Mirror image supramolecular helical tapes formed by the enantiomeric-depsipeptide derivatives of the amyloidogenic peptide amylin(20–29)

Conformation and Morphology of 4-(NH2/OH)-Substituted l/d-Prolyl Polypeptides: Effect of Homo- and Heterochiral Backbones on Formation of β-Structures and Nanofibers

The relative stereochemistry of C2 and C4 in 4-substituted prolyl polypeptides plays an important role in defining the derived conformation in solution. cis-(2S,4S)-Amino/hydroxy-l-prolyl polypeptide (lC-Amp 9/lC-Hyp 9) shows a PPII conformation in phosphate buffer and a β-structure in a relatively hydrophobic solvent, trifluoroethanol (TFE). It is now demonstrated that the homochiral enantiomeric cis-substituted d-prolyl polypeptide (dC-Amp 9/dC-Hyp 9) exhibits mirror image β-structures in TFE. In the case of alternating heterochiral prolyl peptides, it is the trans-substituted [lT(2S,4R)-dT(2R,4S)] n prolyl polypeptide that shows β-structures in TFE, while the cis-substituted [lC(2S,4S)-dC(2R,4R)] n prolyl polypeptide is disordered in both phosphate buffer and TFE. The results highlight the important chirality-specific structural requirements for β-structure formation. The observed conformation in solution (circular dichroism (CD)) is also correlated with the morphology of the self-assemblies (field emission scanning electron microscopy (FESEM)), with the PPII form leading to spherical nanoparticles and β-structures leading to nanofiber formation. The results shed light on the role of relative stereochemistry at C2 and C4 in defining the polyproline peptide conformation in solution and how different conformations drive self-assemblies of peptides toward specific nanostructures.

Polymorphism complexity and handedness inversion in serum albumin amyloid fibrils

Protein-based amyloid fibrils can show a great variety of polymorphic structures within the same protein precursor, although the origins of these structural homologues remain poorly understood. In this work we investigate the fibrillation of bovine serum albumin--a model globular protein--and we follow the polymorphic evolution by a statistical analysis of high-resolution atomic force microscopy images, complemented, at larger length scales, by concepts based on polymer physics formalism. We identify six distinct classes of coexisting amyloid fibrils, including flexible left-handed twisted ribbons, rigid right-handed helical ribbons and nanotubes. We show that the rigid fibrils originate from flexible fibrils through two diverse polymorphic transitions, first, via a single-fibril transformation when the flexible left-handed twisted ribbons turn into the helical left-handed ribbons, to finally evolve into nanotube-like structures, and second, via a double-fibril transformation when two flexible left-handed twisted ribbons wind together resulting in a right-handed twisted ribbon, followed by a rigid right-handed helical ribbon polymorphic conformation. Hence, the change in handedness occurs with an increase in the level of the fibril's structural organization.