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.