Aluminium(III) complexes of S-histidine: synthesis, characterization and potentiometric and spectroscopic study of solution equilibria

Abstract

Complex formation between S-histidine (HHis) and aluminium(III) ion in aqueous solution was studied by potentiometric measurements, ESI-MS, 27Al and 13C NMR spectroscopy at 298 K. Potentiometric titrations were made at 298 K over the pH range 1.90 to 6.40 on solutions with total aluminium concentrations from 1.0 to 20.0 × 10−3 mol dm−3. Ionic strength of the solutions was maintained at 0.1 mol dm−3 with LiCl. A neutral solution of HHis was used as titrant. Non-linear least-squares treatment of the pH 3.0–6.2 data indicated the formation of one main complex, Al2(OH)His4+, and two minor ones, Al(HHis)3+ and Al(HHis)His2+, with overall formation constants, β p , q , r (p, q, r being stoichiometric coefficients for metal, ligand and proton, respectively), of log β2,1,−1 = 6.15 ± 0.09, log β1,1,1 = 12.15 ± 0.10, log β1,2,1 = 20.1 ± 0.08, respectively. The complex Al(His)2+, with a stability constant log β1,1,0 = 7.21 ± 0.08 was at the limit of the detection and is probably the mixed hydroxo complex, Al(OH)HHis2+. ESI-mass spectra generally confirmed the equilibrium model though a variety of polynuclear hydrolytic complexes was observed. 27Al NMR spectra of solutions with aluminium concentrations of 5–50 × 10−3 mol dm−3 and histidine concentrations of 25–260 × 10−3 mol dm−3 were recorded. In the pH interval 4.0–4.5 a resonance at 4.7 ppm was assigned to Al2(OH)His4+, while at pH 5.0–6.1 two resonances at 8.2 and 12.0 ppm were assigned to Al(HHis)2+ and Al(HHis)(His)2+ [or ], respectively. In 13C NMR spectra the upfield chemical shift difference of the carboxyl carbon resonance of free and bound histidine of 0.8 ppm, and that of aliphatic α- and β-carbons of 0.3–0.4 ppm, confirmed the formation of the complex in which both the carboxyl and amino groups of histidine participate in coordination. An isolated complex has the composition [(AlOH)(HHis)2]Cl2. IR spectra showed changes in position and profile of carboxyl and amino bands as compared to those of free S-histidine, again indicating the involvement of both groups in coordination to aluminium.