Interfacial properties of protein nanofibrils with different morphology prepared using aqueous solvent with ethanol: Part I. preparation and characterization

β-lactoglobulin can self-assemble into nano-fibrils with high specific surface area and hydrophobic groups, resulting in excellent interfacial activity. In this paper, β-lactoglobulin fibrils with conformation from semi-flexible to worm-like were prepared by controlling the polarity of the self-assemble precursor solution. The effects of ethanol on the surface properties, microscopic interaction forces and secondary structure of the proteins in the precursor solution, and the morphology, secondary structure, mechanical properties, and surface properties of the formed fibrils were investigated, using techniques including atomic force microscopy, Fourier transform infrared spectroscopy, circular dichroism spectroscopy and molecular dynamic simulation, etc. It was found that with the increase of ethanol concentration in the aqueous solvent, both the fibril length and diameter became smaller, the content of β-sheet in the fibrils gradually decreased whereas the content of random curl increased, resulting in decrease of the modulus of the fibrils (5.47 ± 0.54 GPa for semi-flexible fibrils and 0.52 ± 0.20 GPa for worm-like fibrils). With increasing ethanol concentration, the polarity of the precursor solution decreased, and both hydrophobic interactions and disulfide bonds of the proteins decreased whereas the electrostatic interactions with solvent were enhanced, which resulted in the increase of particle size of the precursor proteins and thus facilitated the final formation of fibrils with worm-like morphology. However the fibrillation process mainly occurs in the heating stage but not in the precursor solution. The surface hydrophobicity of the formed fibrils increased, whereas the amount of free sulfhydryl groups and surface charge decreased slightly with the increase of ethanol concentration. Their effects on the interfacial functional properties will be investigated further.