Self-Assembly of a Designed Alternating Arginine/Phenylalanine Oligopeptide

A model octapeptide peptide consisting of an alternating sequence of arginine (Arg) and phenylalanine (Phe) residues, namely, [Arg-Phe]4, was prepared, and its self-assembly in solution studied. The simple alternating [Arg-Phe]4 peptide sequence allows for unique insights into the aggregation process and the structure of the self-assembled motifs. Fluorescence and UV-vis assays were used to determine critical aggregation concentrations, corresponding to the formation of oligomeric species and β-sheet rich structures organized into both spheroidal aggregates and highly ordered fibrils. Electron and atomic force microscopy images show globular aggregates and long unbranched fibers with diameters ranging from ∼4 nm up to ∼40 nm. Infrared and circular dichroism spectroscopy show the formation of β-sheet structures. X-ray diffraction on oriented stalks show that the peptide fibers have an internal lamellar structure, with an orthorhombic unit cell with parameters a ∼ 27.6 Å, b ∼ 9.7 Å, and c ∼ 9.6 Å. In situ small-angle X-ray scattering (SAXS) shows the presence of low molecular weight oligomers in equilibrium with mature fibers which are likely made up from 5 or 6 intertwined protofilaments. Finally, weak gel solutions are probed under gentle shear, suggesting the ability of these arginine-rich fibers to form networks.