Adjustable twisting periodic pitch of amyloid fibrils

We present compelling experimental evidence supported by theoretical arguments demonstrating that the periodic twisting pitch in protein amyloid fibrils arises from the fine balance between competing electrostatic energy and torsional elastic energy stored along the fibrils contour length. To construct the present picture we have used increasing ionic strengths to progressively screen the electrostatic interactions and observed the corresponding pitch variations in mature beta-lactoglobulin amyloid fibrils using single-molecule atomic force microscopy (AFM). Because the ionic strength is changed after fibrils formation, this does not affect the mechanism by which fibrils grow up to their mature structure. For each individual population of the multi-stranded fibrils family, the pitch is found to increase systematically with the salt content, leading to the gradual untwisting of the fibrils and to the establishment of a controllable pitch up to virtually infinite values.