Conformation of cyclic heptapeptides: Conformational analysis of segetalins D and 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-) and 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 nuclear Overhauser effect (NOE) constraints resulted in uniquely determined backbone conformations of segetalins D and E: each had two beta-turns, 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 classical P-bulge conformation, as suggested by calculations and NMR studies.