SOY PROTEINS AT OIL-WATER INTERFACE : A FLUORESCENCE STUDY

Soy proteins are one of the most attractive plant food proteins for human and animal nutrition for their good nutritional (they exhibit hypocholesterolemic effect and prevention of cardiovascular diseases) and physicochemical proprieties (such as gel-forming and emulsifying abilities) [1-3]. Glicinin (11S) and-conglycinin (7S) constitute the 65-80% of the total amount of soybean proteins, and they are present in different ratios depending on the cultivar and growing condition [3]. Glycinin is a a heterohexamer with two symmetric trimers stacked on top of one another, with a molecular mass of approximately 300-380 kDa. β-conglycinin, (molecular mass ≈180-200 kDa) is a heterogeneous trimeric glycoprotein, composed by three subunits, , ’, and β with an estimated molecular weight of 67, 71, and 50 kDa, respectively [4-5]. -conglycinin can also form supramolecular aggregates as function of pH and ionic strength [6]. Soy proteins readily adsorb at the interface of an oil water emulsion with homogenization, but very little is yet understood on the details of the structural changes at the interface [7]. The aim of this work is to study the structural changes of soy proteins in solution and compare it to those at the oil-water interface, with focus on heat-induced changes. Fluorescence spectroscopy was applied on solutions and emulsions containing -conglycinin or glycinin in isolation, as well as soy protein isolate (SPI). Intrinsic fluorescence spectroscopy was used to evaluate tertiary structural changes, along with the binding of fluorescent dyes (ANS), and accessibility of reactive cysteine thiols. Protein conformational changes after interaction with the hydrophobic oil surface were compared with those ensuing from physical (heat) or chemical denaturation (by added chaotropes). Results from solution denaturation experiments indicate that denaturation of -conglycinin solutions by both heat and chaotropes is reversible under appropriate conditions, and results in a rearrangement of the supramacromolecular assembly of the protein structure. On the other hand, glycinin treated under the same conditions underwent irreversible denaturation in solution. Results demonstrated that -conglycinin undergoes partial denaturation after adsorption on the lipid surface. This denaturation is reversible after protein displacement from the interface. Glycinin denaturation at a lipid interface reflected its solution behaviour. Glycinin undergoes a partial denaturation at the surface of hydrophobic droplets, and gave no indication of structural recovery after displacement from the interface.