The selective oxidative modification of proteins can have significant consequences for structure and function. Here, we show that protein cysteine thiyl radicals (CysS(center dot)) can reversibly abstract hydrogen atoms from the C-alpha-H bonds of selected amino acids in a protein (insulin). CysS(center dot) were generated photolytically through homolysis of cystine and through photoionization of an aromatic residue, followed by one-electron reduction of cystine. Intramolecular hydrogen transfer was monitored through the covalent incorporation of deuterium into specific amino residues. Of 51 insulin amino residues, only six incorporated significant levels of deuterium: Leu(B6), Gly(B8), Ser(B9), Val(B18), Gly(B20), and Cys(A20). All these amino acids are located at the beginning/end or outside of alpha-helices and beta-sheets, in accordance with theory, which predicts that specifically the C-alpha-H bonds of amino acids in these secondary structures have higher homolytic C-H bond dissociation energies compared to the C-alpha-H bonds of amino acids in extended conformations. Through such hydrogen transfer mechanisms, thiyl radicals are able to catalyze the oxidation of amino acids in a protein through oxidants, which would not necessary directly react with these amino acids. This feature has important consequences for protein stability under conditions of oxidative stress and/or protein production in pharmaceutical biotechnology.