Possible insights into metal ion recognition in calcium-binding proteins provided by complexing properties of ligands containing amide oxygen donors

The formation constants of the ligand N,N,N',N'-tetrakis(carbamoylmethyl)-ethylenediamine suggest that the amide oxygen is a stronger Lewis base in water than the alcoholic oxygen, or water, and that part of the selectivity for Ca2+ over Mg2+ shown by calcium-binding proteins such as calmodulin or annexin may be due to the higher affinity of Ca2+ for the O-donor of the Ca-binding amide groups present in such proteins.