Structural and molecular properties of dialdehyde starch

Abstract

Dialdehyde starch (DAS) of degree of oxidation (DO) of 1.5% (DAS1-5), 5% (DAS5), 15% (DAS15) and 25% (DAS25) was prepared from potato starch in the periodate oxidation reaction with electrochemical recovery of the oxidant. Molecular weights ( M w) and radii of gyration ( R g) of polysaccharide chains of native and oxidized starches were measured by means of a high-performance size exclusion chromatography coupled with multiangle laser light scattering and refractive index detectors. The weight average molecular weight ( M w) and radius of gyration ( R g), calculated for the whole complicated peak of the eluate of native starch were 30.0×10 6 and 119.6 nm, respectively. The oxidation induced instant degradation of starch polysaccharide chains. M w value of the chains of the polysaccharide molecules eluted from the sample of DO 1.5% (DAS1-5, 2.2×10 6) was by one order lower than the average molecular weight of native starch molecules (30×10 6). Even more significant decrease in molecular weight up to 6.4×10 4 was observed for the DAS5 sample. The chain length distribution of amylopectin structural units of native and DAS was characterized by the hydrolysis with isoamylase followed by treatment with β-amylase. Chain distribution profiles of completely debranched samples were available from a high-pressure size exclusion chromatography with multiangle laser light scattering and refractometric index detectors. The oxidation slightly changed the average molecular weight ( M w) of the A-type chains although it evoked changes in the mass ratio of the A-type chains. The weight average molecular weight M w and mass ratio of B-type chains significantly differed depending on DO of the sample. X-ray diffraction patterns indicated that crystallinity of DAS1-5 and DAS5 decreased in relation to native starch. Oxidised starches of the highest DO were fully amorphous.