The complexation of reduced glutathione (GSH) in its free and Al(III)-bound species in acidic aqueous solutions was characterized by means of multi-analytical techniques: pH-potentiometry, multinuclear ((1)H, (13)C and (27)Al) and two-dimensional nuclear Overhauser enhancement NMR spectroscopy ((1)H, (1)H-NOESY), electrospray mass spectroscopy (ESI-MS), and ab initio electronic structure calculations. The following results were found. In the 25 degrees C 0.1M KCl and 37 degrees C 0.15M NaCl ionic medium systems, Al(3+) coordinates with the important biomolecule GSH through carboxylate groups to form various mononuclear 1:1 (AlHL, AlH(2)L and AlH(-1)L), 1:2 (AlL(2)) complexes, and dinuclear (Al(2)H(5)L(2)) species, where H(4)L(+) denotes totally protonated GSH. Besides the monodentate complexes through carboxylate groups, the amino groups and the peptide bond imino and carbonyl groups may also be involved in binding with Al(3+) in the bidentate and tridentate complexes. The present data reinforce that the glycine carboxylate group of GSH has a higher microscopic complex formation constant than gamma-glutamyl carboxylate. Compared with simple amino acids, the tripeptide GSH displays a greater affinity for the Al(3+) ion and thus may interfere with aluminum's biological role more significantly.