Isolation and Structural Elucidation of DDMP-Conjugated Soyasaponins as Genuine Saponins from Soybean Seeds

The composition and the structures of native “group B saponin” in soybean seeds were reinvestigated. Five kinds of saponins named soyasaponins αg, βg, βa, γg, and γa, according to elution order from HPLC, were isolated and the structures were characterized as 2,3-dihydro-2,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP) attaching through an acetal linkage to the C-22 hydroxyl of the aglycones of soyasaponins V, I, II, III, and IV, respectively, by UV, IR, MS, and NMR. DDMP-conjugated saponins were detected as major saponin constituents by extraction under mild conditions, and soyasaponins I–V were not detected. Therefore it was strongly suggested that these DDMP-conjugated saponins were genuine saponins in the intact soybeans.