Solubility enhancement of isoflavonoids by complexation with acyclic hexadecasaccharides, succinoglycan dimers isolated from Sinorhizobium meliloti

Sinorhizobium meliloti produces succinoglycan, an acidic exopolysaccharide composed of a monomeric octasaccharide repeating unit with acetyl, succinyl, and pyruvyl groups, in both low- and high-molecular-weight forms. Among the low-molecular-weight succinoglycans, dimers were isolated from S. meliloti and purified using various chromatographic techniques. The dimers were classified as four types (D1, D2, D3, and D4) based on the number of succinyl moieties in their structure. The effect of succinoglycan dimers on the aqueous solubility of isoflavonoids, daidzein and genistein was investigated. The solubility of isoflavonoids increased in the presence of succinoglycan dimers, and the complexation between isoflavonoids and succinoglycan dimers was analyzed by UV–Vis (ultraviolet–visible) and NMR (nuclear magnetic resonance) spectroscopy. In the phase solubility study, succinoglycan dimer D3 was shown to have the highest stability constants (4951 M^?1 for daidzein, and 4452 M^?1 for genistein) among the four succinoglycan dimers. The morphological structures of daidzein and genistein with D3 were studied using scanning electron microscopy, and X-ray powder diffractometry. The results showed the natures of the complexes significantly different from the free isoflavonoids. Herein, we suggest that the succinoglycan dimers are able to act as effective complexing agents for the isoflavonoids.