Design, synthesis, and biological activities of four angiotensin II receptor ligands with gamma-turn mimetics replacing amino acid residues 3-5

Disulfide cyclization is a powerful method for reducing the conformational space of a peptide. This in turn may enable the study of its bioactive conformation. Several analogues of angiotensin II (Ang II) containing a disulfide bridge between amino acids 3 and 5 have been reported. Among these the cyclic octapeptides c[Hcy(3,5)]-Ang II, c[Cys(3,5)]-Ang II, and c[Pen(3,5)]Ang II showed significant activity at Ang II receptors. We have performed conformational analysis studies using theoretical calculations and H-1-NMR spectroscopy on tripeptide model compounds of these cyclic octapeptides which show that the cyclic moieties of c[Cys(3,5)]-Ang II and c[Pen(3,5)]-Ang II preferentially assume an inverse gamma-turn conformation. On the basis of these results, we substituted amino acid residues 3-5 in Ang II with two different gamma-turn mimetics giving four diastereomeric Ang II analogues. Interestingly, two of these are equipotent to Ang II in binding to AT(1) receptors. In the contractile test using rabbit aorta rings, one of the analogues is an agonist with full contractile activity approximately equipotent to c[Pen(3,5)]-Ang II but 300-fold less potent than Ang II. This low potency may suggest that Ang II does not adopt a gamma-turn in the 3-5 region when interacting with the receptor.