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 β_2,1,−1 = 6.15 ± 0.09, log β_1,1,1 = 12.15 ± 0.10, log β_1,2,1 = 20.1 ± 0.08, respectively. The complex Al(His)²⁺, with a stability constant log β_1,1,0 = 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.

Keywords:

• Aluminium
• S-Histidine
• Complex formation
• Synthesis
• Stability constants

Acknowledgements

The authors would like to thank Dr Istvan Banyai (Debrecen, Hungary) for help with NMR measurements. The financial help from the Serbian Ministry of Science and Technology under project 1941 is gratefully acknowledged.