LIGAND SUBSTITUTION-REACTIONS OF AQUOCOBALAMIN - REACTIONS WITH PRIMARY AMINES

Enthalpies and entropies of activation were determined from the temperature variation of the spectrophotometrically determined pH-independent rate constants for the reaction of nine neutral primary amines with aquocobalamin (vitamin B-12a) in aqueous solution (ionic strength, I = 1.00 mol dm-3). There are compensating changes in DELTA-H double-ended-dagger and DELTA-S double-ended-dagger for this series of ligands, with DELTA-H double-ended-dagger decreasing from ca. 81 to 58 kJ mol-1 as DELTA-S double-ended-dagger decreases in parallel from ca. 46 to -46 J K-1 mol-1. There is, however, no apparent isokinetic relationship for the series. Plots of DELTA-H double-ended-dagger and DELTA-S double-ended-dagger against the ligand pK(a) reveal that the ligands fall into two distinct classes; for a given pK(a) value, ligands in the first class [NH3, NH2(CH2)3OH, NH2Me and NH2Pr] have significantly larger DELTA-H double-ended-dagger and DELTA-S double-ended-dagger values than those in the second class [NH2OMe, NH2OH, NH2CH2CO2Me, NH2CH2CH(OH)CH2OH and NH2(CH2)2OH]. This is ascribed to ligands in the second class interacting by hydrogen bonding with the acetamide side-chains of the corrin ring and so having their amino groups favourably oriented for interactions with the metal ion.