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This study investigated relationship between secondary structure and surface hydrophobicity of soy protein isolate (SPI) subjected to a thermal treatment at 70~90°C. Heat denaturation increased the surface hydrophobicity and surface hydrophobicity decreased as aggregate formed. Heat caused an increase in the relative amount ofα-helix structures and an overall decrease in the amount ofβ-sheet structures when compared with nontreated SPI. The relative amounts of secondary structures varied with time, temperature, and intensity of heat treatment applied. Theβ-sheet structure was most important for its significant role in denaturation of 7S globulin and following formed aggregates and even in denaturation of 11S globulin. The amount ofβ-sheet structure in SPI had an inverse correlation with the surface hydrophobicity when the temperature was kept below 90°C. Besides,β-turn structure increased asβ-7S/B-11S aggregate formated.

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This study aimed to purify and identify antioxidant peptides from the low-molecular-weight fraction (SPH-I, MW < 3 kDa) of Alcalase-hydrolyzed soybean (Glycine max L.) hydrolysate and further evaluate the cytoprotective effects of synthesized peptides against oxidative stress in human intestinal Caco-2 cells. After purification by gel filtration chromatography and reversed-phase HPLC, four major peptides were sequenced by nano-LC-ESI-MS/MS as VVFVDRL (847 Da, SPH-IA), VIYVVDLR (976 Da, SPH-IB), IYVVDLR (877 Da, SPH-IC), and IYVFVR (795 Da, SPH-ID). The antioxidant peptides were synthesized and displayed desirable DPPH radical-scavenging activity (from 16.5 ± 0.5 to 20.3 ± 1.0 μM Trolox equivalent (TE)/μM), ABTS •+ radical-scavenging activity (from 3.42 ± 0.2 to 4.24 ± 0.4 mM TE/μM), ORAC (from 143 ± 2.1 to 171 ± 4.8 μM TE/μM), and FRAP (from 54.7 ± 1.2 to 79.0 ± 0.6 mM Fe 2+ /μM). Moreover, the synthesized peptides protected Caco-2 cells against H 2 O 2 -induced oxidative damage via significantly downregulating intracellular ROS generation and lipid peroxidation (p < 0.05). Additionally, SPH-IC and SPH-ID statistically upregulated total reduced glutathione synthesis, enhanced activities of catalase and glutathione reductase, and suppressed ROS-mediated inflammatory responses via inhibiting interleukin-8 secretion (p < 0.05).

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