Surface shear viscometry as a probe of protein-protein interactions in mixed milk protein films adsorbed at the oil-water interface.
Time-dependent surface viscosities are reported for films adsorbed from binary mixtures of the proteins alpha-lactalbumin, beta-lactoglobulin and beta-casein. The measurements were made at a planar interface between n-tetradecane and various protein solutions (10(-3) wt% of each protein, pH 7, 25 degrees C) using a Couette-type torsion-wire surface viscometer operating at very low shear-rate. Differences in behaviour between simultaneous and sequential exposure of the pairs of proteins to the interface were investigated. Some experiments were performed with chemically modified beta-lactoglobulin samples whose disulphide bonds had been cleaved and blocked. Displacement of one protein by another (e.g. alpha-lactalbumin by beta-casein) is indicated by a sudden drop in surface viscosity immediately after addition of the second protein. In systems containing beta-lactoglobulin, the long-time surface viscosity is very sensitive to the adsorption time of beta-lactoglobulin prior to addition of the second protein. Blocking the disulphide bonds in beta-lactoglobulin leads to a much faster approach to a steady-state surface viscosity. This is interpreted in terms of a much more rapid unfolding of the disordered molecules of modified beta-lactoglobulin at the oil-water interface. We conclude that surface viscosity experiments give useful and sensitive information about competitive adsorption and cooperative interactions in mixed protein films.
Dickinson
E
E
Procter Department of Food Science, University of Leeds, UK.
Rolfe
S E
SE
Dalgleish
D G
DG
D002364:Caseins / Q000737:chemistry*; D004220:Disulfides / Q000737:chemistry; D007768:Lactalbumin / Q000737:chemistry*; D007782:Lactoglobulins / Q000737:chemistry*; D009821:Oils / Q000737:chemistry; D011485:Protein Binding; D013500:Surface Tension; D013997:Time Factors; D014783:Viscosity; D014867:Water / Q000737:chemistry
D002364:Caseins; D004220:Disulfides; D007782:Lactoglobulins; D009821:Oils; D014867:Water; D007768:Lactalbumin