Cyclic heptapeptides 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 molecular-dynamics (MD) calculations in explicit solvents. The synthesized secondary metabolites were examined in (D-6)DMSO. Axinastatin 2 was also investigated in CD3OH. In all structures, Pro(2) is in the i + 1 position of a beta I turn and Pro(6) occupies the i + 2 position of a beta VIa 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 I-g 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 beta-bulge motif with two beta 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 beta I/beta VI(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.