Liu Jun, Pu Huimin, Chen Chong, Liu Yunpeng, Bai Ruyu, Kan Juan, Jin Changhai
College of Food Science and Engineering, Yangzhou University , Yangzhou, 225127 Jiangsu, China.
Testing Center, Yangzhou University , Yangzhou, 225009 Jiangsu, China.
J Agric Food Chem. 2018 Jan 10;66(1):279-289. doi: 10.1021/acs.jafc.7b05135. Epub 2017 Dec 19.
The ascorbic acid (AA) and hydroxyl peroxide (HO) redox pair induced free radical grafting reaction is a promising approach to conjugate phenolic groups with chitosan (CS). In order to reveal the exact mechanisms of the AA/HO redox pair induced grafting reaction, free radicals generated in the AA/HO redox system were compared with hydroxyl radical (OH) produced in the Fe/HO redox system. Moreover, the structural and physicochemical properties of caffeic acid grafted CS (CA-g-CS) synthesized in these two redox systems were compared. Results showed that only ascorbate radical (Asc) was produced in the AA/HO system. The reaction between Asc and CS produced novel carbon-centered radicals, whereas no new free radicals were detected when OH reacted with CS. Thin layer chromatography, UV-vis, Fourier transform infrared, and nuclear magnetic resonance spectroscopic analyses all confirmed that CA was successfully grafted onto CS through Asc. However, CA could be hardly grafted onto CS via OH. CA-g-CS synthesized through Asc exhibited lower thermal stability and crystallinity than the reaction product obtained through OH. For the first time, our results demonstrated that the synthesis of CA-g-CS in the AA/HO redox system was mediated by Asc rather than OH.
抗坏血酸（AA）与过氧化氢（HO）氧化还原对引发的自由基接枝反应是一种将酚基与壳聚糖（CS）共轭的很有前景的方法。为了揭示AA/HO氧化还原对引发接枝反应的确切机制，将AA/HO氧化还原体系中产生的自由基与Fe/HO氧化还原体系中产生的羟基自由基（OH）进行了比较。此外，还比较了在这两种氧化还原体系中合成的咖啡酸接枝壳聚糖（CA-g-CS）的结构和物理化学性质。结果表明，AA/HO体系中仅产生抗坏血酸自由基（Asc）。Asc与CS之间的反应产生了新的碳中心自由基，而OH与CS反应时未检测到新的自由基。薄层色谱、紫外可见光谱、傅里叶变换红外光谱和核磁共振光谱分析均证实CA通过Asc成功接枝到CS上。然而，CA很难通过OH接枝到CS上。通过Asc合成的CA-g-CS比通过OH获得的反应产物表现出更低的热稳定性和结晶度。我们的结果首次证明，在AA/HO氧化还原体系中CA-g-CS的合成是由Asc而非OH介导的。
