DETERMINATION OF THE AVERAGE END‐TO‐END DISTANCE OF TWO ANGIOTENSIN II ANALOGS BY RESONANCE ENERGY TRANSFER

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Abstract
The angiotensin II analogs H. Tyr-Arg-Val-Phe-Val-His-Pro-Trp. OH (I) and H. Tyr-Arg-Val-Phe-Val-His-Pro-Trp. OH (II) were synthesized and their conformations in dilute aqueous solution (3 × 10-5 M) were studied by fluorescence techniques. Evaluation of singlet-singlet energy transfer between tyrosine (donor) and tryptophan (acceptor) in the biologically active analog I resulted in a low transfer efficiency (E ˜ 0.1) Since transposition of the tyrosyl and tryptophanyl residues (analog II) produced a transfer efficiency similar to that observed in compound I, the orientation factor did not present a serious problem for the determination of the intramolecular Tyr-Trp distances in these peptides on the basis of the Förster equation. Similar average Tyr-Trp separations above 15 Å were obtained for analogs I and II at pH 5.2 and no drastic titration effects on the distance were observed with compound I in the pH range 1.5–8.5. The observed end-to-end distances indicate that the conformations of analogs I and II are not quite as compact as some of the models which have been proposed for angiotensin II. Furthermore, the results exclude an electrostatic head-to-tail interaction between the terminal NH+3- and COO- groups as well as several proposed β- and γ-turn models.