Adhesion Performance of Amino Groups (ε-NH2) of Soybean Glycinin Proteins

Abstract

Amino groups of a protein have been considered as a major factor contributing to adhesion strength and water absorption. Positively charged amino groups (−NH2+) of soybean glycinin protein were replaced with negative charged carboxyl groups (−COO−) by using succinylation modification. The amino groups also were replaced with hydrophobic methyl groups (−CH3) by using methylation modification. Adhesion strength and water resistance were reduced when the −NH2 groups were substituted with −COOH groups at pH 7.6. But adhesive performance at the same pH environment was improved when the −NH2 group was replaced with −CH3, which suggests that hydrophobic groups of protein contribute to adhesion and that hydrophilic groups reduce adhesive strength. Both methylation and succinylation modifications caused the fluorescence reading (λmax) of the protein samples to shift to a higher wavelength. Native glycinin protein λmax was 344.84 nm. Methylated glycinin at 6% substitute degree (SD) had a λmax of 350.92 nm, and at 8% SD of succinylated glycinin λmax was 354.92 nm. The native glycinin has a denaturation temperature (Tp) of 165.7 °C and enthalpy (ΔH) of 16.3 J/g of glycinin. The Tp of methylated glycinin was 152 °C and ΔH was 13.9 J/g of glycinin at 4% SD. At 6% SD, Tp was 147.4 °C and the ΔH was 11.3 J/g of glycinin. The Tp for succinylated glycinin was 165.6 °C and the ΔH was −2.5 J/g of glycinin when SD = 3%. At 8% SD, the Tp was 173.6 °C and ΔH was −4.8 J/g of glycinin (SD = 8%).