Conformational changes to deamidated wheat gliadins and β-casein upon adsorption to oil–water emulsion interfaces

The conformation of deamidated gliadins and β-casein in solution and adsorbed at the interface of oil-in-water emulsions was studied using synchrotron radiation circular dichroism (SRCD) and front-face-fluorescence spectroscopy. Deamidation led to partial unfolding of gliadins in solution. The α-helix content of the protein decreased from 35% (in the native form) to 16.3% while the percentage of β-sheet and unordered structure increased upon deamidation. The secondary structure of deamidated gliadins was largely unchanged upon adsorption to both tricaprin/water and hexadecane/water interfaces. In contrast, β-casein adopted a more ordered structure upon adsorption to these two oil/water interfaces, the α-helix content increased from 5.5% (in solution) to 20% and 22.5% respectively after adsorption to tricaprin/water and hexadecane/water interfaces. Both deamidated gliadins and β-casein have distinctive N-terminal hydrophilic and C-terminal hydrophobic domains. Unlike β-casein which contains no cysteine residue, gliadins have a large number of intramolecular disulphide bonds located in the C-terminal hydrophobic domain which constrains the conformational freedom of this protein upon adsorption to oil/water interfaces. The hydrophobicity of the oil phase also has an impact on the conformation of each protein upon adsorption to the oil/water interfaces – systematic trends were observed between oil phase polarity from: i) tryptophan fluorescence emission maxima, and ii) the α-helix content in the adsorbed state. Our results demonstrate that conformational re-arrangement of proteins upon adsorption to emulsion interfaces is dependent not only on the hydrophobicity of the oil phase, but more importantly on the conformational flexibility of the protein.