Complex formation of calcium(II) with amino acids under physiological conditions

Abstract

The complex formation of calcium(II) with glycine(HL), DL-alanine(HL), β-alanine(HL), and DL-aspartic acid(H₂L) was investigated under physiological conditions (at 37 °C in 0.15 mol dm^–3 NaCl aqueous solution) by potentiometric titrations using both calcium-ion selective and glass electrodes. The e.m.f. data for the glycine, DL-alanine, and β-alanine systems were explained with only one species [CaL]⁺ and those for the DL-aspartic acid system with two, [Ca(HL)]⁺ and [CaL]. Nitrogen-14 and ¹⁷O n.m.r. spectral measurements suggested that both the amino and carboxylate groups co-ordinate to Ca^II within the complexes in the glycine, DL-alanine, and β-alanine systems. N.m.r. spectra could not be obtained for the complexes in the DL-aspartic acid system because of the low solubility of that acid.