Unraveling the Link between Molecular Conformation and Morphology and Mechanics of Amyloid Fibrils

Abstract

Nearly all proteins and peptides have the ability to self-assemble into amyloid fibrils when they are denatured. These highly ordered nanofibrils exhibit superior mechanical properties, and are therefore attractive candidates for applications in materials science and food industry. The flipside of the remarkable stability is their accumulation in tissues in the context of conformational diseases.It is thought that the high stability and rigidity of amyloid fibrils is caused by β-sheets, which are stabilized with hydrogen bridges. However, spectroscopic measurements show that amyloids contain not only β-sheets, but also have a pronounced α-helical and random coil content, and morphological measurements show that amyloids are highly polymorphic. The link between molecular conformation and the mesoscopic fibril structure and mechanical rigidity is still not understood. Our strategy to elucidate this link is to measure both the mechanical properties and the molecular structure of amyloid fibrils prepared from the model protein β-lactoglobulin (β-lg).The average β-sheet intensity of amyloid fibrils prepared under different conditions is studied by Fourier transform infrared (FT-IR) and sum-frequency generation (SFG) spectroscopy and varies between about 40 and 90%. Tip-enhanced Raman spectroscopy (TERS) is used to determine with nanometer resolution the structure and amino acid content of single fibrils, which shows that the surface of the fibrils is heterogeneous in terms of its molecular structure. Bending dynamics of single, fluctuating fibrils enable us to correlate the secondary structure of fibrils to their morphology and mechanical properties. Together, our results show that although the fibrils are composed of the same peptides, they are highly polymorphic and their shape and mechanical rigidity is strongly correlated with the underlying molecular structure.