NMR Study of Amino Acids and Their Derivatives. V. Structures and Formation Constants of Zinc- l -Aspartic Acid Complexes in Aqueous Solution

NMR spectra of l-aspartic acid coordinated with Zn ions were measured in aqueous solution at various pH values. Three species of zinc-l-aspartic acid complexes were identified i.e., 1 : 1 zinc–l-aspartic acid(zwitter ionic form) complex (MAH), and 1 : 1 and 1 : 2 zinc–aspartic acid(anionic form) complexes (designated as MA and MA2, respectively). It is shown that, in MAH, the amino acid coordinates with Zn as a monodentate ligand, whereas in MA and MA2, approximately 65% of the molecules of amino acid coordinate as a tridentate ligand. From the chemical shift data, the equilibrium constants for these complexes are determined as follows: pKa-(MAH)=5.3±0.2 and logK2=4.5±0.2 where Ka(MAH)=[MA][H+]/[MAH] and K2=[MA2]/[MA][A].