Water-soluble squaramide functionalised peptides for sulfate recognition in aqueous media

Abstract

Neutral anion receptors based on a short peptide backbone functionalised with both squaramide hydrogen bond donors and triethyleneglycol solubilising units were synthesised and their water solubility and anion binding properties investigated. The appendage of two triethyleneglycol units per squaramide was demonstrated to provide water solubility of >12 mM, enabling anion binding to be evaluated in DMSO/water mixtures with a water content up to 100%. In 50% water/DMSO mixtures, selective binding for sulfate over monovalent anions and up to 5-fold stronger binding to sulfate than selenate was observed. Interactions between the peptide backbone amide protons and sulfate contribute to the observed selectivity, demonstrating that selective sulfate recognition in highly competitive solvents is achievable with non-preorganised, small molecule receptors.