Axinastatin 1 or malaysiatin? proof of constitution and 3D structure in solution of a cyclic heptapeptide with cytostatic properties

Recently, a cyclic heptapeptide with cytostatic activity was isolated from marine sources by three different groups independently. The sequence of the isolated peptide was ambiguous, since two different isomers have been proposed: cyclo (‐Asn 1 ‐Pro 2 ‐Phe 3 ‐Val 4 ‐Val 5 ‐Pro 6 ‐Val 7 ‐) ( 1 ) also called axinastatin 1 resp. pseudoaxinellin and cyclo (‐Asn 1# ‐Pro 2# ‐Pro 3# ‐Phe 4# ‐Val 5# ‐Val 6# ‐Val 7# ‐) ( 2 ) called malaysiatin. We synthesized both peptides 1 and 2 and compared their optical rotation, FAB‐MS, 1 H‐ and 13 C‐NMR data with those of the native compounds. Our results prove that peptide 1 has been assigned correctly, whereas the data of 2 differ significantly from those of the natural peptide. Peptide 1 adopts two conformations (90:10 ratio) in DMSO, interconverting slowly on the NMR time scale. According to MD simulations, using NOEs and J couplings as experimental restraints, a βVIa‐turn with a cis peptide bond between Val 5 ‐Pro 6 and a βI‐turn in the Asn 1 ‐Val 4 region are the characteristic secondary structural elements of the major conformer. Its backbone conformation is very similar to the X‐ray structure of a related peptide cyclo (‐Asn a ‐Leu b ‐Ser c ‐Phe d ‐Leu e ‐Pro f ‐Val g ‐) called evolidine .