Sequence-defined peptoids, or N-substituted glycines, are an attractive class of bioispired polymer due to their biostability and efficient synthesis. However, the de novo design of folded peptoids with precise three-dimensional structures has been hindered by limited means to deterministically control backbone conformation. Peptoid folds are generally destabilized by the cis/trans backbone-amide isomerization, and few side-chains are capable of enforcing a specific amide conformation. Here, we show that a novel class of cationic alkyl ammonium ethyl side-chains demonstrates significant enforcement of the cis-amide backbone (K cis/trans up to 70) using an unexpected ensemble of weak intramolecular CH−O and/or NH−O hydrogen bonds between the sidechain and the backbone carbonyl moieties. These interactions are evidenced by X-ray crystallography, variable-temperature NMR spectroscopy, and DFT calculations. Moreover, these side-chains are inexpensive, structurally diverse, hydrophilic, and can be integrated into longer peptoid sequences via solid-phase synthesis. Notably, we extended these concepts to synthesize a water-soluble peptoid 10-mer that adopts one predominant fold in solution, as determined by multidimensional NMR spectroscopy. This decamer, to the best of our knowledge, is the longest linear peptoid sequence atomically characterized to retain a well-folded structure. These findings fill a critical gap in peptoid folding and should propel the development of peptoid applications in a broad range of contexts, from pharmaceutical to material sciences.

Conformational control in peptoids, N‐substituted glycines, is crucial for the design and synthesis of biologically‐active compounds and atomically‐defined nanomaterials. While there are a growing number of structural studies in solution, most have been performed with conformationally‐constrained short sequences (e.g., sterically‐hindered sidechains or macrocyclization). Thus, the inherent degree of heterogeneity of unconstrained peptoids in solution remains largely unstudied. Here, we explored the folding landscape of a series of simple peptoid tetramers in aqueous solution by NMR spectroscopy. By incorporating specific 13C‐probes into the backbone using bromoacetic acid‐2‐13C as a submonomer, we developed a new technique for sequential backbone assignment of peptoids based on the 1,n‐Adequate pulse sequence. Unexpectedly, two of the tetramers, containing an N‐(2‐aminoethyl)glycine residue (Nae), had preferred conformations. NMR and molecular dynamics studies on one of the tetramers showed that the preferred conformer (52%) had a trans‐cis‐trans configuration about the three amide bonds. Moreover, >80% of the ensemble contained a cis amide bond at the central amide. The backbone dihedral angles observed fall directly within the expected minima in the peptoid Ramachandran plot. Analysis of this compound against similar peptoid analogs suggests that the commonly used Nae monomer plays a key role in the stabilization of peptoid structure via a side‐chain‐to‐main‐chain interaction. This discovery may offer a simple, synthetically high‐yielding approach to control peptoid structure, and suggests that peptoids have strong intrinsic conformational preferences in solution. These findings should facilitate the predictive design of folded peptoid structures, and accelerate application in areas ranging from drug discovery to biomimetic nanoscience.