Conformational Studies of Two Bradykinin Antagonists by Using Two-dimensional NMR Techniques and Molecular Dynamics Simulations

Abstract

The solution conformations of two potent antagonists of bradykinin (Arg¹-Pro²-Pro³-Gly⁴- Phe⁵-Ser⁶-Pro⁷-Phe⁸-Arg⁹), Aca-¹, DArg⁰, Hyp³, Thi⁵, DPhe⁷,(N-Bzl)Gly⁸]BK (1) and Aaa- ¹, DArg⁰, Hyp³, Thi⁵,(2-DNal)⁷, Thi⁸]BK (2), were studied by using 2D NMR spectroscopy in DMSO-dg and molecular dynamics simulations. The NMR spectra of peptide 1 reveals the existence of at least two isomers arising from isomerization across the DPhe⁷-(N-Bzl)Gly⁸peptide bond. The more populated isomer possesses the cis peptide bond at this position. The ratio of cis/trans isomers amounted to 7:3. With both antagonists, the NMR data indicate a β-turn structure for the Hyp³-Gly⁴ residues. In addition, for peptide 2, position 2,3 is likely to be occupied by turn-like structures. The cis peptide bond between DPhe⁷ and (N- Bzl)Gly⁸ in analogue 1 suggests type VI β-turn at position 7,8. The molecular dynamics runs were performed on both peptides in DMSO solution. The results indicate that the structure of peptide 1 is characterized by type VIb β-turn comprising residues Ser-Arg⁹ and the βI or βII-turn involving the Pro²-Thi⁵ fragment, whereas peptide 2 shows the tendency towards the formation of type I β-turn at position 2,3. The structures of both antagonists are stabilized by a salt bridge between the guanidine moiety of Arg¹ and the carboxyl group of Arg⁹. Moreover, the side chain of DArg⁰ is apart of the rest of molecule and is not involved in structural elements except for a few calculated structures.