Anion Recognition by Aliphatic Helical Oligoureas

Anion binding properties of neutral helical foldamers consisting of urea type units in their backbone have been investigated. ¹ H NMR titration studies in various organic solvents including DMSO suggest that the interaction between aliphatic oligoureas and anions (CH₃ COO^- , H₂ PO₄^- , Cl^- ) is site-specific, as it largely involves the urea NHs located at the terminal end of the helix (positive pole of the helix), which do not participate to the helical intramolecular hydrogen-bonding network. This mode of binding parallels that found in proteins in which anion-binding sites are frequently found at the N-terminus of an α-helix. ¹ H NMR studies suggest that the helix of oligoureas remains largely folded upon anion binding, even in the presence of a large excess of the anion. This study points to potentially useful applications of oligourea helices for the selective recognition of small guest molecules.