Walnut protein isolates were oxidized by lipid hydroperoxides, which were produced from different levels of linoleic acid oxidation catalysed by lipoxygenase. Effects of hydroperoxides on structural characteristics of walnut protein isolates were studied. The results showed that when 9 mL linoleic acid was added to 100 mL 0.05 mg/mL protein solution, walnut protein solubility decreased to 6.89%. The level of free sulfhydryl group decreased from 2.82 umol SH/g to 1.92 umol SH/g, and the carbonyl level increased from 2.26 nmol/mg to 4.23 nmol/mg. This indicated that the physicochemical properties of isolated walnut protein changed after oxidation. The CD spectrum analysis showed that protein oxidation caused reduction in the ɑ-helix and β-sheet contents, and the contents of β-turn and random-coil increased. With increasing amount of linoleic acid added, the degree of protein oxidation increased and protein surface hydrophobicity decreased from 429.66 to 405.24. Moreover, red shift happened to the highest fluorescent peak, and the intensity of endogenous fluorescence decreased. This suggested that lipid hydroperoxide oxidation could induce aggregation of isolated walnut protein, thus leading to further changes in its secondary and tertiary structures. Furthermore, oxidation caused the contents of small molecule polypeptides decreasing, and the levels of protein aggregates gradually increased in the molecular weight distribution of walnut protein. The results indicated that the structure of walnut protein changed by lipid hydroperoxide oxidation, which caused the formation of oxidative aggregates of walnut protein.
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