In position 7 l- and d-Tic-substituted oxytocin and deamino oxytocin: NMR study and conformational insights

Incorporation of l- or d-Tic into position 7 of oxytocin (OT) and its deamino analogue ([Mpa1]OT) resulted in four analogues, [l-Tic7]OT (1), [d-Tic7]OT (2), [Mpa1,l-Tic7]OT (3) and [Mpa1,d-Tic7]OT (4). Their biological properties were described by Fragiadaki et al. (Eur J Med Chem 42:799–806, 2007). Their NMR study (NOESY, TOCSY, 1H–13C HSQC spectra) is presented here. Analogues 1, 3 and 4 showed partial agonistic activity, analogue 2 was pure antagonist, suggesting that a cis conformation between residues 6 and 7 of the molecule does not result in antagonistic activity. However, the reduction in agonistic activity of analogues 1, 3 and 4 in comparison to oxytocin is consistent with the reduction of the trans conformation form. Binding affinity for the human oxytocin receptor with IC50 value of 130, 730, 103, and 380 nM for peptides 1, 2, 3, and 4, respectively, showed lower affinity in the case of d analogues. Deamination slightly increased the affinity. The existence of both cis and trans configurations of the Cys6-d-Tic7 bond is supported by observation of two sets of cross-peaks for 1H and 13C nuclei for most of the residues of the peptide not only in NOESY and TOCSY but also in 1H–13C HSQC spectra. The MS and HPLC indicate the presence of a single molecule/peptide, and NMR data thus suggest that this second set of peaks is due to the cis conformation.