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 beta-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 similar to 4 nm up to similar to 40 nm. Infrared and circular dichroism spectroscopy show the formation of beta-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 similar to 27.6 angstrom, b similar to 9.7 angstrom, and c similar to 9.6 angstrom. 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 protofilarnents. Finally, weak gel solutions are probed under gentle shear, suggesting the ability of these arginine-rich fibers to form networks.