Preparation, Structure Characterization and Antioxidant Activity of Selenylated Low-Molecular-Mass Pectin

In this study, apple pectin was used as a raw material to prepare selenylated low-molecular-mass pectin by hydrolysis with pectinase followed by ultrasonic-assisted nitric acid-sodium selenite (HNO₃-Na₂SeO₃) treatment. Using pectin, selenylated pectin and low-molecular-mass pectin as the controls, the structure of selenylated low-molecular-mass pectin was determined using size exclusion chromatography with refractive index and multi-angle laser light scattering detection (SEC-RI-MALLS), ion chromatography (IC), Fourier transform infrared (FTIR) spectroscopy, ultraviolet (UV) spectroscopy, thermal gravimetric analysis (TGA), particle size and potential. The selenium content was measured by hydride generation atomic fluorescence spectrometry (HG-AFS), and the galacturonic acid content was detected by the carbazole-sulfuric acid method. The in vitro antioxidant activity and α-glucosidase inhibitory effect were characterized. The results showed that the yield of selenylated low-molecular-mass pectin was (78.07 ± 1.66)%, with selenium content of (148.29 ± 1.97) μg/g, and galacturonic acid content of (68.02 ± 3.21)%. The relative molecular mass was concentrated at 8.905 × 10³. In terms of monosaccharide composition, selenylated low-molecular-mass pectin was the same as pectin, selenylated pectin and low-molecular-mass pectin. They were mainly composed of rhanose, arabinose, galactose, glucose, xylose and galacturonic acid, but there were differences in the molar ratio among monosaccharides. Se=O and C–O–Se bonds were found in the structure by FTIR analysis, suggesting the formation of selenylated low-molecular-mass pectin. An absorption peak was found at around 200 nm in the UV spectrum, which is characteristic of pectin polysaccharides. After enzymatic hydrolysis and selenylation treatment, the thermal stability of pectin decreased, the degree of dispersion became higher and the stability of the system was enhanced. Meanwhile, pectin, selenylated pectin, low-molecular-mass pectin, and selenylated low-molecular-mass pectin had high scavenging effects on hydroxyl and superoxide anion radicals and α-glucosidase inhibitory effect in a dose-dependent manner, and selenylated low-molecular-mass pectin had the strongest effect.