Torsion Angles in the Cystine Bridge of Oxytocin in Aqueous Solution. Measurements of Circumjacent Vicinal Couplings between ¹H, ¹³C, and ¹⁵N

The six couplings from ¹H^α, ¹³C', and ^15N' to the two β protons have been measured for the two half-cystyl residues in oxytocin, using a series of specifically designed and synthesized isotopic isomers. In the case of half-cystyl 1, the observed couplings strongly suggest that the torsion angle x¹ (N'-C^α-C^β-S) has the eclipsed value of 120°. The value for this angle in the half-cystyl residue 6 is less clearly defined, but is probably also fixed. Using the same analysis as for x1¹, X6¹ is approximately + 120°. The stereochemical assignments of the ff protons of the two residues have been confirmed by stereoselective deuteration.