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

A tripodal urea and tripodal thiourea with the same cyclic peptide core have been synthesised and their anion binding ability investigated. In CDCl(3), the tripodal urea self-associates whereas the thiourea does not. Neither compound shows self-association in the more polar 10% v/v DMSO-d(6)/CDCl(3). Both compounds bind strongly and selectively to sulfate ions in CDCl(3) and 10% v/v DMSO-d(6)/CDCl(3). 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.