Peptides containing a single free cysteine will be oxidized at pH>7 to form dimers. This oxidation can be reverted. Peptides containing two or more thiol moieties may yield a mixture of products upon oxidation. A pH 7.5-8 is the best condition for the disulfide bond formation. Hence, peptides containing free cysteines are best dissolved in degassed solvents, e.g. buffers pH<7, diluted acetic acid, 0.1% trifluoroacetic acid in aqueous acetonitrile. DMSO should be avoided, especially with peptide trifluoroacetate.
