Preparation and NMR Structure of the Cyclo-β-tripeptide [β3-HGlu]3 in Aqueous Solution: A New Class of Enterobactin-TypeC3-Symmetrical Ligands?, Preliminary Communication

To date, cyclo-β-tripeptides (twelve-membered-ring trilactams, C) have resisted structural investigations because of their extreme insolubility. Modelling and comparison with the corresponding tetramers indicate that the rings stack to form tube-like hydrogen-bonded aggregates (D). By exploiting the solubilizing effect of LiCl on peptides in THF, we were able to prepare the water-soluble title compound and determine its structure (E) by NMR spectroscopy. Structural similarities and differences between the trilactam ring of [β3-HGlu]3 and the corresponding trilactone ring, such as that of enterobactin (A), are discussed, and structures (`elongated'; F, G) are proposed that should be able to serve as tridentate or trivalent ligands for metal centers.