Cis-Trans Amide Bond Rotamers in β-Peptoids and Peptoids: Evaluation of Stereoelectronic Effects in Backbone and Side Chains

Non-natural peptide analogs have significant potential for the development of new materials and pharmacologically active ligands. One such architecture, the beta-peptoids (N-alkyl-beta-alanines), has found use in a variety of biologically active compounds but has been sparsely studied with respect to folding propensity. Thus, we here report an investigation of the effect of structural variations on the cis-trans amide bond rotamer equilibria in a selection of monomer model systems. In addition to various side chain effects, which correlated well with previous studies of alpha-peptoids, we present the synthesis and investigation of cis-trans isomerism in the first examples of peptoids and beta-peptoids containing thioamide bonds as well as trifluoroacetylated peptoids and beta-peptoids. These systems revealed an increase in the preference for cis-amides as compared to their parent compounds and thus provide novel strategies for affecting the folding of peptoid constructs. By using NMR spectroscopy, X-ray crystallographic analysis, and density functional theory calculations, we present evidence for the presence of thioamide-aromatic interactions through C-sp2-H...S-amide hydrogen bonding, which stabilize certain peptoid conformations.