Effects of polyphenols from walnut pellicle on the structure and allergenicity of walnut globulin

Abstract

This study investigated the effects of polyphenols from walnut pellicle on the structure and allergenicity of walnut globulin (WG). The allergenicity-guided assay revealed walnut globulin (WG) from walnut defatted powder without pellicle was the most allergenic protein fraction of walnut. Complexes of phenolic extracts from walnut pellicle (PEWP) and WG were prepared in different ratio and pH conditions to further investigate the effect of PEWP on the protein structure and allergenicity of globulin. The covalent interaction between PEWP and WG in pH 7 was lower than pH 11, according to the free amino groups' content and SDS-PAGE analysis. Comprehensive methods characterized the complexes, including UV–Visible, fluorescence, Fourier transform infrared, and circular dichroism spectroscopy, and indicated that combination of polyphenols with protein altered the secondary and tertiary structure of the protein, leading to the unfolding of the protein structure. Meanwhile, the combination with PEWP reduced the IgG-binding capacity of WG in a dose-dependent manner, reaching a minimum of 26.90 ± 4.31% (at pH 7), which was significantly (P < 0.05) lower compared to that at pH 11 (41.79 ± 1.25%). Correlation analysis revealed a significant correlation between IgG-binding capacity and protein structure, as well as the addition concentration of PEWP (r = −0.85, P < 0.01), and pH conditions may indirectly influence the structure and allergenicity of the protein by affecting the interaction between WG and PEWP. The interaction between WG and PEWP was characterized through fluorescence-quenching mechanism analysis, indicating that polyphenols from PEWP mainly interact with WG through hydrogen bond and hydrophobic interaction.