Multinuclear magnetic resonance study of equilibria between aluminum(III) and sulfur-containing amino acids in aqueous solutions

Abstract

Aluminum(III) is generally found in small amounts in living organisms where it interacts with several biomolecules. Low mass biomolecules can form complexes associated with absorption and distribution processes of this toxic ion. However, investigating aluminum(III) complexes in solution poses singular difficulties, because it is highly hydrolyzable. Amino acids constitute possible ligands for aluminum(III). Methionine, cysteine and homocysteine are found in healthy human body. Penicillamine is a drug used in several circumstances, such as in treating metal poisoning. These four biomolecules share similar donor atoms involved in complexation reactions: a carboxylate oxygen, an amine nitrogen and sulfur. In this study, four binary complexes formed by aluminum(III) and amino acids in solution at a metal-to-ligand ratio of 1:1 were analyzed through potentiometry and multinuclear magnetic resonance (¹³C and ²⁷Al). Potentiometry data consistently demonstrated complexion formation. Distributions of species revealed the coexistence of several species in ample pH ranges and pointed out the preferable value to perform the NMR analyses. ¹³C NMR data were used to confirm complexion formation, while ²⁷Al NMR data indicated the geometric arrangement adopted by the complexes. Structures were proposed for each complex.

GRAPHICAL ABSTRACT

Abstract

• Amino acids can form complexes related to the absorption and distribution of Al³⁺.

• The techniques used were potentiometry, ¹³C NMR and ²⁷Al NMR in solution.

• The potentiometry reveals distribution of species according to pH of binary systems.

• The ¹³C NMR results indicate complexion formation.

• The ²⁷Al NMR results indicate hexa-coordinated aluminum in the complex species.

Keywords:

• aluminum(III)
• amino acids
• complexes
• ²⁷Al NMR
• ¹³C NMR

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplemental data

Supplemental data for this article can be accessed at 10.1080/17415993.2015.1018909

Additional information

Funding

The study was supported by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) under Grant [141582/2010-3].