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\uf020\uf062-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. \u3b2-conglycinin, (molecular mass 48180-200 kDa) is a heterogeneous trimeric glycoprotein, composed by three subunits, \uf061, \uf061\u2019, and \u3b2 with an estimated molecular weight of 67, 71, and 50 kDa, respectively [4-5]. \uf062-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 \uf062-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 \uf062-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 \uf062-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

Denaturation of soy proteins in solution and at the oilewater interface: a fluorescence study

Structural changes ensuing from denaturation of soy proteins in solution or occurring at the oil-water interface were studied by fluorescence spectroscopy. Studies were carried out on solutions and emulsions stabilized with \u3b2-conglycinin or glycinin. Tryptophan fluorescence spectroscopy was used to evaluate tertiary structural changes. The binding of fluorescent dyes and the accessibility of reactive cysteine thiols were also used to better identify structural changes of these proteins in solution. Protein conformational changes after interaction with the hydrophobic oil surface were compared with those ensuing from physical (temperature) or chemical denaturation (chaotropes). Results from solution denaturation experiments indicate that structural changes of \u3b2-conglycinin by both temperature and chaotropes are reversible under appropriate conditions, and result in a rearrangement of the supramacromolecular assembly of the protein structure. On the other hand, glycinin treated under the same conditions undergoes irreversible denaturation in solution at temperatures well below 90\ub0C. Both proteins undergo partial denaturation after adsorption on the lipid surface, and no further denaturation occurs upon heating of the emulsions prepared with either protein

Structural changes of soy proteins at the oil\u2013water interface studied by fluorescence spectroscopy

Fluorescence spectroscopy was used to acquire information on the structural changes of proteins at the oil/water interface in emulsions prepared by using soy protein isolate, glycinin, and beta-conglycinin rich fractions. Spectral changes occurring from differences in the exposure of tryptophan residues to the solvent were evaluated with respect to spectra of native, urea-denatured, and heat treated proteins. The fluorescence emission maxima of the emulsions showed a red shift with respect to those of native proteins, indicating that the tryptophan residues moved toward a more hydrophilic environment after adsorption at the interface. The heat-induced irreversible transitions were investigated using microcalorimetry. Fluorescence spectroscopy studies indicated that while the protein in solution underwent irreversible structural changes with heating at 75 and 95 degrees C for 15 min, the interface-adsorbed proteins showed very little temperature-induced rearrangements. The smallest structural changes were observed in soy protein isolate, probably because of the higher extent of protein-protein interactions in this material, as compared to the beta-conglycinin and to the glycinin fractions. This work brings new evidence of structural changes of soy proteins upon adsorption at the oil water interface, and provides some insights on the possible protein exchange events that may occur between adsorbed and unadsorbed proteins in the presence of oil droplets

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 nutritional and physicochemical proprieties. Glicinin and\uf020\uf062-conglycinin are the most abundant soybean storage proteins. Glycinin (11S) is an heterohexamer with two symmetric trimers stacked on top of one another while \u3b2-conglycinin (7S) is a heterogeneous trimeric glycoprotein, composed by three subunits. 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. 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 \uf062-conglycinin or glycinin, and on soy protein isolate (SPI) 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 show that denaturation of \uf062-conglycinin solutions by both heat and chaotropes is reversible under appropriate conditions, leading to 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. \uf062-conglycinin underwent 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