Determination of the average end-to-end distance of two angiotensin II analogs by resonance energy transfer.

The angiotensin II analogs H . Tyr-Arg-Val-Phe-Val-His-Pro-Trp . OH (I) and H . Trp-Arg-Val-Phe-Val-His-Pro-Tyr . OH (II) were synthesized and their conformations in dilute aqueous solution (3 X 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 approximately 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 A 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 NH3+- and COO- groups as well as several proposed beta- and gamma-turn models.