核桃酚类物质和蛋白间的相互作用及其抗氧化活性

酚类物质和蛋白质之间存在多种相互作用力，因此难以被充分提取。该研究以核桃蛋白为研究对象，用不同量的尿素和氯化钠定向破坏核桃酚类物质和核桃蛋白之间的离子键、氢键和疏水作用力，以研究核桃酚类物质和核桃蛋白之间的非共价键结合机制。结果显示在pH值3.0时，77.7%的酚类物质与蛋白以共价键结合，而有34.6%的缩合单宁与核桃蛋白以共价键结合。在pH值8.0时，有65.4%酚类物质和59.4%的缩合单宁与核桃蛋白以非共价键结合。其中疏水作用力是主要的非共价键结合方式。对以不同非共价键与核桃蛋白相结合的酚类物质的抗氧化活性进行测定，结果显示以离子键与蛋白结合的酚类物质的抗氧化活性最强。其中，在pH值3.0条件下与WPI（walnut protein isolates）以离子键结合的酚类物质的DPPH自由基清除能力为5.7%，Fe2+螯合率为5.9%，在pH值8.0条件下与WPI以离子键结合的酚类物质的DPPH自由基清除能力为10.9%， Fe2+螯合率为11.5%。另外，该研究还探讨了在pH值3.0和8.0时，70%(v/v)乙醇和60%(v/v)甲醇提取核桃蛋白中酚类物质的能力，结果表明这两种提取剂不能显著提高酚类物质的提取率，且不能有效得到具有较强抗氧化能力的酚类物质。

Walnut phenolic compounds: Binding with proteins and antioxidant activities

Abstract: Walnut (Juglans regia L.) is rich in phenolic compounds (POHs) which show profound antioxidant capacity. But the extraction of POHs could not be completed as they usually interact with walnut proteins. In this study, different amount of urea or NaCl were added to walnut protein solutions in order to analyze the interactions between the POHs and proteins. Results showed that 77.7% total phenolic compounds and 34.6% total condensed tannins (at pH 3.0), 65.4% total phenolic compounds and 59.4% condensed tannins (at pH 8.0) bonding with proteins through various non-covalent bonds, among which hydrophobic bonds were predominant. Besides, the antioxidant activities of POHs were investigated. It showed that the POHs bonding with proteins through ionic bonds could scavenge 5.7% DPPH radical and chelate 5.9% Fe2+ at pH 3.0 while scavenge 10.9% DPPH radical and chelate 11.5% Fe2+ at pH 8.0, demonstrating the strongest antioxidant activities. In addition, the efficiency of 70% (v/v) ethanol and 60% (v/v) methanol in extracting POHs and condensed tannins from WPI at pH 3.0 and 8.0 were tested. Results showed the above two commonly used extractants could only slightly promote the extraction of POHs and were not efficient in extracting the POHs with higher antioxidant capacity.