Structural and conformational analysis of hydroxycyclochlorotine and cyclochlorotine, chlorinated cyclic peptides from Penicillium islandicum.

A new chlorinated cyclic pentapeptide, hydroxycyclochlorotine (1), has been isolated from Penicillium islandicum, and the structure including absolute stereochemistry of 1 and conformational properties of 1 and cyclochlorotine (2) in DMSO-d6 were elucidated by using extensive 2D NMR and chemical means. Hydroxycyclochlorotine (1) and astin B (3) from Aster tataricus, each containing an allo threonine at residue 2, have a cis proline configuration, whereas cyclochlorotine (2) has two conformational states in solution, which may be produced from cis-trans isomerization of the proline amide bond. The presence of an intramolecular hydrogen bond between Ser (3)-NH and a hydroxyl oxygen atom of alloThr (2) may serve to maintain the backbone conformation with a cis proline amide bond.