Synthesis, characterization, and antioxidant activity of caffeic-acid-grafted corn starch

The antioxidant molecule of caffeic acid was grafted onto corn starch by an N,N-carbonyl diimidazole-mediated reaction. The structural, crystalline, thermal, and antioxidant properties of caffeic-acid-grafted starch (caffeic acid-g-starch) were investigated. The grafting ratio of caffeic acid-g-starch was determined as 66.1 mg/g by the Folin-Ciocalteu method. The Fourier-transform infrared spectrum of caffeic acid-g-starch exhibited a band at 1720 cm(-1) attributed to the vibration of the esterified carboxyl group, and bands from 1440 to 1633 cm(-1) attributed to the stretching of the aromatic ring. The H-1 nuclear magnetic resonance spectrum of caffeic acid-g-starch showed all the characteristic signals of starch at 3.30-5.47 ppm, and methine protons of caffeic acid moieties at 6.77-7.33 ppm. Caffeic acid-g-starch was piece or virgate in shape with a rough surface as observed by scanning electron microscopy. The X-ray diffraction pattern of caffeic acid-g-starch showed an amorphous character. The small-angle X-ray scattering pattern further indicated the semi-crystalline growth rings in native starch were disrupted during the synthetic processes. Thermogravimetric analysis showed caffeic acid-g-starch had a higher thermal stability than native starch. Moreover, the reducing power and free-radical scavenging activity of native starch were greatly enhanced after grafting with caffeic acid. These results suggest the potential of caffeic acid-g-starch as a novel antioxidant agent.