQUACOBALAMIN WITH AZIDE ION AND HYDRAZOIC ACID - PH PROFILE AND ACTIVATION PARAMETERS

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 stopped-flow spectrophotometry. The pH-independent rate constants (25-degrees-C, ionic strength l = 1.0 mol dm-3) and activation parameters are k1 = 525 +/- 14 dm3 mol-1 s-1, DELTA-H double-ended dagger 1 = 65 +/- 1 kJ mol-1, DELTA-S double-ended dagger 1 = 26 +/- 2 J K-1 mol-1, and DELTA-V double-ended dagger 1 = 6.4 +/- 0.1 cm3 mol-1 for the reaction with N3-, and k2 = 103 +/- 13 dm3 mol-1 s-1, DELTA-H double-ended dagger 2 = 65 +/- 3 kJ mol-1, DELTA-S double-ended dagger 2 = 13 +/- 8 J K-1 mol-1, and DELTA-V double-ended dagger 2 = 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.