The photodissociation of intrachain disulfide bonds in I model peptide and salmon calcitonin generates a series of cyclic peptide products following the generation of a CysS(center dot) thiyl radical pair. Key to the formation of these cyclic products are disproportionation and reversible hydrogen atom transfer reactions as well as secondary photoreactions, which lead to C-S bond breakage of primary photoproducts. Depending oil the wavelength of the incident light, disulfides Ultimately convert into cyclic thioethers. Ail important photolytic product is H,S, which is highly relevant for the production and storage of protein pharmaceuticals, where H,S can catalyze disulfide scrambling and protein degradation.