A novel conformation in a highly potent, constrained gonadotropin-releasing hormone antagonist

Through design, synthesis, and biological testing of constrained gonadotropin releasing hormone (GnRH) antagonists, we are studying the structural requirements for biological activity. Here we describe the conformational analysis in solution of a highly potent, dicyclic GnRH antagonist, dicyclo(4-10/5,5'-8)[Ac-D-2Nal¹,D-pClPhe²,D3Pal³,Asp⁴,Glu⁵(Gly),D -Arg⁶,Dbu⁸,Dpr¹⁰]GnRH (1), using NMR spectroscopy. The dicyclic part of this molecule adopts a preferred conformation containing a type II β turn around residues 5-6, nested with a type I' β turn around residues 6-7,and a type II β-turn-like structure involving residue 9 and the side chain of residue 10, which is stabilized by hydrogen bonds between Leu⁷ NH/Asp⁴ CO, Dbu⁸ NH(δ)/Glu⁵ CO, and Dpr¹⁰ NH(γ)/Dbu⁸ 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 another dicyclic(4-10/5-8) GnRH antagonist with very similar sequence, dicyclo(410/5-8)[Ac-D-2Nal¹,D-pClPhe²,D-Trp³,Asp⁴,Glu⁵,D-Asp⁶,Ly s⁸,Dpr¹⁰]GnRH (2) (Bienstock et al. J. Med. Chem. 1993, 36, 3265-3273). The conformation of 2 contains a type II' β turn around residues 6-7, which had been proposed to be essential for GnRH activity. These results are important for our general understanding of polypeptide conformation, since they show that the dicyclo(4-10/5-8) backbone can adopt more than one family of conformations despite its dicyclic nature, and from the point of view of the design of GnRH antagonists, since they suggest that the presence of a turn around residues 6-7, rather than the type of β turn, may be necessary for biological activity.