Laser Raman spectroscopic analysis of angiotensin peptides' conformation

In this study we examined the conformation and side chain environments of angiotensins I, II, III, and [Sar¹-Ile⁵-Ala⁸]angiotensin II using laser Raman spectroscopy. The positions of the amide I bands for all four peptides were found between 1664 and 1673 cm^?1. D₂O exchange studies confirmed the positions of the amide I and amide III bands. The positions of the amide I bands for all the angiotensins were found at approximately 1665 cm^?1 and the amide III bands were all located between 1265 and 1278 cm^?1. From the positions and intensities of the amide I and III bands we concluded that all peptides share the same overall conformation consisting of β-turn structure. Spectral analysis indicated that although the spectra for all the peptides were qualitatively identical there was evidence that the angiotensin conformations were more flexible in the aqueous phase than the solid phase. Examination of the $\text{850}\text{830}$ cm^?1 tyrosine doublet suggested that the tyrosine residue in the peptides is exposed to the solvent environment and becomes more exposed as the peptide length is decreased. Therefore, there are some localized conformational differences among the angiotensins. The conformational data yielded by this study leads us to conclude that the various biological properties ascribed to the angiotensins are not due to different conformations of the peptides. The biological differences could perhaps be attributed to localized interactions of the individual amino acid residues with themselves and with the hormone receptors.