Controlling the equilibria between backbone cis- and trans-amides in peptoids, or N-substituted glycine oligomers, constitutes a significant challenge in the construction of discretely folded peptoid structures. Through the analysis of a set of monomeric peptoid model systems, we have developed new and general strategies for controlling peptoid conformation that utilize local noncovalent interactions to regulate backbone amide rotameric equilibria, including n→π*, steric, and hydrogen bonding interactions. The chemical functionalities required to implement these strategies are typically confined to the peptoid side chains, preserve chirality at the side chain N-α-carbon known to engender peptoid structure, and are fully compatible with standard peptoid synthesis techniques. Our examinations of peptoid model systems have also elucidated how solvents affect various side chain-backbone interactions, revealing fundamental aspects of these noncovalent interactions in peptoids that were largely uncharacterized previously. As validation of our monomeric model systems, we extended the scope of this study to include peptoid oligomers and have now demonstrated the importance of local steric and n→π* interactions in dictating the structures of larger, folded peptoids. This new, modular design strategy has guided the construction of peptoids containing 1-naphthylethyl side chains, which we show can be utilized to effectively eliminate trans-amide rotamers from the peptoid backbone, yielding the most conformationally homogeneous class of peptoid structures yet reported in terms of amide rotamerism. Overall, this research has afforded a valuable and expansive set of design tools for the construction of both discretely folded peptoids and structurally-biased peptoid libraries, and should shape our understanding of peptoid folding.

We report that n→π* interactions are operative in peptoids and play a major role in controlling amide isomerism. These interactions can be tuned using α-chiral amide side chains known to promote peptoid folding. To our knowledge, this is the first report of n→π* interactions between amides in non-prolyl systems. Furthermore, we have characterized an n→π* interaction between backbone carbonyls and side chain aromatic rings that can dramatically stabilize the cis-amides required for peptoid helix formation. The tunability of both types of n→π* interactions in peptoids has significant implications for peptoid folding and could be exploited for the design of new peptoid architectures.