Selective recognition of sulfate ions by tripodal cyclic peptides functionalised with (thio)urea binding sites

Abstract

A tripodal urea and tripodal thiourea with the same cyclic peptide core have been synthesised and their anion binding ability investigated. In CDCl₃, the tripodal urea self-associates whereas the thiourea does not. Neither compound shows self-association in the more polar 10% v/v DMSO-d₆/CDCl₃. Both compounds bind strongly and selectively to sulfate ions in CDCl₃ and 10% v/v DMSO-d₆/CDCl₃. This selectivity is attributed to a unique binding mode for sulfate, in which this tetrahedral anion forms nine hydrogen bonds to the receptors, with three of these coming from the amide protons of the cyclic peptide.