Amino acid side chain conformation in angiotensin II and analogs: Correlated results of circular dichroism and 1 H nuclear magnetic resonance

Abstract

[1-Sar,8-Ile]angiotensin II [AII] (Sar-Arg-Val-Tyr-Ile-His-Pro-Ile) was a potent antagonist of the pressor action of AII. With a view to increase half-life in vivo of this peptide, the amino acid residue at position 4 (Tyr) or position 5 (Ile) was replaced with the corresponding N-methylated residue. This change drastically reduced the antagonistic properties of this analog. The present work was undertaken to investigate the effect of N-methylation on overall conformation of these peptides and to determine the conformational requirements for maximum agonistic or antagonistic properties. Conformation studies were carried out by circular dichroism and proton NMR spectroscopy in aqueous solution as a function of pH. The results indicated that: AII and [1-Sar,8-Ile]AII gave practically identical spectroscopic data; and N-methylation in either position 4 or position 5 resulted in remarkable changes in the peptide backbone and a severe limitation in rotational freedom of side chains in Tyr, Ile and His residues. Rotational restriction of the Tyr side chain was found to be less pronounced in [1-Sar,4-N-methyltyr,8-Ile]AII than in [1-Sar,5-N-methyleu,8-Ile]AII. The backbone and side chain structure of a potent AII antagonist should resemble that of the hormone, AII, so that it can mimic the hormone in recognizing and binding with the receptor on the cell membrane; and greater impact of N-methylation in position 5 on the overall conformation of these peptides points to the controlling influence of position 5 (Ile) in aligning the residues in the central segment (Tyr-Ile-His) of AII and its potent agonist and antagonist analogs in a nearly extended structure. Any change in this arrangement may lead to reduced biological activity.