Structural Characterization, Stability and Hypoglycemic Activity in Vitro of Roselle Anthocyanins and Its Esterified Product

To investigate the structure, stability properties and hypoglycemic activity in vitro of roselle anthocyanins and its esterification product, anthocyanins were modified by esterification with succinic anhydride. The anthocyanins with and without esterification were characterized by UV spectroscopy (UV-Vis), infrared spectroscopy (FTIR), X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The anthocyanins content, color value, color and stability were determined and compared. The inhibitory effects and types of anthocyanins on α-amylase and α-glucosidase before and after esterification were explored. The results showed that after esterification, the maximum ultraviolet absorption wavelength of anthocyanins was blue-shifted, ester carbonyl was increased, the structure became more loosely and the thermal stability decreased. In addition, the content and color value of anthocyanins reduced after esterification, thus the solution color became lighter, but the stability under light, high temperature and different pH conditions of esterified anthocyanins improved. At 37 ℃ and pH6.8 for 15 min, the anthocyanins and its esterification product could effectively inhibit the activities of two enzymes. The two kinds of anthocyanins both demonstrated a mixed-type inhibition on α-amylase and α-glucosidase with IC₅₀ values of 4.507, 3.708 mg/mL and 4.045, 2.950 mg/mL, respectively. Furthermore, the esterification product had higher enzyme inhibitory activities. Anyway, after esterification, the molecular structure, color and its stability properties of roselle anthocyanins were changed, which obviously enhanced its hypoglycemic activity in vitro.