In vitro study reveals calcium(II) forms specific complexes with amino acids under physiological conditions, indicating both amino and carboxylate groups coordinate in biological fluids.
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 CaII 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.
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Maeda et al. (1990) studied this question.
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