On the mechanism of degradation of oxytocin and its analogues in aqueous solution.

Oxytocin (OT) is a cyclic nonapeptide containing one internal disulfide bond between its Cys(1) and Cys(6) residues. Although OT is one of the most commonly used peptidic drugs, the mechanism of its degradation in aqueous solution and the identity of its degradants have not been fully elucidated. To investigate the pathways and products of OT degradation in slightly acidic to neutral solutions, we prepared the peptides: OT, [D-Cys(1)]OT, a series of N-alkylated OT analogues, [[(13)C3,(15) N]Cys(1)]OT, and OT where each sulfur atom was systematically replaced by either methylene, (34)S, or Se.