Structural and physicochemical characterization of an aminosugar-rich exopolysaccharide isolated from a Chlorella sp.

Abstract

In the present work an aminosugar-rich exopolysaccharide (EPS) of a novel Chlorella sp. strain was isolated and characterized. Isolation of the EPS matrix was a fast and financially viable procedure based upon consecutive centrifugation and resuspension steps without the use of hazardous chemicals. After acidic hydrolysis, the EPS matrix was examined by Nuclear Magnetic Resonance (NMR) Spectroscopy as well as Electrospray Ionization Tandem Mass Spectrometry (ESI-MS/MS), and it was shown that it is a heteropolysaccharide. Aminosugar moieties, mainly glucosamine and galactosamine, constituted >40 % of the structure's relative molar ratio, followed by rhamnose, arabinose, galactose, xylose and mannose. Differential scanning calorimetry (DSC) analysis showed that the EPS matrix exhibited specific thermal transitions in the range of 59–88 °C corresponding to crystallization steps, while thermogravimetric analysis showed that the polysaccharide presented exceptional thermal stability at temperatures up to 600 °C. Powder X-ray diffraction analysis showed that the polymer exhibited high crystallinity after one DSC cycle. The isolated EPS exhibited a maximum total antioxidant capacity of 11.5 mg ascorbic acid equivalents·g−1 and a reducing power of 11.2 mg ascorbic acid equivalents·g−1, while the ferric reducing antioxidant power of the EPS reached 37.2 μM Fe2+·g−1. In all cases the antioxidant and reducing power of the EPS was dose-dependent. The EPS exhibited a moderate hydrogen peroxide and hydroxyl radical scavenging activity reaching a maximum of approximately 40 % at 2 mg mL−1 concentration. All these structural and physicochemical characteristics, along with the strong antioxidant profile, make this macromolecule a strong candidate for further exploitation in commercial applications in the pharmaceutical, nutraceutical and bioplastic industries.