Limited conformational change of β‐lactoglobulin when adsorbed at the air–water interface

Abstract

Detailed insight can be obtained from proteins at and near the air-water interface using external reflection IR and circular dichroism techniques. Besides information on local protein concentrations and surface layer thickness, it is shown that beta-lactoglobulin displays a limited unfolding at the interface. The conformational change is comparable to that observed upon heat-induced aggregation of the protein and can be understood in view of the high surface concentration of the protein (approximately 40% volume fraction). The layer thickness and the conformational properties of the protein do not depend on the bulk concentration. After adsorption of beta-lactoglobulin to a preformed lipid monomolecular layer a similar conformational change is induced, suggesting that the folding properties of the protein itself determine the extent of conformational changes at the interfaces.

Interfacial and foaming properties of sulfydryl-modified bovine β-lactoglobulin

“…The conformational change of β-lg at the air-water interface is very rapid, at a time-scale of (sub)-second [39], but as for some other globular proteins [34,40] is rather limited: molecules of β-lg keep most of their secondary structure after adsorption and during aging of the film [41,42]. Moreover, whatever the protein (native or sulfydryl-modified β-lg), intermolecular interactions start to take place when the protein concentration at the interface was in the range 0.5-0.8 mg m −2 .…”

Emulsion Drops with Complex Interfaces: Globular Versus Flexible Proteins

“…At a bulk concentration of 10 mgÁmL À1 , the thickness was about 30 nm and the concentration in the surface layer still about 15Â above the subphase. Meinders and De Jongh [83] also used infrared reflection absorption spectroscopy (IRRAS) and external reflection circular dichroism to investigate globular proteins at the air/water interface and found a top layer thickness of 8 nm for b-lactoglobulin at various subphase conditions. In contrast to h, the ''pseudo bulk modulus'' E of the layer material is not known and a comparative bulk value cannot be measured easily since Figure 9.…”

“…However, Meinders and De Jongh [83] gave such values for the adsorption layer structure of b-lactoglobulin: they measured the local protein concentration at the solution/water interface using IRRAS and compared the interfacial properties to equivalent bulk conditions of the same protein corresponding to about 40% v/v protein/solvent.…”

The adsorption and unfolding kinetics determines the folding state of proteins at the air–water interface and thereby the equation of state

“…With this method, the spectrum of an infrared (IR) beam is analyzed after specular reflection at the interface. The sensitivity of the amide I region to changes in the secondary structure can be used to gain insight in the protein conformation at a secondary folding level [30][31][32][33][34][35]. More recently also external reflection circular dichroism has been used to assess this information [33].…”

“…More recently also external reflection circular dichroism has been used to assess this information [33]. From a combination of these techniques, several authors conclude that only limited changes in the conformation of proteins occurs upon adsorption at the air-water interface [28,32,34,[36][37][38][39][40]. Maximum changes of up to 10% are observed in the secondary structure, but the globular folding state of the protein is generally found to remain intact [34].…”