Speciation of Aluminum(III) Complexes with Oxidized Glutathione in Acidic Aqueous Solutions

Abstract

The structural speciation aspects, including the binding sites, species, complexation abilities and effects of the oxidized glutathione (GSSG) with aluminum(III) in aqueous solutions, have been studied by means of many analytical techniques: pH-potentiometry (25°C, 0.1 M KCl and 37°C, 0.15 M NaCl medium) was used to characterize the stoichiometry and stability of the species formed in the interactions of the Al(III) ion and the peptide GSSG, while multinuclear (¹H, ¹³C, ²⁷Al) nuclear magnetic resonance (NMR) and electrospray mass spectroscopy (ESI-MS) were applied to characterize the binding sites and species of the metal ion in the complexes. Two-dimensional (¹H, ¹H-NOESY) was also employed to reveal the difference in the conformational behavior of the peptide and its complexes. The following results were obtained: (1) Aluminum(III) can coordinate with the important biomolecule GSSG through the following binding sites: glycyl and glutamyl carboxyl groups to form various mononuclear 1:1 (AlLH₄, AlLH₃, AlLH₂, AlLH, AlL, AlLH_-1, AlLH_-2) and several binuclear 2:1 (Al₂LH₄, Al₂LH₂, Al₂L) species (where H₆L²⁺ denotes the totally protonated oxidized glutathione) in acidic aqueous solutions. (2) It indicates that the COO^- groups at low level of preorganization in such small peptide are not sufficient to keep the Al(III) ion in solution and to prevent the precipitation of Al(OH)₃ in the physiological pH range. (3) It also suggests that the occurrence of an Al-linked complexation, the conformation of the peptide GSSG in aqueous solutions appeared to change a little, relative to the initial structure.