Determination of the elastic modulus of β-lactoglobulin amyloid fibrils by measuring the Debye-Waller factor.

Although amyloid fibrils are associated with amyloidoses, they are now being considered as novel biomaterials for industrial use due to their structural stability in the matured state. Therefore, the physical characteristics of these materials need to be clarified prior to their industrial application. In the present study, the mechanical properties of amyloid fibrils precursored by β-lactoglobulin were investigated. Previous studies have examined the stiffness or modulus values of these fibrils using atomic force microscopy. However, the modulus values reported, even for amyloid fibrils from the same precursor proteins, range over three orders of magnitude, from a few MPa to GPa, depending on the experimental methods employed under specific loading conditions. We determined the elastic modulus of amyloid fibrils by measuring spontaneous thermal fluctuations in the material, the Debye-Waller factor. This method does not require any contact between the probe and material or any loading. The vibrational modes of a fibril were considered in order to estimate mechanical parameters. The modulus value determined along the fibril axis for single amyloid fibrils was slightly smaller than those reported in the literature. The smaller modulus value suggests the existence of less ordered proto-fibrils in our specimen, which was confirmed by the AFM images.