Spectrophotometric and 27-Al NMR characterization of aluminum(III) complexes with L-histidine

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/dm(3) 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)His(2+), and Al-2(OH)His(4+) with the overall formation constants beta(p,q,r): log beta(1,1,1) = 11.90 +/- 0.04, log beta(1,1,0) = 7.25 +/- 0.08, log beta(1,2,1) = 20.1 +/- 0.1, and log beta(2,1,1) = 5.92 +/- 0.12 ( p, q, r are stoichiometric indices for metal, ligand, and proton, respectively). Al-27-NMR spectra were taken on solutions with the concentration of aluminum 50 mmol/dm(3) and that of histidine 250 mmol/dm(3). 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