Understanding the Structure of Proteins at the Oil-Water Interface
Due to their amphiphilicity, proteins are commonly utilised in both the pharmaceutical and food industries to form emulsions. Their adsorption to an emulsion’s oil-water interface results in proteins forming an interfacial layer that works to stabilise emulsions against flocculation through steric and/or electrostatic repulsions. The proteins’ structural properties when adsorbed at the oil-water interface is a key factor in the determination of an emulsion’s stability, as well as various other physicochemical properties. As a result, gaining a better knowledge of the stability and structure of proteins adsorbed at oil-water interfaces with the aid of particle size analysers has been identified by researchers as crucial to improving emulsion-based products.
The limitations of old techniques
While it’s common knowledge that proteins go through a degree of conformational change when being adsorbed, the change’s extent and its impact on the stability of the emulsion is still unclear because of limitations in measuring techniques. It’s important, then, that new approaches be explored to help identify the protein geometric structure and conformation in situ at oil-water interfaces at the molecular level.
Some of the techniques that have been used include:
- Fourier transform infrared spectroscopy (FTIR)
- Circular dichroism spectroscopy (couple with the refractive index matching method)
- Fluorescence spectroscopy
These have all shown that proteins go through a level of conformational change when adsorbed at the oil-water interface level.
There is also a limited molecular knowledge of proteins’ geometric structure in relation to their molecular dimensions (once again, when adsorbed at oil-water interfaces). Some of the methods that have been used include:
- X-ray/neutron reflectivity
- Interfacial rheology
- Small-angle X-ray scattering
- Dynamic light scattering
However, it is difficult to acquire high-res structural information on proteins at oil-water interfaces with these techniques being employed.
The emergence of DPI
Recently, though, the technique of dual polarisation interferometry (DPI) has been employed to characterise surface-adsorbed proteins’ and lipid layers’ geometric dimensions. It’s an optical biosensing technique which analyses thin layers that are adsorbed to a planar surface within an aqueous environment. The instrument used (a dual optical waveguide interferometer) enables measurement of both thickness and density of adsorbed protein monolayers at the same time and in real time.
Studies that have utilised DPI
Even without the acquisition of high-res data on proteins in emulsions’ structure, there have been many studies into the stability of protein-stabilised emulsions for naturally unfolded proteins and globular proteins. There have also been studies into characterising the geometric structure and conformation of various proteins adsorbed at the oil-water interface. Some of these studies utilised DPI to measure geometric dimensions of the adsorbed protein monolayer and then linked this data with analysis of conformation through the process of synchrotron radiation circular dichroism (SRCD), as well as front-face tryptophan fluorescence spectroscopy.