13 C NMR study of the ion‐binding selectivity of a new ammonium ionophore

A bicyclic peptide, cyclo ( L ‐Glu 1 — D ‐Leu 2 —Aib 3 — L ‐Lys 4 — D ‐Leu 5 — D ‐Ala 6 )‐cyclo‐(1γ‐4ε) (I), was designed and synthesized to provide an ammonium ion complexation site in a tetrahedral geometry. Molecular modeling, dynamics and electrostatic studies for I indicated that it exhibits some selectivity for ammonium ions over potassium and sodium ions. NMR measurements in CDCl 3 /CD 3 OD (1:1) show that for those carbonyl groups involved in cation binding, 13 C resonances shifted downfield with increasing cation concentration. The resonance that exhibited the largest change in chemical shift between uncomplexed and complexed forms was used to determine the selectivity. Selectivity values obtained were log K = − 2.4 and log K = − 0.6. Copyright © 2008 John Wiley & Sons, Ltd.