Formation and Characterization of Amyloid-like Fibrils from Soy β-Conglycinin and Glycinin

The fibrillar aggregation at pH 2.0 of soy β-conglycinin, glycinin, and the 1:1 mixture thereof, induced by heating at 80 °C for various periods of time, was investigated using Th T and Congo Red spectroscopic techniques. The morphology of the formed fibrillar aggregates was characterized using atomic force microscopy (AFM), whereas the conformational changes and the polypeptide hydrolysis of the proteins upon heating were also evaluated. Th T fluorescence analysis indicated that β-conglycinin had a much higher potential to form heat-induced amyloid-like aggregates than glycinin. AFM analyses showed that all of the soy globulins could form twisted screw-structure fibrils with heights of 1.4-2.2 nm, but the morphology of the amyloid-like fibrils considerably varied among various soy proteins. Significantly lower width at half-heights and higher coil periodicity values were observed for the β-conglycinin fibrils than the glycinin counterpart. Far-UV CD spectral analysis indicated that upon heating, the secondary conformations of the proteins changed considerably, especially during initial heating (e.g., <4 h), and the changes were much more distinct in the β-conglycinin case than in the glycinin case. Furthermore, reducing electrophoresis analyses indicated that progressive polypeptide hydrolysis occurred upon heating, but the polypeptide hydrolysis for the β-conglycinin was much more severe than that of glycinin. The data suggest that soy β-conglycinin exhibited a much higher potential to form thermally fibrillar aggregates than glycinin, and the differences seem to be mainly associated with the differences in their conformational changes and extent of polypeptide hydrolysis by the heating. The results would be of vital importance for the utilization of soy proteins to produce thermally induced fibrillar gels with excellent properties.