Synthesis, Characterization, and Conformational Analysis of the D/L-Tic⁷ Stereoisomers of Bradykinin Receptor Antagonist D-Arg⁰[Hyp³,Thi⁵,D-Tic⁷,Oic⁸]bradykinin

D-Arg⁰[Hyp³,Thi⁵,D-Tic⁷,Oic⁸]bradykinin (HOE-140) is a potent (K_i= 0.11 nM) inhibitor of [³H]bradykinin binding to bradykinin B₂ receptors found on human IMR-90 fetal lung fibroblasts. During the synthesis of this compound, we isolated and unambiguously identified the L-Tic⁷ stereoisomer (WIN 65365), which exhibits a 2000-fold lower binding affinity (K_i = 130 nM) than HOE-140 to the bradykinin receptor. A similar decrease in potency is observed for WIN 65365 inhibition of bradykinin-stimulated ⁴⁵Ca²⁺ efflux from IMR-90 cells. Both HOE-140 and WIN 65365 appear to be competitive antagonists at the IMR-90 bradykinin receptor. This is the first documentation of bradykinin binding and functional antagonist activity by a bradykinin peptide analogue with an L amino acid replacing Pro⁷. In an attempt to rationalize the differences in binding affinities of HOE-140 and WIN 65365, a conformational analysis of the peptides was undertaken using annealed molecular dynamics (AMD). Conformational analysis of HOE-140 reveals a strong preference for the formation of a type II′ β-turn in the carboxy-terminal region. Analogous modeling of WIN 65365 reveals that its conformation is strikingly different from HOE-140 in that the four carboxy-terminal residues of WIN 65365 do not form a β-turn. These differences in lowenergy conformations between the two peptides may lead to a better understanding of the molecular interaction of antagonists with the bradykinin receptor.