Preferred Solution Conformation of des‐Arg9‐Bradykinin and Analysis of Structure‐Conformation‐Activity Relationships in the Series [Alan]des‐Arg9‐Bradykinin

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Abstract
The solution conformation of the vasoactive peptide hormone des-Arg9-bradykinin (Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe) is described. By 1H-NMR spectroscopy des-Arg9-bradykinin and its fragments bradykinin-(1-5), bradykinin-(1-6) were studied in aqueous solution as a function of pH (titration) and in dimethylsulfoxide solution at 2 ionic states (cation and neutral ion species). The preferred solution conformation which is most strongly stabilized in dimethylsulfoxide in the neutral ion species includes a distorted .beta.-turn II involving the N-terminal sequence of Pro2-Pro3-Gly4-Phe5 and most likely a C7-type bend in the C-terminal part Ser-Pro-Phe. A complete series of analogs ([Alan]des Arg9-bradykinin, with n = 1,2.cntdot..cntdot..cntdot.8) was then investigated by circular dichroism and 1H-NMR spectroscopy in order to study the conformational role played by each residue and to delineate the local and the long-range effects on conformation brought about by the Xaa .fwdarw. Ala substitutions. Chosen spectral parameters (circular dichroic spectra, chemical shift variations and vicinal coupling constants) characteristics of the preferred solution conformation of the native sequence of des-Arg9-bradykinin are followed from analogue to analogue. The important conformational role of the arginine-1 side chain and its positive charge and the spatial proximity of the N-terminal and C-terminal groups, i.e., the folded structure of the peptide can be inferred from these data. A comparison of the biological activities of the analogs with the conformative perturbations caused by the chemical alterations shows des-Arg9-bradykinin conformation and receptor affinity to be equally sensitive to single-residue substitutions. The correct orientation of the arginine-1 side chain, the precise geometry of the turn involving residues 2-5 and of the C-terminal Pro-Phe sequence are of primary importance.