Interligand CH · · · π interaction in binary cobalt(III) complexes of N-pyridoxy-L-amino acids. Biological significance of the pyridoxal 2-methyl group

Abstract

Reaction of K₃[Co(CO₃)₃] with 2 equivalents of N-pyridoxy-L-amino acid (pyridoxy = 3-hydroxy-5-hydroxymethyl-2-methylpyridin-4-ylmethyl; amino acid = alanine, leucine, phenylglycine, phenylalanine, p-nitrophenylalanine, p-methoxyphenylalanine, or tryptophan) afforded the bis(N-pyridoxy-L-amino acidato)cobalt(III) complex as a single band on anion-exchange chromatography. Each complex has been isolated as its sodium salt and characterized in aqueous solution by UV/VIS circular dichroism (CD), and ¹H NMR spectra and by X-ray crystallographic analysis in the solid state. The cobalt(III) atom is co-ordinated to two trans(N)-meridional N-pyridoxy-L-amino acid ligands. The 2-methyl group of one pyridoxal moiety lies over the aromatic ring of another pyridoxal ring with separations of 3.67 and 3.56 Å. The ¹H NMR resonance of the 2-methyl group shows an upfield shift when compared with that of the free pyridoxyamino acid. The selective formation of the trans(N)-meridional-Λ-[R(N), R(N)] diastereoisomer is interpreted in terms of an interligand CH · · · π interaction. The CD and ¹H NMR spectra suggest that addition of 1 equivalent HCl weakens this interaction. The biological significance of the pyridoxy 2-methyl group in vitamin B₆ coenzyme is discussed.