2.B.56. The Cyclic Azapeptide Fluoride Carrier(CAFC) Family
Zhao et al. 2020 designed and synthesized a cyclic azapeptide (a peptide-based N-amidothiourea) as a transporter for fluoride, through a confined cavity that encapsulates the fluoride ion, together with acyclic control compounds, the analogs 2 and 3. Cyclic receptor 1 exhibits more stable beta-turn structures than the control compounds 2 and 3 and affords a confined cavity containing multiple inner -NH protons that serve as hydrogen bond donors to bind anions. It is noteworthy that the cyclic receptor 1 shows the capacity to selectively transport fluoride across a lipid bilayer on the basis of the osmotic and fluoride ion-selective electrode (ISE) assays, during which an electrogenic anion transport mechanism is operative. No transmembrane transport activity was observed with 2 and 3, despite the fact that 2 and 3 are also able to bind fluoride via the thiourea moieties. These results demonstrate that the encapsulation of an anionic guest within a cyclic host compound is key to enhancing the anion transport activity and selectivity (Zhao et al. 2020).