A NOVEL CONFORMATION IN A HIGHLY POTENT, CONSTRAINED GONADOTROPIN-RELEASING-HORMONE ANTAGONIST

Through design, synthesis, and biological testing of constrained gonad otropin releasing hormone (GnRH) antagonists, we are studying the stru ctural requirements for biological activity. Here we describe the conf ormational analysis in solution of a highly potent, dicyclic GnRH anta gonist, 3),Asp(4),Glu(5)(Gly),D-Arg(6),Dbu(8),Dpr(10)]GnRH (i), using NMR spectroscopy. The dicyclic part of this molecule adopts a preferre d conformation containing a type II beta turn around residues 5-6, nes ted with a type I' beta turn around residues 6-7, and a type II beta-t urn-like structure involving residue 9 and the side chain of residue 1 0, which is stabilized by hydrogen bonds between Leu(7) NH/Asp(4) CO, Dbu8 NHdelta/Glu(5) CO, and Dpr(10) NHgamma/Dbu(8) CO. This is a novel conformation that had not been observed previously in any constrained GnRH antagonist and is remarkably different from that found for anoth er dicyclic (4-10/5-8) GnRH antagonist with very similar sequence, -Tr p(3),Asp(4),Glu(5),D-Arg(6),Lys(8),Dpr(10)]GnRH (2) (Bienstock et al. J. Med. Chem. 1993, 36, 3265-3273). The conformation of 2 contains a t ype II' beta turn around residues 6-7, which had been proposed to be e ssential for GnRH activity. These results are important for our genera l understanding of polypeptide conformation, since they show that the dicyclo(4-10/5-8) backbone can adopt more than one family of conformat ions despite its dicyclic nature, and from the point of view of the de sign of GnRH antagonists, since they suggest that the presence of a tu rn around residues 6-7, rather than the type of beta turn, may be nece ssary for biological activity.