Preparation, characterization and hypoglycemic activity of banana condensed tannin-inulin conjugate

To enhance the hypoglycemic effects of inulin, banana condensed tannins (BCT) were grafted onto inulin via a free radical method to synthesize the novel BCT grafted inulin (BCT-g-inulin) complex. Spectroscopic methods, XRD, TGA, 1 H NMR, GPC and morphology analyses were utilized to characterize the structural properties of the BCT-g-inulin complex, and our results confirmed the conjugation of BCT and inulin. The conjugation possibly occurred between the hydroxyl group attached at the C 6 position of inulin and the C 6 /C 8 position of flavon-3-ol units of BCT. The grafting ratio and grafting efficiency of the BCT-g-inulin complex were 357.54 ± 2.98 g kg −1 complex and 74.57 ± 1.44%, respectively. The data of the antioxidant assays indicated that the BCT-g-inulin complex showed a significantly higher antioxidant activity than native inulin. Also, the grafting reaction remarkably improved the in vitro anti-diabetic activity of inulin. The glucose adsorption capacity and glucose dialysis retardation index of the BCT-g-inulin complex were remarkably higher than those of inulin, while the BCT-g-inulin complex showed much stronger inhibitory effects against α-amylase and α-glucosidase compared with inulin. Notably, the inhibition of both α-amylase and α-glucosidase by the BCT-g-inulin complex occurred through mixed-competitive mode. On the basis of fluorescence spectroscopy, the fluorescence of α-amylase and α-glucosidase could be quenched by the BCT-g-inulin complex through a static quenching mechanism. Hence, the BCT-g-inulin complex might have the potential to be developed as an effective anti-diabetic agent. This is the first systematic study to enhance the anti-diabetic effects of inulin by grafting with condensed tannins in vitro , and the results were meaningful for broadening the application of inulin in the treatment of diabetes mellitus.