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

Neutral anion receptors based on a short peptide backbone functionalised with both squaramide hydrogen bond donors and triethylene glycol solubilising units were synthesised and their water solubility and anion binding properties were investigated. The appendage of two triethylene glycol 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 of up to 100%. In 50% v/v water/DMSO mixtures, selective binding for sulfate over monovalent anions and up to 5-fold stronger binding to sulfate than selenate were 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.