Self-Assembled Cyclic Structures from Copper (II)-Peptoid

Metal-ligand coordination is a key interaction in the self-assembly of both biopolymers and synthetic oligomers. Although binding of metal ions to synthetic proteins and peptides is known to yield high-order structures, self-assembly of peptidomimetic molecules upon metal binding is still challenging. Peptide–metal assemblies are typically composed of a peptide with a given sequence containing metal binding residues and a specific metal ion. Among peptidomimetic foldamers, peptoids – N-substituted glycine oligomers – represent an important class of various applications including metal binding, catalysis and medicine. Peptoids were also shown to adopt a range of secondary structures and self assemble into helical bundles and sheets. Moreover, macrocyclization has been widely demonstrated to be an effective strategy for establishing conformational order in peptoid sequences, but up to date it was only achieved via the formation of covalent bonds between pendant and/or backbone groups within the oligomer. Herein we explore the coordination of Cu(II) to peptoid trimers bearing a bipyridine center and a alkoxyl group (R) [where R = OH (hydroxyl) or OCH₃ (methoxyl)] by the metal-directed self-assembly of a short peptoid. X-ray diffraction analysis revealed unique, highly symmetric, cyclic (R =OH, Complex 1) and aqua bridged cyclic (R =OCH₃, Complex 2) bimetallic structures formed by the self-assembly of two peptoid molecules with two Cu(II) ions. Spectroscopic and theoretical calculation data showed that upon crystals solvation in acetonitrile, Complex 1 is deformed such that a different, monometallic, complex exists in this solution, while Complex 2 remains intact also in solution.