The Effect of Enzymatic-TGase Cross-Linking on the Structure of Soy Protein Isolate Based on Multiplex Spectroscopy

The functional properties of soy protein isolate (SPI) after alkaline protease digestion and transglutaminase (TGase) catalyzed polymerization is correlated with its internal molecule structure. The structure of SPI modified by alkaline protease digestion-TGase by using ultraviolet (UV) and Fourier transform infrared (FT-IR) and circular dichroism spectroscopy were characterized. The UV spectroscopy analysis showed that the cross-linking of SPI by alkaline protease digestion-TGase induced the reorganization and polymerization of small polypeptide chains, and the absorption intensity of Tyr and Trp increased, which demonstrated that the side chain structure of the SPI polypeptide chains modified by TGase was changed. The results of FT-IR showed that the absorption peaks of amide I and II segments shifted after TGase cross-linked SPI, and part of beta-sheet secondary structures was transformed into alpha-helix, indicated that the intramolecular hydrogen bonds of the modified SPI were changed. By circular dichroism spectroscopy showed that the content of random coil and alpha-helix secondary structure of SPI modified by TGase increased and the content of beta-sheet structure decreased.