NMR studies on the structure of some cyclic and linear antagonists of luteinizing hormone‐releasing hormone (LHRH)

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Abstract
The structure of cyclic antagonists of luteinizing hormone-releasing hormone (LHRH), Ac-D-Phe(p-Cl)1-D-Phe(p-Cl)2-D-Trp3-Ser4-c(Asp5-D-Arg6-Leu7- Lys8)-Pro9- D-Ala10-NH2 (I), Ac-D-Phe(p-Cl)1-D-Phe(p-Cl)2-D-Trp3-Ser4-c(Glu5-D-Arg6-Leu7- Lys8)-Pro9-D-Ala10-NH2 (II) and their linear analogues, Ac-D-Phe(p-Cl)1-D- Phe(p-Cl)2-D-Trp3-Ser4-Asp5-D-Arg6-Leu7-Lys8-Pro9-D-++ +Ala10-NH2 (III) and Ac-D-Phe(p-Cl)1-D-Phe(p-Cl)2-D-Trp3-Ser4-Glu5-D-Arg6-Leu7-++ +Lys8-Pro9-D-Ala10-NH2 (IV), have been studied by NMR spectroscopy. The cyclic peptides I and II are more potent antagonists than the corresponding linear peptides in an in vivo assay. All the peptides show propensity of an unusual type II' beta-turn involving residues 3-6. Cyclic analogues also show some additional structure around residues 7 and 8 which is absent in the linear peptides. This additional structure in the cyclic peptides may be due to a minor conformation with a beta-turn between residues 5 and 8.