Molecular structures of two crystalline forms of the cyclic heptapeptide antibiotic ternatin, cyclo[‐β‐OH‐d‐Leu‐d–Ile–(NMe)Ala (NMe)Leu‐Leu‐(NMe)Ala‐d–(NMe)Ala‐]

The crystal structures of two solvated forms of ternatin, cyclo[‐β‐OH‐d‐Leu‐d‐Ile‐(NMe)Ala‐(NMe)Leu‐Leu‐(NMe)Ala‐d‐(NMe)Ala‐] are reported. The first crystallizes with two molecules of peptide and one of dioxane in the asymmetric unit: P2₁2₁2₁, a= 11.563(1), b= 21.863(2), c= 36.330(4) Å. The second crystallizes with two molecules of peptide and one of water in the asymmetric unit: P2₁2₁2₁, a= 14.067(2), b= 16.695(1), c= 36.824(6) Å. N‐Methylation of four of the seven residues of ternatin appears to reduce the number of low‐energy conformations the molecule can assume. The same H‐bonded macrocyclic ring conformation is adopted by the backbone of each of the four molecules observed here. All the amino‐acid side chains, with the exception of d‐Ile², have similar orientations in each of the four conformers. The heptapeptide macrocycle is characterized by: (i) a cis peptide between (NMe)Ala³ and (NMe)Leu⁴, (ii) a type II β‐bend, involving residues Leu⁵‐(NMe)Ala⁶‐d‐(NMe)Ala⁷‐β‐OH‐d‐Leu², stabilized by two H‐bonds, N1′05 and N5′01, between Leu⁵ and β‐OH‐d‐Leu¹ residues, (iii) a third intramolecular H‐bond, observed in each of the four molecules, between the hydroxyl group of β‐OH‐d‐Leu¹ and the carbonyl oxygen of d‐Ile².