Efficient preparation process and fine structural analysis of polysaccharides from black soybean (Glycine max (L.) Merr.) hull

Dietary polysaccharides, widely present in plants, fungi, and algae, have been utilized as functional foods because of their excellent bioactivity. In this study, four polysaccharides (BSPS) from black soybean hulls, designated as BSPS-N-a, BSPS-N-b, BSPS-A-c, and BSPS-A-d, were efficiently extracted and purified. Their fine structures were subsequently investigated through a combination of Carbon-13 Nuclear Magnetic Resonancea ( 13 C NMR), Matrix-Assisted Laser Desorption/Ionization Time of Flight Mass Spectrometry (MALDI-TOF-MS), and enzymatic hydrolysis. Monosaccharide analysis, Fourier transform infrared spectroscopy (FT-IR) analysis, and 13 C NMR analysis indicated that BSPS-N-a was a galactomannan composed of β-1,4-D-Man p and β-1,4-D-Gal p , BSPS-N-b was a galactoglucomannan composed of β-1,4-D-Gal p , β-1,4-D-Glc p and β-1,4-D-Man p ; Both BSPS-A-c and BSPS-A-d were composed of homogalacturonan (HG) with minor rhamnogalacturonan-I (RG-I) domains composed of α-1,4-D-Gal p A and β-1,4-D-Gal p , the difference was that BSPS-A-c exhibited a higher degree of branching, mainly containing a large amount of α-1,2,4-L-Rha p . To verify the results of 13 C NMR, enzymatic hydrolysis combined with MALDI-TOF-MS analysis was determined. The results showed that BSPS-N-a and BSPS-N-b could be degraded into hexose (Hex) with a degree of polymerization ranging from 2 to 9 by mannanase, and BSPS-A-c and BSPS-A-d were degraded into galacturonic acid (GalA) with a degree of polymerization ranging from 2 to 3 by endo-polygalacturonase. These results were consistent with the above results from monosaccharide analysis, FT-IR analysis, and 13 C NMR analysis. This study will provide a basis for the structure-activity relationships of black soybean hull polysaccharides (BSPS) and the development of BSPS as new functional dietary polysaccharides, natural functional foods and pharmaceuticals.