Kinetic and thermodynamic study of the interaction of vitamine B₁₂a with ligands in aqueous and micellar environments

Abstract

Rate constants for the formation, k₁(superscript app) and decomposition, k₋ ₁(superscript app), of glycine, imidazole, sodium azide, cysteine, N-butanoyl-D,L-cysteine, N-octanoyl-D,L-cysteine, and N-decanoyl-D,L-cysteine adducts of vitamin B₁₂(subscript a) have been determined in water, in aqueous micellar sodium dodecyl sulfate and hexadecyltrimethylammonium bromide as well as in dodecylammonium propionate solubilized water in benzene. Effects in aqueous micellar systems are relatively small, with the exception of the N-alkanoyl-D,L-cysteine derivatives, and are explicable in terms of electrostatic and hydrophobic considerations. Using the obtained linear correlations between the glycine solubilities versus solvent polarity parameter E (subscript T)(30) , the effective environment was determined to depend on both the concentrations of water and of surfactant. Vitamin B₁₂(subscript a) effectively shielded from the apolar solvent by some 300 surfactant molecules. Values for k₁ (superscript app) and k₋₁(superscript app) for the interaction of vitamin B ₁₂(subscript a) a with ligands in the polar cavities of surfactants in benzene are significantly greater than those in bulk water and also depend on both the water and surfactant concentrations. Not unexpectedly, the largest rate enhancements were found with the smallest water pools. For the reaction of glycine with vitamin B₁₂(subscript a) at constant concentration of solubilized water for k₁ (superscript app) and k₋₁ (superscript app) increase with increasing dodecylammonium propionate concentration up to a maximum after which they decrease logarithmically. The significance of these results is discussed.