Sequence length determinants for self-assembly of amphipathic β-sheet peptides

Amphipathic peptides composed of alternating hydrophobic and hydrophilic amino acids are a privileged class of peptide, which have a high propensity to self-assemble into β-sheet fibrils. The Ac-(FKFE)₂-NH ₂ peptide has been extensively studied and forms putative β-sheet bilayer fibrils in which the hydrophobic Phe side chains are organized to a single face of each constituent sheet; upon bilayer formation, these hydrophobic benzyl groups are sequestered in the hydrophobic core of the resulting fibril. In order for the Phe side chains to be uniformly displayed on one face of Ac-(FKFE)₂-NH₂ β-sheets, an antiparallel packing orientation in which one amino acid residue is unpaired must be adopted. Based on molecular models, we hypothesized that truncated seven amino acid derivatives of Ac-(FKFE)₂-NH₂ in which either the N-terminal Phe residue (Ac-KFEFKFE-NH₂) or the C-terminal Glu residue (Ac-FKFEFKF-NH₂) is eliminated should readily self-assemble into β-sheet bilayers in which all hydrogen bond and hydrophobic/charge interactions are satisfied. We found, however, that the seminute changes in peptide sequence have unanticipated and dramatic effects on the self-assembly of each peptide. Ac-FKFEFKF-NH₂ self-assembled into fibrils with unique morphology relative to the parent peptide, whereas the Ac-KFEFKFE-NH ₂ peptide had a strongly reduced propensity to self-assemble, even failing to self-assemble altogether under some conditions. These findings provide significant insight into the effect of sequence length and strand registry as well as hydrophobicity and charge on the self-assembly of simple amphipathic peptides to illuminate the possibility of tuning self-assembly processes and the resulting structures with minute changes to peptide sequence.