Amyloid-like fibrils formed from intrinsically disordered caseins: physicochemical and nanomechanical properties

Amyloid-like fibrils are studied because of their significance in understanding pathogenesis and creating functional materials. Amyloid-like fibrils have been studied by heating globular proteins at acidic conditions. In the present study, intrinsically disordered alpha-, beta-, and kappa-caseins were studied to form amyloid-like fibrils at pH 2.0 and 90 degrees C. No fibrils were observed for alpha-caseins, and acid hydrolysis was found to be the rate-limiting step of fibrillation of beta- and kappa-caseins. An increase of beta-sheet structure was observed after fibrillation. Nanomechanic analysis of long amyloid-like fibrils using peak-force quantitative nanomechanical atomic force microscopy showed the lowest and highest Young's modulus for beta-casein (2.35 +/- 0.29 GPa) and kappa-casein (4.14 +/- 0.66 GPa), respectively. The dispersion with beta-casein fibrils had a viscosity more than 10 and 5 times higher than those of kappa-casein and beta-lactoglobulin, respectively, at 0.1 s(-1) at comparable concentrations. The current findings may assist not only the understanding of amyloid fibril formation but also the development of novel functional materials from disordered proteins.