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.
