Soy glycinin: Influence of pH and ionic strength on solubility and molecular structure at ambient temperatures

This study describes the relationship between the solubility of glycinin, a major soy protein, and its structural properties at a quaternary, tertiary , and secondary folding level under;conditions representative for food prod ucts. When the ionic strength is lowered from 0.5 to 0.2 or 0.03, the basic polypeptides shift more to the exterior of the glycinin complex, as determ ined at pH 7.6 by labeling solvent-exposed lysines, supported by the study of the proteolytic action of clostripain on glycinin. This structural reorg anization caused the pH of minimal solubility to shift to higher values. Ul tracentrifugational analysis shows that at pH 7.6 and an ionic strength of 0.5 glycinin forms hexameric complexes (11S), whereas at pH 3.8 and at an i onic strength of 0.03 glycinin exists as trimers (7S). Intermediate situati ons are obtained by modulation of pH and ionic strength. The observed quate rnary dissociation correlates with an increased amount of nonstructured pro tein at a secondary level and with changes in tertiary folding as determine d using circular dichroism. Tryptophan fluorescence shows no significant st ructural changes for different ionic strengths but demonstrates a more tigh tly packed fluorophore environment when the pH is lowered from 7.6 to 3.8.