Adhesion performance of amino groups (epsilon-NH2) of soybean glycinin proteins

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 (lambda(max)) of the protein samples to shift to a higher wavelength. Native glycinin protein lambda(max) was 344.84 nm. Methylated glycinin at 6% substitute degree (SD) had a lambda(max) of 350.92 nm, and at 8% SD of succinylated glycinin lambda(max) was 354.92 nm. The native glycinin has a denaturation temperature (T-p) of 165.7 degrees C and enthalpy (Delta H) of 16.3 J/g of glycinin. The T-p of methylated glycinin was 152 degrees C and Delta H was 13.9 J/g of glycinin at 4% SD. At 6% SD, T-p was 147.4 degrees C and the Delta H was 11.3 J/g of glycinin. The T-p for succinylated glycinin was 165.6 degrees C and the Delta H was -2.5 J/g of glycinin when SD = 3%. At 8% SD, the T-p was 173.6 degrees C and Delta H was -4.8 J/g of glycinin (SD = 8%).