Preparação e caracterização de compostos de l-triptofano com La3+, Nd3+ e Eu3+

Abstract

The study and characterization of organic ligands, such as amino acids and peptides, associated with metals, has been receiving considerable attention in the scientific community. This fact can be associated with the structural diversity provided by the formed compounds, especially with amino acids, enabling a wide variety of applications of these compounds. Among the amino acids, it can be highlighted the L-Tryptophan (Trp), besides being an essential amino acid, it has up to four bonding sites, has low toxicity, and is obtained with high purity (above 99% w/w). The Trp can be found naturally in several foods, such as milk, eggs, potatoes, tomatoes and others. Due to the diversity of chemical properties presented by Trp, it becomes an interesting ligand for coordination chemistry, however there are few studies of its association with lanthanides (Ln). In this way, the objective of this work was to prepare and characterize solid compounds of the Trp with some trivalent lanthanides (La3+, Nd3+ e Eu3+). The preparation of the compounds were carried out in aqueous medium by the direct mixing of the soluble salts of Ln with aqueous solution of Trp. The characterization of the obtained compounds was accomplished by means of thermoanalytical techniques (TGA/DTG, TGA/DTA), X-ray diffraction (XRD), absorption spectroscopy in the medium infrared region (FTIR), and the evaluation of radical sequestering activity Was performed with the 1,1-diphenyl-2-picryl-hydrazyl (DPPH) reagent. The thermal characterization of the compounds made it possible to evaluate thermal stability, residue formation and thermal decomposition stages, besides establishing the respective minimum formulas: La2(Trp)3(OH)3.2H2O, Nd2(Trp)2(OH)4.2H2O and Eu4(Trp)3(OH)9.6H2O. The spectroscopic analysis with FTIR allowed to suggest that the coordination of the metallic ions with the Trp occurred by two binding sites of the Trp, being they, the nitrogen of the amino group and the oxygen atoms of the carboxylate ion. XRD analysis indicated that the synthesized compounds have lower crystallinity than Trp. The compounds synthesized had low free radical scavenging activity, this fact may be associated with the use of an aprotic solvent (Dimethyl sulfoxide-DMSO), due to the low solubility in other solvents. The low crystallinity, associated with low solubility, the presence of hydroxyls and the interactions between the carboxylate ions, indicate the formation of bridging bonds between the metal ions.