Novel Amphi-ionophore in Aqueous Solution:  Cyclohexaalanyl

To find a novel amphi-ionophore which strongly binds a cation or an anion alternatively in aqueous solution, we have investigated the host−guest complexation of a cyclohexapeptide composed of six alanine molecules with either alkali metal ions or halide anions, using ab initio calculations, molecular mechanics, and molecular dynamics simulations. The cyclohexapeptide is indeed found to be a novel amphi-ionophore in the aqueous phase as well as in the gas phase. In the presence of cations (Li +, Na + ), the C O group tends to fold inward to capture a cation, whereas in the presence of anion, the N−H group tends to fold inward to capture an anion (F -, Cl - ). In the aqueous phase, the ions tend to preferentially bind both cyclohexaalanyl and water molecules at the surface of cyclohexaalanyl.