Cyclic 12-, 13- and 14-membered ring angiotensin II analogues related to disulfides but encompassing methylene-dithioether bridges have been prepared. The affinity data from these derivatives were compared to those from the disulfides. The methylenedithioether analogues displayed good binding affinities to rat liver AT1 receptors although in most cases somewhat lower than their disulfide counterparts
The endogenous angiotensin II (Ang II) and the synthetic AT(2) selective agonist 4-aminoPhe(6)-Ang II respond very differently to identical cyclizations. Cyclizations of Ang II by thioacetalization, involving the 3 and 5 amino acid residue side chains, provided ligands with almost equipotent binding affinities to Ang II at the AT(2) receptor. In contrast, the same cyclization procedures applied on the AT(2) selective 4-aminoPhe(6)-Ang II delivered significantly less potent AT(2) receptor ligands, although the AT(2)/AT(1) selectivity was still very high. The fact that different structure-activity relationships are observed after imposing conformational restrictions on Ang II and 4-aminoPhe(6)-Ang II, respectively, suggests that the peptides, despite large similarities might adopt quite different backbone conformations when binding to the AT(2) receptor. (C) 2003 Elsevier Science Ltd. All rights reserved.