Peptoid oligomers with alpha-chiral, aromatic side chains: sequence requirements for the formation of stable peptoid helices.

The achiral backbone of oligo-N-substituted glycines or "peptoids" lacks hydrogen-bond donors, effectively preventing formation of the regular, intrachain hydrogen bonds that stabilize peptide alpha-helical structures. Yet, when peptoids are N-substituted with alpha-chiral, aromatic side chains, oligomers with as few as five residues form stable, chiral, polyproline-like helices in either organic or aqueous solution. The adoption of chiral secondary structure in peptoid oligomers is primarily driven by the steric influence of these bulky, chiral side chains. Interestingly, peptoid helices of this class exhibit intense circular dichroism (CD) spectra that closely resemble those of peptide alpha-helices. Here, we have taken advantage of this distinctive spectroscopic signature to investigate sequence-related factors that favor and disfavor stable formation of peptoid helices of this class, through a comparison of more than 30 different heterooligomers with mixed chiral and achiral side chains. For this family of peptoids, we observe that a composition of at least 50% alpha-chiral, aromatic residues is necessary for the formation of stable helical structure in hexameric sequences. Moreover, both CD and 1H-13C HSQC NMR studies reveal that these short peptoid helices are stabilized by the placement of an alpha-chiral, aromatic residue on the carboxy terminus. Additional stabilization can be provided by the presence of an "aromatic face" on the helix, which can be patterned by positioning aromatic residues with three-fold periodicity in the sequence. Extending heterooligomer chain length beyond 12-15 residues minimizes the impact of the placement, but not the percentage, of alpha-chiral aromatic side chains on overall helical stability. In light of these new data, we discuss implications for the design of helical, biomimetic peptoids based on this structural motif.