The peptide alkaloid anachelin:: NMR spectroscopic evidence for β-turn formation in aqueous solution

Anachelin, a complex secondary metabolite isolated from Anabaena cylindrica, possesses an unusual structure combining polyketide, peptide and alkaloid building blocks. This natural product was postulated to act as siderophore with catecholate and salicylate groups for putative iron binding with the three-dimensional solution structure of anachelin remaining unknown. In this preliminary communication, the conformation of anachelin in aqueous solution is investigated. ROESY experiments revealed several interresidual NOEs characteristic of a compact, folded structure. A beta-turn type structure of the central tripeptide sequence L-Thr-D-Ser-L-Ser structure involving D-Ser at position (i+1) and L-Ser at position (i+2) would be in agreement with the observed experimental data. In particular, this conformation of the free ligand would align the putative metal binding groups (structural preorganization) and thus lower the free energy for Fe binding. As it is highly unusual to see such a small tripeptide fragment folding into a compact structure, it appears reasonable to assume that both the cationic tetrahydroquinolinium ring as well as the trihydroxylated epsilon-amino acid contribute to the stabilization of the folded structure.