Design, synthesis and characterization of bradykinin antagonists via cyclization of the modified backbone

With the aim of synthesizing cyclic antagonists of the nonapeptide hormone bradykinin with minimal side chain modification, we performed backbone to backbone and backbone to side chain cyclization. To probe and compare different strategies for this new kind of cyclization, the branched peptide bonds were formed by both reductive alkylation on the solid phase and by using preformed building units. Lactam bridges between the modified amide groups were formed by the use of the phenylalanine derivatives N(CH2COOH)Phe and N(CH2CH2NH2)Phe. The best results in the formation of the N-alkylamide bond were obtained with the coupling reagent PyBrop. The coupling rate was monitored by estimation of the N-terminal Fmoc-group. The cyclization was performed on the solid support. Unexpected difficulties resulted from the instability of the N-alkylamide bond under strong acidic conditions, as used for deprotection and for removal from the resin. We synthesized peptides with backbone to backbone cyclization between positions 2 and 5, as well as backbone to side chain cyclizations between positions 0 and 5, and between 2 and 6. The relatively high biological activities of some of the cyclic analogues support the supposed receptor-bound conformation of bradykinin antagonists with a beta-turn in the N-terminal sequence.