Reversibility and regioselectivity in thiol/disulfide interchange of tocinoic acid with glutathione in lyophilized solids.

Thiol/disulfide interchange is one of the most common routes of aggregation of lyophilized protein drug products, but the mechanisms of the reaction in the solid state have not been established. Here, we report perturbations in thiol/disulfide interchange upon lyophilization, using tocionic acid (cycloCYIQNC; TA(ox)) and glutathione (GSH) as model peptides. The results suggest that thiol/disulfide interchange reactions differ in aqueous solution and in dry lyophilized solids with regard to reversibility and regioselectivity. In solids, the reaction of TA(ox) with GSH is effectively irreversible and the less-hindered single mixed disulfide is formed exclusively.