Changes in Structural Characteristics of Antioxidative Soy Protein Hydrolysates Resulting from Scavenging of Hydroxyl Radicals

Antioxidant activity of soy protein (SP) and its hydrolyzed peptides has been widely reported. During scavenging of radicals, these antioxidative compounds would be oxidatively modified, but their fate is not understood. The objective of this study was to evaluate the structural characteristics of SP hydrolysates (SPHs), compared to intact SP, when used to neutralize hydroxyl radicals (center dot OH). SPHs with degree of hydrolysis (DH) 1 to 5 were prepared with Alcalase. Antioxidant activity of SPHs was confirmed by lipid oxidation inhibition measured with thiobarbituric acid-reactive substances, ability to scavenge 2,2-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) radicals, and ferrous ion chelation capability. Oxidation of SPHs was initiated by reaction with center dot OH generated from 0.1 mM FeCl3, 20 mM H2O2, and 1.0 mM ascorbate. After oxidative stress, carbonyl content of SPHs increased by 2- to 3-fold and sulfhydryl groups decreased by up to 42% compared to nonoxidized samples (P < 0.05). Methionine, histidine, and lysine residues were significantly reduced as a result of inactivating center dot OH (P < 0.05). Attenuated total reflectance-Fourier transform infrared and circular dichroism spectroscopy suggested the conversion of helical structure to strands and turns. Oxidatively modified SPHs had a lower intrinsic fluorescence intensity but similar solubility when compared to nonoxidized samples. These structural changes due to center dot OH stress may impact the ingredient interaction and functionality of SPHs in food products.