Análise da associação de aminoácidos e receptores de carboidratos com alguns carboidratos representativos através de RMN e cálculos teóricos

Abstract

Carbohydrates are involved in many biological process and they are recognized by proteins through multiples interactions like as hydrogen bonds, C-H ? ? and van der Waals between the carbohydrate and amino acids residues of the protein. However, from a molecular point of view, this recognition is not yet well understood. Firstly, the stereo electronics effects involved in conformations showed by anomers ? and ? of the carbohydrates D-Glicopyranoside (Glc) and D-Galactopyranoside (Gal) and by D-Sucrose were investigated at M06-2X/6-31++G(d,p) level, both in vacuum and in water with the continuous solvation model IEF-PCM, using the natural bond orbital theory (NBO) and non-covalent interactions (NCI). Orbital interactions revealed how each conformation of mono-saccharides was stabilized and what were the predominant effect observed. Different conformations for D-Sucrose regarding the glycosidic bond were evaluated and the interactions of the type hydrogen bonding were important to explain the relative energy observed. The interaction between a carbohydrate and amino acid was investigated by analyzing the 1H chemical shift variation of the carbohydrate signals in water with the addition of amino acid using the selective excitation technique DPFGSE 1D TOCSY and theoretical calculations at the same level of theory applied in the analysis of carbohydrates. The results indicated that the amino acid L-Tryptophan on the upper face of the carbohydrate was more stable for anomer ? -Glc but was more stable in the lower face for anomer ?-Glc. For D-Galactose, the L-Tryptophan in the lower face was more stable in both anomers. Intermolecular interactions analyzed by NCI surfaces and orbital interactions (NBO) were mainly responsible for the relative stability observed in complexes between carbohydrate and amino acid. Synthetic carbohydrate receptors with recognition units based in metil ester amino acid derivatives of L-Tryptophan and L-Alanine were synthesized and complexes with carbohydrates were investigated by 1H NMR titration and theoretical calculations. 1H NMR analyses showed a moderate affinity between the receptor and the carbohydrate. The analyses of NBO and NCI?s surfaces allowed a good understanding of how the interaction between the receptor and carbohydrate takes place, so that it was possible to describe the intermolecular interactions of the type hydrogen bond, C-H ? ? or van der Waals