Physicochemical and functional properties of acidic and basic polypeptides of soy glycinin

The amino acid composition, some selected physicochemical and functional properties of acidic and basic polypeptides of soy glycinin were investigated and compared. Large amount of these polypeptides were obtained by DEAE-Sepharose fast flow column chromatography. Free sulphydryl contents, surface hydrophobicity, solubility and emulsifying activities (at different pH values) were evaluated. Different polypeptides had different patterns of amino acid composition, especially contents of acidic (and basic) and hydrophobic amino acids. The free sulphydryl contents (including total and exposed) and surface hydrophobicity considerably varied with the type of polypeptides. Compared with glycinin, isoelectric point (pI) of individual polypeptides shifted towards a more acidic pH. At a given pH value (e.g. above or below pI), the solubility and emulsifying ability index of these polypeptides were closely related to their relative contents of acidic (and basic) amino acids. The results indicated that glycinin polypeptides with different amino acid character have different physicochemical and functional properties, especially solubility and emulsifying ability.