A Designed Receptor for pH-Switchable Ion Binding in Water

Abstract

Molecules with conditional (switchable) properties are of considerable fundamental interest and are potentially useful for a broad range of applications, including chemical sensing. We have prepared a novel receptor, derived from the peracylation of cyclohexane 1,3,5-trimethanol with tyrosine, that suggests that the phenol-amine hydrogen bond may be an effective structural tool in the preparation of molecules with pH-switchable conformations. This conclusion is based on several key observations. First, the proton longitudinal relaxation rates for this molecule change in a pH-dependent fashion, while those for closely related structures do not. Second, NOESY spectra for the receptor change markedly depending on pH, with the spectrum acquired at pH 9.5 displaying NOEs consistent with the calculated "closed" conformation. Third, this molecule serves as a receptor for anions and cations in aqueous solution at high pH, but not at low pH, as demonstrated by UV-vis titrations and isothermal titration calorimetry (ITC).