Interactions of a vitamin B12 model complex with amino-acids and oligopeptides. A visible and nuclear magnetic resonance spectroscopic study

Abstract

Substitution of the water molecule of [MeCo(tn)H₂O] ClO₄ by some amino-acids and simple oligopeptides has been studied by ¹H n.m.r. and visible absorption spectroscopy. Amino-acids bind to cobalt through –NH₂, imidazole, nitrogen and –S^–; formation constants are higher for the sulphur ligands while no Co–CH₃ bond breaking is observed. Amino-acid protons nearest to cobalt undergo anomalous high fields shifts; for the trans methyl and the equatorial ligand proton shifts, the ring current effect of the entering ligand is the major factor. The existence of µ-amino-acido-complexes and of preferred orientation in solution of some ligands in the sixth co-ordination position is proposed.