CONFORMATION OF CYCLIC HEPTAPEPTIDES - CONFORMATIONAL-ANALYSIS OF SEGETALIN-D AND SEGETALIN-E BY DISTANCE GEOMETRY CALCULATIONS

Three-dimensional structures in dimethyl sulfoxide-d(6) of two cyclic heptapeptides, segetalin D(1): cycle (-Gly-Leu-Ser-Phe-Ala-Phe-Pro-) a nd segetalin E (2): cyclo (-Gly-Tyr-Val-Pro-Leu-Trp-Pro-), which have been isolated from the seeds of Vaccaria segetalis, were elucidated by computational and NMR methods. Distance geometry calculations using n uclear Overhauser effect (NOE) constraints resulted in uniquely determ ined backbone conformations of segetalins D and E: each had two beta-t urns, a beta II turn at Pro(7)-Gly(1) and a beta I turn at Phe(4)-Ala( 5) for segetalin D, and a beta II turn at Pro(7)-Gly(1) and a beta VI turn at Val(3)-Pro(4) for segetalin E, respectively. In addition, each had three intramolecular hydrogen bonds, which constructed a classica l P-bulge conformation, as suggested by calculations and NMR studies.