Reversible hydrogen transfer reactions of cysteine thiyl radicals in peptides: the conversion of cysteine into dehydroalanine and alanine, and of alanine into dehydroalanine.

The photodissociation of disulfide bonds in model peptides containing Ala and Ala-d(3) generates a series of photoproducts following the generation of a CysS(•) thiyl radical pair. These photoproducts include transformations of Cys to dehydroalanine (Dha) and Ala, as well as Ala to Dha. Intramolecular Michael addition of an intact Cys with a photolytically generated Dha results in the formation of cyclic thioethers. The conversion of Cys into Dha likely involves a 1,3-H-shift from the Cys (α)C-H bond to the thiyl radical, followed by elimination of HS(•). The conversion of Dha into Ala most likely involves hydrated electrons, which are generated through the photolysis of Cys, the photoproduct of disulfide photolysis. Prior to stable product formation, CysS(•) radicals engage in reversible hydrogen transfer reactions with (α)C-H and (β)C-H bonds of the surrounding amino acids. Especially for the (β)C-H bonds of Ala, such hydrogen transfer reactions are unexpected on the basis of thermodynamic grounds; however, the replacement of deuterons in Ala-d(3) by hydrogens in H(2)O provides strong experimental evidence for such reactions.