Zinc(II) complexes of phosphonic acid analogues of glutamic acid

The stoichiometries and stability constants of proton and zinc(II) complexes of two phosphonic acid analogues Of glutamic acid and three analogues of pyroglutamic acid (5-oxopyrrolidine-2-carboxylic acid) have been determined potentiometrically at 25 degrees C and at ionic strength 0.2 mol dm(-3) (KCl). Complementary studies by means of P-31 and C-13 NMR spectroscopy were also undertaken and indicated that the analogue obtained by the replacement of the gamma-carboxylic group of glutamic acid,by a phosphonic acid moiety and the phosphonic acid analogue of pyroglutamic acid exhibit predictable properties when complexing zinc(II) ions. The analogue obtained by the replacement of the alpha-carboxylic group by a phosphonic acid group underwent chemical transformation upon reaction with zinc(II) ions under-neutral and alkaline conditions yielding most probably a cyclic phosphonamidate composed of two ligand molecules. The crystal structures of some analogues of pyroglutamic acid were also determined.