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

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 H^γ ones) present remarkably upfield shifted resonances, because of ring current effects induced by the naphthyl moiety. The upfield shifted resonances of the Ilys⁸ H^γ 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 H^γ 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.