Optically active co-ordination compounds. Part XXI. The oxygenation of cobalt(II)–tripeptide complexes

Abstract

The binding sites of glycyl-glycyl-glycine (triglycine) with cobalt(II) in neutral and alkaline solutions have been investigated by contact shifts in the proton magnetic resonance spectrum and by titrations in absence of air. The binding is via the amino-group and adjacent amide linkage, probably via oxygen. In neutral or slightly alkaline solution (pH ⩽ 9) the amide nitrogen is protonated but at higher pH ( 11) it becomes fully deprotonated.

The oxygen uptake by alkaline solutions of tripeptide and cobalt(II) has the stoicheiometry, cobalt(II): oxygen:peptide = 2 : 1 : 4. Two oxygen-carrying intermediates have been identified, one reversible, the second irreversible. The latter may be obtained by another route and has been shown to contain a hydroperoxo-bridge by titration with acid and identification of the hydrogen peroxide liberated. The final product of this and other preparative routes is the cobalt bis-tripeptide anion, whose structure is related to the dipeptide analogue. The resistance to hydrolysis of the bound peptide except in strong base is demonstrated. Similar complexes of L-alanylglycylglycine, glycylglycyl-D-leucine and L-alanyl-L-alanyl-L-alanine have been prepared and their optical activity discussed.