Selective complexation of divalent cations by a cyclic α,β-peptoid hexamer: a spectroscopic and computational study

We describe the qualitative and quantitative analysis of the complexation properties towards cations of a cyclic peptoid hexamer composed of alternating α- and β-peptoid monomers, which bear exclusively chiral (S)-phenylethyl side chains (spe) that have no noticeable chelating properties. The binding of a series of monovalent and divalent cations was assessed by 1H NMR, circular dichroism, fluorescence and molecular modelling. In contrast to previous studies on cations binding by 18-membered α-cyclopeptoid hexamers, the 21-membered cyclopeptoid cP1 did not complex monovalent cations (Na+, K+, Ag+) but showed selectivity for divalent cations (Ca2+, Ba2+, Sr2+ and Mg2+). Hexacoordinated C-3 symmetrical complexes were demonstrated for divalent cations with ionic radii around 1 Å (Ca2+ and Ba2+), while 5-coordination is preferred for divalent cations with larger (Ba2+) or smaller ionic radii (Mg2+)

Effects of Thionation and Fluorination on Cis–Trans Isomerization in Tertiary Amides: An Investigation of <i>N</i>‑Alkylglycine (Peptoid) Rotamers

Peptoids constitute a class of peptidomimetics with potential as protease resistant, biologically active ligands. To harness the full potential of such compounds, however, detailed predictive insight into their propensity to adopt well-defined secondary structures is highly desirable. In this work we present an investigation of the effects of thioamides and/or fluorides in peptoid monomer model systems using chemical synthesis, NMR spectroscopy, and X-ray crystallography. We find that the steric environment surrounding the tertiary amide bonds is the key promoter of conformational preference, and X-ray crystallographic interrogation of our model systems did not suggest the presence of stabilizing <i>n</i> → π* interactions unless the carbonyls were altered electronically by α-halogenation or thioamide formation. In addition to the function as an investigative tool, these two types of modification may thus be utilized as stabilizers of secondary structure in future oligomer designs, such as the <i>cis-</i>amide-based polypeptoid helices that resemble the polyproline type-I helix