Conformational comparison of (Val5, Trp8)-angiotensin II and (Val4, Trp7)-angiotensin III by fluorescence measurements.

Abstract

Intramolecular Tyr–Trp distances in the biologically active analogs (Val5, Trp8)-angiotensin II and (Val4, Trp7)-angiotensin III were determined by evaluation of singlet–singlet resonance energy transfer in dilute aqueous solution (3 × 10−5 M) at pH values of 1.5, 5.2, and 8.5. The higher transfer efficiencies obtained from donor (tyrosine) fluorescence quenching compared with those resulting from measurement of acceptor (tryptophan) fluorescence enhancement indicate the existence of additional quenching aside from energy transfer in both peptides. Taking into account additional quenching, similar Tyr–Trp distances in the range from 12.1 to 12.7 Å (1 Å = 0.1 nm) were obtained for the heptapeptide and the octapeptide at all three pH values. This finding provides evidence for a folding of the backbone in the C-terminal segment of both peptides. Furthermore, it can be concluded that titration of the carboxyl groups, of the histidine side chain, and of the α-amino group does not engender drastic changes in the overall conformation; however, subtle changes in intramolecular distance and fluorophore environment are observed. Small differences in conformational parameters are also apparent between (Val5, Trp8)-angiotensin II and (Val4, Trp7)-angiotensin III under identical conditions. The latter observation is of interest in view of the difference in specificity between angiotensin II and angiotensin III in their interaction with distinct classes of angiotensin receptors.