On the requirement of oxidizing reagent for the formation of a disulfide bond

Cyclization of a peptide through the formation of a disulfide bond between the SH groups of cysteines on the N- and C-terminals of peptide was studied in degassed water solution under vacuum. Cyclization went to completion although the solution was oxygen deficient (the number of oxygen molecules available for the reaction was at least 16 times less than the number of peptide molecules). This result indicates that, contrary to the common assumption, disulfide bond formation does not necessarily require an oxidant (O(2), I(2), etc.) to occur.