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(‐Asn¹‐Pro²‐Phe³‐Val⁴‐Val⁵‐Pro⁶‐Val⁷‐) (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, ¹H‐ and ¹³C‐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 Val⁵‐Pro⁶ and a βI‐turn in the Asn¹‐Val⁴ 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.