Transmembrane Fluoride Transport by a Cyclic Azapeptide With Two β-Turns

Diverse classes of anion transporters have been developed, most of which focus on the transmembrane chloride transport due to its significance in living systems. Fluoride transport has, to some extent, been overlooked despite the importance of fluoride channels in bacterial survival. Here, we report the design and synthesis of a cyclic azapeptide (a peptide-based N-amidothiourea, 1), as a transporter for fluoride transportation through a confined cavity that encapsulates fluoride, together with acyclic control compounds, the analogs 2 and 3. Cyclic receptor 1 exhibits more stable β-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 found operative, whereas no transmembrane transport activity was found 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.

Diversity-Oriented Synthesis of Cyclic Azapeptides by A3 -Macrocyclization Provides High-Affinity CD36-Modulating Peptidomimetics

Macrocyclization has enabled the use of peptides in drug discovery creating a need for methods to synthesize diverse peptide macrocycles. Azapeptides have advanced to clinically used drugs, however, few cyclic azapeptides have been studied. A multiple component "A³ -macrocyclization" strategy is described for the preparation of diverse cyclic azapeptides and is demonstrated by the synthesis of 15 growth hormone releasing hormone-6 (GHRP-6) analogs. Certain cyclic aza-GHRP-6 analogs exhibited unprecedented affinity for the CD36 receptor, and capacity to modulate Toll-like receptor agonist-induced overproduction of nitric oxide, and reduce pro-inflammatory cytokine and chemokine production in macrophages.