Evolution of the amyloid fiber over multiple length scales.

The amyloid is a natural self-assembled peptide material comparable in specific stiffness to spider silk and steel. Throughout the literature there are many studies of the nanometer-sized amyloid fibril; however, peptide mixtures are capable of self-assembling beyond the nanometer scale into micrometer-sized fibers. Here, atomic force microscopy (AFM) and scanning electron microscopy (SEM) are used to observe the self-assembly of the peptide mixtures in solution for 20 days and the fibers upon drying. Beyond the nanometer scale, self-assembling fibers differentiate into two morphologies, cylindrical or rectangular cross-section, depending on peptide properties. Microscopic observations delineate a four stage self-assembly mechanism: (1) protofibril (2-4 nm high and 15-30 nm wide) formation; (2) protofibril aggregation into fibrils 6-10 nm high and 60-120 nm wide; (3) fibril aggregation into large fibrils and morphological differentiation where large fibrils begin to resemble the final fiber morphology of cylinders (WG peptides) or tapes (Gd:My peptides). WG large fibrils are 50 nm high and 480 nm wide and Gd:My large fibrils are 10 nm high and 150 nm wide; (4) micrometer-sized fiber formation upon drying at 480 h resulting in 18.0 μm diameter cylindrical fibers (WG peptides) and 14.0 μm wide and 6.0 μm thick flat tapes (Gd:My peptides). Evolution of the large fiber morphology can be rationalized on the basis of the peptide properties.