Ligand Exchange Reactions of Hemin and Vitamin B12a in the Presence of Surfactants in Water and in Nonpolar Solvents

Abstract

Hemin is present as monomers in alcohol, aqueous micellar hexadecyltrimethylammonium bromide, CTAB, and sodium dodecyl sulfate, NaLS, as well as in the polar solvent pools in cyclohexane and in benzene, solubilized by polyoxyethylene(6) nonylphenol, Igepal CO-530. Conversely in water hemin exists as dimers. There are significant and specific micellar effects on the ligand exchange reactions of monomeric hemin with imidazole and cyanide ion. Vitamin B12a is completely insoluble in nonpolar solvents. In the presence of surfactants, capable of forming reversed micelles vitamin B12a is entrapped in the surfactant solubilized water pool and is shielded from the bulk apolar solvents by some 300 surfactant molecules. Depending on the size of the surfactant solubilized water pool, the effective polarity of the environment of vitamin B12a can vary considerably. Rate constants for the formation and decomposition of glycine, imidazole, azide and cysteine adducts of vitamin B12a in micellar environments differ dramatically and specifically from those in bulk water. Ligand exchange reactions with functionalized thiols are also affected. These data are discussed.