Conformational Characteristics Affect Emulsifying And Interfacial Properties Of Globulins

Many plant proteins show excellent emulsifying properties, however, there are fewknowledge about the molecular mechanism on the emulsifying and interfacial properties ofmajor polymeric storage (7S/11S globulins). Emulsifying properties of these proteins notonly depend on their characteristics of tertiary and quaternary conformation but also relate totheir interfacial properties. Knowing the relationship between flexibility of tertiary andquaternary conformation and emulsifying and interfacial properties is very important todevelop protein products with high emulsifying properties. In this thesis, BSA was chosen asa model monomeric globular protein to investigate the importance of the flexibility of tertiaryconformation on its emulsifying and interfacial properties. Three types of legume vicilins withdifferent flexibility of quaternary conformation. The main results are as follows:Flexibility of proteins can be controlled by the concentration of2-ME and protein. Withthis method, the effects of tertiary conformation on the emulsifying properties of BSA wassymmetrically studied. The result showed that the conformation of BSA molecules dependson the concentration (0.001%-1.0%). As for BSA molecules, the cleavage of disulfide bondscan make the conformation more unfolded below a critical concentration; but above thecritical concentration, the unfolded molecules will rearrange to form a new tertiaryconformation. Such a change will lead to corresponding changes of emulsion stability. Ingeneral, with increase of cleavage disulfide bonds, the activity and stability of BSA emulsionwas improved at the concentration of0.25%; while the contrary happened when theconcentration is0.5%-0.75%. Therefore, the flexibility of BSA molecules has great effects onthe properties of emulsions.With pendent drop method, we further studied the change of BSA conformation on theadsorption of BSA molecules on the oil-water interface and dilatational properties ofinterfacial films. The result showed that at low concentrations (0.01%-0.5%), interfacialpressure increase with the cleavage of disulfide bonds; the contrary happens at higherconcentration (1%). The adsorption of protein at oil-water interface can, to some extent, reflect the conformation of protein. When the concentration was less than0.5%, the cleavageof disulfide bond can make BSA molecules form more viscoelastic interface film; the reverseoccurs above the concentration of0.5%. So, the interfacial properties of BSA molecules isclosely related to the emulsifying properties.Then three vicilins with significantly different conformations were chosen to study theemulsifying and interfacial properties at different concentrations (0.25%-2.5%). It showedthat the increase of concentration can increase emulsifying activity, while decrease thestability of emulsion formed by kidney bean and red bean; as for mung bean, the stability ofthe emulsion was worst. It seems that the quaternary conformation greatly influences theemulsifying and interfacial properties.All the results showed that the emulsifying properties of globulins are closely related tointerfacial properties, and both of them depend on the flexibility of tertiary and quaternaryconformation below a critical concentration. As for monomer protein, flexible conformationfavors the formation of more viscoelastic films, thus leading to more stable emulsions.However, for mutipolymer proteins, the more flexible the quaternary conformation is, themore stable the emulsion is.