100. Cyclic heptapcptides axinastatin 2, 3, and 4: Conformational analysis and evaluation of the biological potential

The conformational analysis of naturally occurring cytostatic cyclic heptapeptides axinastatin 2, 3, and 4 was carried out by two-dimensional NMR spectroscopy in combination with distance-geometry (DG) and moleculardynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D ₆)DMSO. Axinastatin 2 was also investigated in CD ₃OH. In all structures, Pro ² is in the i + 1 position of a βI turn and Pro ⁶ occupies the i + 2 position of a βVIa turn about the cis amide bond between residue 5 and Pro ⁶. In all peptides, a bifurcated H-bond occurs between residue 4 CO and the amide protons of residue 1 and 7. For axinastatin 2 and 3, an Asn Iθ turn was found about Asn ¹ and Pro ². We compared these structures with conformations of cyclic heptapeptides obtained by X-ray and NMR studies. A β-bulge motif with two β turns and one bifurcated H-bond is found as the dominating backbone conformation of cyclic all-L-heptapeptides. Axinastatin 2, 3, and 4 can be characterized by six trans and one cis amide bond resulting in a βI/βVI(a)-turn motif, a conformation found for many cyclic heptapeptides. Detailed biological tests of the synthetic compounds in different human cancer cell lines indicates these axinastatins to be inactive or of low activity.