Conformational studies of peptide hormones using laser Raman and circular dichroism spectroscopy. A comparative study of oxytocin agonists and antagonists

A comparative study of the conformational properties of the peptide hormone agonists oxytocin and [4-glycine]oxytocin, and of the oxytocin inhibitors (antagonists) [1-penicillamine]oxytocin, and [1-penicillamine,2-leucine]oxytocin was made using laser Raman and circular dichroism spectroscopy. Conformational information regarding the disulfide, peptide amide, and tyrosine chromophores was obtained, and indicated differences for the hormone agonists and antagonists. [Gly ⁴]oxytocin and oxytocin have very similar Raman spectra with the v(S-S) as a singlet at 510 cm ^-1 and amide III bands in the range of 1260 to 1272 cm ^-1, while the oxytocin antagonists [Pen ¹]oxytocin and [Pen ¹, Leu ²]oxytocin have a singlet or doublet v(S-S) band and the amide III band at 1255 cm ^-1. The circular dichroism spectra of the compounds were examined over a wide range of pH values (2 to 11.5) in aqueous solution. [Gly ⁴]oxytocin had very similar CD spectra to those previously obtained for oxytocin. The CD spectra of [Pen ¹]oxytocin and [Pen ¹, Leu ²]oxytocin had important similarities to each other, and important differences with ocytocin and [Gly ⁴]oxytocin. Using [Pen ¹, Leu ²]oxytocin which does not contain a tyrosine moiety, it was possible to assign the disulfide and amide chromophores in this compound and in [Pen ¹]oxytocin. In these latter compounds the longest wavelength disulfide transition was found at 274 to 279 nm as a negative band, indicating that the disulfide C-S-S-C dihedral angle was about 100-115° with a right-handed chirality. The intensity of this band and especially that of the peptide amide transition at about 220 nm indicated that [Pen ¹, Leu ²]oxytocin and [Pen ¹]oxytocin were more rigid (less flexible) molecules than oxytocin and [Gly ⁴]oxytocin. These results are in agreement with the results of previous NMR studies on oxytocin and [Pen ¹]oxytocin in aqueous solution. A conformation for the ring moiety of [Pen ¹]oxytocin is suggested.