The Synthesis, Structure And Antioxidant Capacity Of Baishouwu Starch-quercetin Graft Copolymer

Cynanchum auriculatum Royle ex Wight(also called ’Baishouwu’ in Chinese)is a traditional Chinese medicine and healthy food.The dry root of C.auriculatum contains about 40-70%of starch,which is a potential raw material for food industry.Therefore,it is essential to investigate the morphology and structural characterization of C.auriculatum starch,which is helpful to widen the applications of C.auriculatum starch in food industry.In this study,native starch was isolated from the root of C.auriculatum.The granule morphology and structural characterizations of C.auriculatum starch was first investigated by several instrumental methods.Then acid hydrolysis was applied as a useful tool to reveal the inner structure of C.auriculatum starch granule.Finally,a novel method was developed to conjugate quercetin with Cynanchum auriculatum starch.The structural,physicochemical and biological properties of quercetin grafted starch(QU-g-ST)were evaluated.(1)The morphology and structural and physicochemical properties of C.auriculatum starch were investigated.C.auriculatum starch exhibited both spherical and polygonal shapes with granule size ranging from 2 to 12 μm.Some void cavities and serpentine channels were observed in the inner of starch granules.XRD pattern revealed that C.auriculatum starch was a CB type with relative crystallinity of 25.19%.SAXS spectrum indicated C.auriculatum starch had a lamellar repeatdistance of 9.21 nm.The proportions of single helix,double helix and amorphous components in C.auriculatum starch were 3.42%,27.11%and 69.47%,respectively.The amylose content of C.auriculatum starch was 28.0%with the gelatinization temperature ranging from 59.3 to 70.1℃.The maximum weight loss rate of C.auriculatum starch appeared at 309℃.In addition,C.auriculatum starch showed high swelling power.(2)Effect of acid hydrolysis on the morphology and structure of C.auriculatum starch was investigated.The hydrolysis degree of C.auriculatum starch rapidly increased to 63.69%after 4 days and reached 78.67%at the end of 9 days.Morphology observation showed that the starch granules remained intact during the first 4 days of hydrolysis.However,serious erosion phenomenon was observed after 5 days and starch granules completely fell into pieces after 7 days.During acid hydrolysis process,the crystal type of hydrolyzed starch changed from original CB type to final A type.SAXS patterns showed the semi-crystalline growth rings started to be hydrolyzed after 4 days.The proportions of single helix and amorphous components as well as amylose content in starch gradually decreased,whereas the proportion of double helix components continuously increased during acid hydrolysis.(3)Quercetin was first succinylized and then grafted onto C.auriculatum starch through a N,N’-carbonyldiimidazole mediated reaction.The obtained water soluble QU-g-ST was characterized by several instrumental methods.The grafting ratio of QU-g-ST was 95.1 mg/g.UV-vis spectrum of QU-g-ST aqueous solution exhibited two absorption bands at around 300 and 430 nm.FTIR spectrum of QU-g-ST showed characteristic bands at 1731 and 1568 cm-1,assigning to C=O stretching of esterified carboxyl group and C=C stretching of aromatic ring,respectively.Proton NMR spectrum of QU-g-ST showed partial proton signals of starch(3.30-5.47 ppm)and quercetin(6.19-7.68 ppm).All above results suggested quercetin was successfully grafted onto C.auriculatum starch.Besides,QU-g-ST particles were irregular in shape and were in an amorphous state.Notably,the thermal stability,resistant starch content as well as antioxidant activity of C.auriculatum starch was greatly enhanced by grafting with quercetin.Furthermore,QU-g-ST showed no cytotoxicity to macrophage RAW 264.7 and had protective effects on cellular oxidative damage induced by H2O2.