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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[Hcy3,5]-Ang II, c[Cys3,5]-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 1H-NMR spectroscopy on tripeptide model compounds of these cyclic octapeptides which show that the cyclic moieties of c[Cys3,5]-Ang II and c[Pen3,5]-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 AT1 receptors. In the contractile test using rabbit aorta rings, one of the analogues is an agonist with full contractile activity approximately equipotent to c[Pen3,5]-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.

Analogues of the hypertensive octapeptide angiotensin II, comprising novel constrained 5,8-bicyclic and 5,9-bicyclic tripeptide units adopting nonclassical beta-turn geometries, as deduced from theoretical conformational analysis, have been synthesized. Spontanous bicyclization upon acid-catalyzed deprotection of a model peptide, encompassing a protected omega-formyl alpha-amino acid in position 5 and cysteine residues in positions 3 and 7, revealed a strong preference for bicyclization toward the C-terminus. The bicyclic thiazolidine related angiotensin II analogues synthesized exhibited no affinity for the angiotensin II AT1 receptor.

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A simple experimental procedure on solid phase for the construction of new tripeptidic 5,9- and 5,10-fused thiazabicycloalkane scaffolds that adopt beta-turns has been developed. This N-terminal-directed bicyclization, relying on masked aldehyde precursors derived from glutamic acid as key building blocks, provides a complement to the related bicyclization previously reported, where an aspartic acid-derived precursor was employed to induce cyclization toward the C-terminal end of the peptide. Thus, the regioselectivity of the bicyclization can be altered simply by varying the chain length of the incorporated aldehyde precursor. Four analogues of the hypertensive octapeptide angiotensin II, comprising the new scaffolds in the 3-5- and 5-7-positions, were synthesized. One of these conformationally constrained angiotensin II analogues exhibited AT(1) receptor affinity (K(i) = 750 nM). Results from theoretical conformational analysis of model compounds of the bicyclic tripeptide mimetics are presented, and they demonstrate that subtle differences in geometry have a strong impact on the affinity to the AT(1) receptor.