Solution equilibria between aluminum(III) ion and L-histidine or L-tyrosine

Abstract

Toxic effects due to high aluminum body loads were observed in a number of conditions following ingestion of Al-containing antacids. Bio-availability of aluminum depends not only on the solubility of the ingested salt but also on the physico-chemical properties of the soluble Al complexes formed in body fluids. Amino acids may, upon interaction with Al-salts, form absorbable Al-complexes. Hence, complex formation equilibria between Al3+ and either, L-histidine or L-tyrosine were studied by glass electrode potentiometric (0.1 mol/L LiCl ionic medium, 298 K), proton NMR and uv spectrophotometric measurements. Non linear least squares treatment of the potentiometric data indicates that in the concentration ranges: 0.5 ≤ CAl ≤ 2.0; 1.0 ≤ CHis ≤ 10.0; 2.5 ≤ pH ≤ 6.5, in Al3+ + His solutions, the following complexes (with log overall stability constants given in parenthesis) are formed; Al(HHis)3+ (12.21 ± 0.08); Al(His)2+, (7.25 ± 0.08); and Al(HHis)His2+, (20.3 ± 0.1). In Al3+ + Tyr solutions in the concentration range 1.0 ≤ CTyr ≤ 3.0 mmol/L and ligand to metal concentration ratio from 2:1 to 3:1, in the pH interval from 3.0 to 6.5 the formation of the following complexes was detected: Al(HTyr)2+, (12.72 ± 0.09); Al(Tyr)2+, (10.16 ± 0.03) and Al(OH)2Tyr, (2.70 ± 0.05). Proton NMR data indicate that in Al(His)2+ complex histidine acts as a monodentate ligand but its bidentate coordination is possible with carboxylate oxygen and imidazole 1-nitrogen as donors. In Al(HTyr)3+ complex tyrosine is a monodentate ligand with carboxylate oxygen as donor. The mechanism of the formation of complexes in solution is discussed as well as their possible role in aluminum toxicity.