Spectrophotometric and 27‐Al NMR Characterization of Aluminum(III) Complexes with l‐Histidine

Abstract

The complex formation between l‐histidine (HHis) and aluminum(III) ion in water solutions was studied by UV spectrophotometric and 27‐Al NMR measurements at 298 K. UV spectra were measured on solutions in which the total concentration of histidine was from 15.0 to 50.0 mmol/dm3 and the concentration ratio of histidine to aluminum was varied from 3∶1 to 10∶1 in the pH range between 4.2 and 6.0. The spectra were taken in the wavelength interval 240–340 nm. Nonlinear least‐squares treatment of the spectrophotometric data indicates the formation of the complexes Al(HHis)3+, Al(His)2+, Al(HHis)His2+, and Al2(OH)His4+ with the overall formation constants βp,q,r: log β1,1,1=11.90±0.04, log β1,1,0=7.25±0.08, log β1,2,1=20.1±0.1, and log β2,1,1=5.92±0.12 (p, q, r are stoichiometric indices for metal, ligand, and proton, respectively). 27Al‐NMR spectra were taken on solutions with the concentration of aluminum 50 mmol/dm3 and that of histidine 250 mmol/dm3. In the pH interval 5.0–6.1, two resonances at 9.5 ppm and 12.0 ppm were assigned to Al(HHis)2+ and Al(HHis)(His)2+ (or Al(OH)(HHis)2 2+), respectively.