Obtention and characterization of glycosylated derivatives of low molecular weight chitosan through amide formation

Abstract

Chitosans are cationic biopolymers composed of ß (1-4) linked residues of D-glucosamine and N-acetyl-D-glucosamine which have been described to have broad applications in medicine, cosmetics, foods and agriculture. However, their use is limited since they are only soluble in acidic medium (pH <6.5). The introduction of hydrophilic residues chemically bound to reactive amino groups of chitosan has been shown to improve the solubility as well as functional properties of the polymer and many studies have been conducted in this sense. Due to their high hydrophilicity and specificity, carbohydrates have been used for chitosan modifications. Glycosylation of chitosan with lactobionic acid (4-O-ß-D-galactopyranosyl-D-gluconic acid, LA) through amide formation give rise to branched derivatives with modified characteristics. Different functional properties have been reported for LA such as pH-reducing and strong mineral-complexing effect which makes it suitable for diverse applications in food industry. Moreover, it has been described to be resistant to digestive enzymes being fermented by the intestinal flora, which makes it a potential prebiotic compound. The objective of this work was to synthesize different low molecular weight chitosan derivatives introducing LA chains in order to study their influence in the functional properties of the final products. LA was covalently coupled to low molecular weight chitosan (LMWC) using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxysuccinimide (NHS) as cross linker agents. For the determination of the molecular weight of chitosans and their derivatives, size exclusion chromatography (SEC) was used. Two TSKGel series columns (G2500PWXL × G5000PWXL) were combined and coupled to a RID-10A Shimadzu refractive index (RI) detector. Analyses were performed at 25 ºC in a solution of 0.5M AcOH-0.2M AcONa as mobile phase with a flow rate of 0.8 mL/min. Commercial pullulan samples of different molecular weights were used for the calibration curve. The degree of acetylation was determined by FT-IR and potentiometric titration. Functional properties such as solubility, fat binding capacity (FBC) and water binding capacity (WBC) of the samples were also evaluated. Changes in molecular weight and degree of acetylation of the samples were observed after the reaction, suggesting the development of new derivatives. Moreover, when comparing FTIR spectra of these compounds with those of native chitosan and LA, differences were observed, confirming the successfully introduction of LA into chitosan chains. These derivatives presented different degrees of substitution (11-24%). Functional properties of glycosylated LMWC improved with respect to those of native chitosan.