Studies of Angiotensin‐II Conformations by Circular Dichroism

Abstract

Angiotensinamide I1 has been studied by circular-dichroism spectroscopy in comparison with some of its peptidic fragments and structural analogs. The following observations have been made. The stepwise examination of elongated peptides shows the appearance of a structural organisation at the heptapeptide stage. Evidence that C-terminal phenylalanine residue plays an important role on the overall conformation of the octapeptide is obtained. As already suggested for some other short peptides, circular-dichroism spectra of angiotensinamide I1 show dramatic perturbations of the peptidic region due to side-chain aromatic-chromophore overlapping. The contribution of prolyl residue in position 7 of the peptide appears to be critical; equilibrium between cis and trans forms is believed to be shifted toward one or the other conformer upon binding to receptor site. This would result in the adequate positioning of the phenylalanine residue, the biological importance of which has already been shown.Angiotensin I1 is a natural octapeptide [l] contributing to the regulation of several important physiological functions, among others the regulation of blood pressure [2], whence its name. To investigate the mode of action of angiotensin I1 a t the molecular level, three sets of information are required concerning: (a) the recognition of specific binding sites and their isolation, (b) the processes triggered when the binding sites are properly occupied and (c) the reciprocal adjustment of the hormone to the active sites responsible both for the extreme specificity of binding and for the triggering of the biological response.Recently rapid progress has been made concerning the characterization of specific binding sites [3] and of different classes of receptors [4], the calcium release in smooth muscles [5] and the formation of cyclic AMP [6]. The data available concerning the secondary structure of angiotensin I1 are much less clear cut. Smeby et al. [7] suggested that angiotensin I1 could adopt an a-helix configuration in aqueous solution, whereas Scheraga [8] was in favor of an unordered structure. The work of Paladini et al. [9] and of Paiva [lo], on the other hand, suggested that several conformations are simultaneously present in aqueous solutions of angiotensin 11. All these conclusions might reflect the fact that angiotensin I1 is a small peptide with linear primary structure. A predominant secondary structure may well require precise surrounding conditions. In the present paper we report an analysis by circular dichroism in the ultraviolet region between 300 nm and 185 nm of angiotensin-I1 solutions in various solvents and a t different temperatures.Comparison of the spectra of short peptides and of angiotensin I1 analogous with the spectra of the hormone itself, indicated that angiotensin I1 has a marked tendency to adopt a defined ordered conformation of ,&antiparallel structure. Triiluorethanol was from Fluka. Alkaline methanol was prepared by addition of 1 ml 2 N NaOH to 100 ml methanol. The range of concentrations of ...

Solvent Effects on Luteinizing‐ and Follicle‐Stimulating‐Hormone Releasing Factor Polymorphism Studied by Circular Dichroism

“…If i t would be so in trifluoroethanol, the C-terminal heptapeptide should present a negative band around 220nm and this is not the case. Thus part of the negative band a t 220nm presented by the decapeptide would better correspond to chromophore interactions of the type of antiparallel ,9-structures [20,23,24]. Hence the presence of the shoulder a t 206nm and the intense band a t 220nm, suggest that a t least two types of peptidic chromophore interactions participate in the ordered conformation of the decapeptide.…”

“…It was of interest to investigate whether an increase in temperature would shift the balance between the existing conformations, as observed in other instances [20]. Fig.3 reports experiments carried out a t pH 7.4 between Fig.…”

Angiotensin II Conformations

“…Organic solvents like trifluoroethanol, alkaline methanol or ethanol promoted a considerable structuration of the dissolved angiotensinamide 1 1 into a predominant form, which, by circular dichroism measurements, has been tentatively assigned as a cross /? structure [6], defined according to Pysh [7] and Timasheff et al [S]. The required folding of the molecule was assigned principally to the hydrophobic interactions between the 3 and 5 valine and the 8 phenylalanine side-chains and to the cis conformation of the histidine-proline peptide bond.…”

Correlation of the biological activity and solution conformation of [Asp1,Ile5]- and [Phe4,Tyr8]angiotensin II.

“…Fermandjian et al (23) suggested that a conformational change observed by circular dichroism upon heating angiotensin II at pH 6 was attributable to trans-to-cis isomerization of proline. A later report from this group (24) signed a major circular dichroism change observed upon titration through pH 6.5 to reorientation of the tyrosine side chain rather than a backbone conformational change.…”