NMR conformational analysis of antide, a potent antagonist of the gonadotropin releasing hormone (GnRH)

Antide is a decapeptide [(N-Ac-D-Nal1-D-Cpa2-D-Pal3-Ser 4-Lys(Nic)5-D-Lys(Nic)6-Leu7 -Ilys8-Pro9-D-Ala10-NH2] that acts in vivo as an antagonist of GnRH (gonadotropin-releasing hormone). The conformational behavior of antide has been studied in water, TFE, DMF, and DMSO solutions by means of 2D-NMR spectroscopy and molecular dynamics calculations. Antide adopts in aqueous solution a δ-shaped backbone conformation, which is characterized by an irregular turn around residues D-Pal3-Ser4 and by the close spatial proximity of the side chains belonging to D-Nal1 and Ilys8 (as many as 17 NOE peaks were detected between these side chains). The side-chain protons of Ilys8 (especially the Hγ ones) present remarkably upfield shifted resonances, because of ring current effects induced by the naphthyl moiety. The upfield shifted resonances of the Ilys8 Hγ hydrogen atoms are strictly characteristic of the water δ-shaped conformation and can be considered as structure markers. The observation of ring current shifted Ilys8 Hγ resonances under different conditions (temperature, pH, solvent) indicates a remarkable stability of the water δ-shaped conformation. Such a conformation is at least partially disrupted in solvent mixtures containing high percentages of organic solvents. TFE can induce a well-defined conformation, which is characterized by an S-shaped backbone conformation. In DMF and DMSO solution, the molecule is basically endowed with a random coil conformation and high fluxionality. Antide fulfills the conformational requirements that are known to play a crucial role in receptor recognition, namely (i) the presence of a turn in the backbone and (ii) the all-trans nature of peptide bonds. In addition, the structural rigidity of antide likely adds a further contribution to the receptor binding affinity.