Conformational restriction of angiotensin II: cyclic analogs having high potency

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Abstract
Cyclic analogues of angiotensin II (AII) were synthesized by connecting the side chains of residues 3 and 5 via a disulfide bridge. Appropriate conformational constraints afforded an analogue, [Hcy3,5]AII, having high contractile activity (pD2 = 8.48 vs 8.81 for AII) and excellent binding affinity (IC50 = 2.1 nM vs 2.2 mM for AII). This type of cyclization was also used to prepare a highly potent AII antagonist, [Sar1,Hcy3,5,Ile8]AII (pA2 = 9.09 vs 9.17 for [Sar1,Ile8]AII; IC50 = 0.9 nM vs 1.9 nM for [Sar1,Ile8]AII). Model building suggests that this ring structure is consistent with a receptor-bound conformation having any of a variety of three-residue turns, including a .gamma.-turn. In contrast, the receptor-bound conformation of AII does not appear to accommodate a .beta.-turn or an .alpha.-helix which includes residues 3-5.

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