Preparation and NMR structure of the cyclo-beta-tripeptide [beta(3)-HGlu](3) in aqueous solution: A new class of enterobactin-type C-3-symmetrical ligands?

To date, cyclo-beta-tripeptides (twelve-membered-ring trilactams, C) have r esisted structural investigations because of their extreme insolubility. Mo delling and comparison with the corresponding tetramers indicate that the r ings 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 sp ectroscopy. Structural similarities and differences between the trilactam r ing of [beta(3)-HGlu](3) and the corresponding trilactone ring, such as tha t of enterobactin (A), are discussed, and structures ('elongated'; F, G) ar e proposed that should be able to serve as tridentate or trivalent ligands for metal centers.