Effect of disulfide bond cleavage on the structure and conformation of glycinin

The effect of S‐S bond cleavage on the molecular properties of glycinin has been studied by u.v. difference spectroscopy, fluorescence spectroscopy, circular dichroism, viscometry, tryptic hydrolysis, turbidimetry and electrophoresis. U.v. difference spectra, fluorescence spectra, viscometry and tryptic hydrolysis revealed that the overall protein structure becomes more flexible up to a cleavage of about 15 S‐S bonds. However, the cleavage of all the protein S‐S bonds decreased the structural flexibility. U.v. circular dichroic spectral measurements revealed that the increase in the conformational flexibility (up to 15 S‐S bond cleavage) results from a decrease in both secondary (ordered) and tertiary structure of the protein. These measurements also indicated that the decrease in the conformational flexibility of the glycinin with all the S‐S bonds cleaved was caused by protein refolding. Turbidity measurements indicated that high ionic strength (γ/2 = 0.3) increases protein‐protein interactions among the completely S‐S bond cleaved polypeptides. Electrophoretic experiments suggested that the basic polypeptides are responsible for the protein aggregation at high ionic strength. Hydrophobic associations are primarily involved in these protein‐protein interactions.