Stabilization of the fibrous structure of an α-helix-forming peptide by sequence reversal

The α3-peptide, which comprises three repeats of the sequence Leu-Glu-Thr-Leu-Ala-Lys-Ala and forms an amphipathic α-helix, is unique among various α-helix-forming peptides in that it assembles into fibrous structures that can be observed by transmission electron microscopy. As part of our investigation of the structure-stability relationships of the α3-peptide, we synthesized the r3-peptide, whose amino acid sequence is the reverse of that of the α3-peptide, and we investigated the effects of sequence reversal on α-helix stability and the formation of fibrous structures. Unexpectedly, the r3-peptide formed a more-stable α-helix and longer fibers than did the α3-peptide. The stability of the r3-peptide helix decreased when the ionic strength of the buffer was increased and when the pH of the buffer was adjusted to 2 or 12. These results suggest that the r3-peptide underwent a “magnet-like” oligomerization and that an increase in the charge-distribution inequality may be the driving force for the formation of fibrous structures.