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

The conformation of deamidated gliadins and beta-casein in solution and adsorbed at the interface of oil-in-water emulsions was studied using synchrotron radiation circular dichroism (SRCD) and front-facefluorescence spectroscopy. Deamidation led to partial unfolding of gliadins in solution. The alpha-helix content of the protein decreased from 35% (in the native form) to 16.3% while the percentage of beta-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, beta-casein adopted a more ordered structure upon adsorption to these two oil/water interfaces, the alpha-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 beta-casein have distinctive N-terminal hydrophilic and C-terminal hydrophobic domains. Unlike beta-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 e systematic trends were observed between oil phase polarity from: i) tryptophan fluorescence emission maxima, and ii) the alpha-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. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.