Dedicated to Ernst B q e r on the occasion of his 70th birthday (24.11.97) 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- 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 1, turn was found about Asn' and Pro'. We compared these structures with conformations of cyclic heptapeptides obtained by X-ray and NMR studies. A P-bulge motif with two 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 fruns and one cis amide bond resulting in a PI/PVI(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.
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