Highly dynamic motion of crown ethers along oligolysine peptide chains.

Molecular mobility has attracted considerable attention in supramolecular chemistry and biochemistry, but the simple question of whether a small molecule can move directly between different binding sites of a multitopic host without intermediate dissociation has not been addressed so far. To study such processes, we consider hydrogen/deuterium exchange experiments on a model system comprising complexes formed between 18-crown-6 and oligolysine peptides. Because direct binding-site hopping is indistinguishable in solution from a dissociation/reassociation mechanism, here we show that the high vacuum of a mass spectrometer offers a unique environment for probing such processes. The highly dynamic motion of crown ethers along oligolysine peptide chains proceeds mechanistically by a simultaneous transfer of the crown ether from its ammonium ion binding site to a nearby amino group together with a proton. Furthermore, the exchange experiments unambiguously reveal the zwitterionic structure of the 18-crown-6/oligolysine complexes, highlighting the versatility and potential of gas-phase experiments for investigating non-covalent interactions.