Calcium, Magnesium, and Manganese(II) Complexes of Some O-Phosphorylated Peptides.

Abstract

Complex formation between calcium, magnesium, and manganese(II) ions and O-phosphorylated ethanol-amine, seryllysine, serylglycine, glycylserine, glycylseryglycine and serylglutamic acid has been measured by pH titration at 25[degree] in an ionic medium, whose anion concentration, [Cl~], was kept constant by potassium chloride, and whose initial ionic strength was 0.15 [image]. The experimental data could be explained by assuming that complexes of the type MpHqA are formed, where (p;q) = (1:1), (1:0) for the first two ligands, (p:q) = (1:1) (1:0), (2:0) for the next three ligands ((p;q) = (2;0) was not measurable, when M = Ca2+), and (p;q) = (1;2), (1;1), (l;0), (2;1), (2;0) for the last ligand. The stability constants of the complexes have been computed, and the coordination sites are discussed. The observed differences in complexity between the calcium and magnesium ions support Williams'' conclusion1 that their complexing ability can be correlated to ionic size. The definitely greater tendency of magnesium and manganese(II) to dinuclear complex formation as compared to calcium is discussed in relation to the specificity of enzymes for these metal ions.