Oxytocin dimers

Synthetic FI was treated using conditions known to convert oxytocin to a mixture of parallel and antiparallel dimers RPLC separation gave FI (20.1 min) and two more retained peaks at 42.0 and 45.3 min. The latter were analyzed by size-exclusion LC both had 1,570. 

Walti M, Hope DB. Synthesis of oxytocin suppression of dimer 126. formation. Experientia 1973 29 389-389. 

The triplet lifetimes at 77 °K of proteins showing an oxytocin-t5q)e phosphorescence are in the range of 1.4—1.7 sec 28b,i49) similar to the value 1.7 sec found from oxytocin m), whereas the lifetime of the tyrosine zwitterion itself is —2.9 sec Xhe phosphorescence lifetime of the dipeptide dimer cystinyl-bis-tyrosine also is 1.7 sec 28b) in contrast with the 2.6 sec lifetime found for the reduced dipeptide cysteinyl-tyrosine Thus, the shortened lifetime is due to the neighboring disulfide linkages and accordingly when the disulfide bridges are reduced the observed protein or peptide hfetimes are found to increase 28b), The ribonuclease A phosphorescence is reported to decay as two first order processes at 77 °K with decay lifetimes of 0.5 sec and 1.6 sec 30). The reduced t T osine phosphorescence lifetime may involve electron transfer to disulfide from the triplet ns). 

Dimerization of the oxytocin molecule by incubation in NaHC03 at pH 8.3 at room temperature yields a biologically inactive compound. When vasopressin is incubated with cysteine or glutathione, the reduction of the disulfide bridge diminishes antidiuretic activity to one-third of its original value. The substitution of the 8-lysine of vasopressin by 1-deamino lysine enhances the antidiuretic activity, but substitution of the 8-lysine by 1-acetyllysine abolishes most of the hormonal activity. 

Despite these successes serious side reactions can result from the action of alkali on cysteine or cystine-containing peptides. Open-chain unsymmetrical cystine derivatives [92, 103, 166] and some cyclic molecules including oxytocin [153] and calcitonin [24, 25] are known to undergo rapid base-catalyzed disulphide interchange yielding symmetrical disulphides or mixtures of polymer and parallel and antiparallel dimers [107, 153]. Furthermore the possibility of /3-elimination of trityl, benzhydryl, and benzyl mercaptide ion from the corresponding peptide bound cysteine residue is known [154] the extent of... 

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