The effect of alkyl ammonium ionic liquids on thermal denaturation aggregation of β-lactoglobulin

Abstract

Differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), small-angle neutron scattering (SANS) measurements, and scanning and transmission electron microscope (SEM and TEM) of β-lactoglobulin (β-LG) are performed in alkyl (methyl, ethyl, and propyl) ammonium nitrate ionic liquids (RAN-ILs) – water mixtures at 25–100 °C. The addition of these RANs decreased the thermal denaturation temperature of β-LG, and the effect is larger with an increase of the hydrophobic part of the cation of the ILs. In ethyl ammonium nitrate (EAN) and propyl ammonium nitrate (PAN) mixtures, the fractal aggregate structure of β-LG was found after the thermal denaturation occurred. On the other hand, in methyl ammonium nitrate (MAN) mixture, an amorphous aggregate was formed as well as in NaCl and NaNO3 solutions. The difference of the effect between MAN and EAN (and PAN) could arise from the balance of hydrophobic/hydrophilic parts of the cation of RAN. These results are similar to those of alcohol-water mixtures. The nanoheterogeneous structure of the RAN-water mixture which consists of the hydrophobic and hydrophilic parts might affect the aggregation structure of β-LG.