Polymorphism in bovine serum albumin fibrils: morphology and statistical analysis

We investigate the self-assembly process of the globular protein bovine serum albumin (BSA) into fibrillar structures upon incubating the protein solution at high temperature (90 °C) and in an acidic environment (pH 2) for several days. The investigation is performed by atomic force microscopy (AFM) on the self-assembled fibrillar structures, adsorbed on mica substrates from a solution at different fibrillation time snapshots. A rigorous study of structural morphology reveals a sophisticated hierarchy of the BSA fibrils, where two major classes can be identified: flexible and rigid fibrils, with an order of magnitude of difference in their stiffness. Furthermore, each main class can be divided into two subclasses of thin and thick fibrils according to their average height. It is also shown that all types of flexible ribbon-like fibrils at some stage can wrap and close into nanotubes, that is into a rigid class of fibril. A precise statistical analysis of all the subclasses identified is developed throughout the manuscript. The determination of height and contour length distributions, persistence lengths, and other topological characteristics is carried out by processing the coordinates of BSA fibrils acquired from AFM imaging using in-house developed software. The resulting statistical analysis allows better understanding the fibrillation process and the structural properties of the BSA fibrils.