The development of an optical trigger for protein folding is described. The optical trigger is an aryl disulfide embedded in a polypeptide such that the aryl disulfide constrains the peptide in a nonhelical conformation. Upon photocleavage of the SS bond, the peptide can then commence to fold into an a-helical conformation. Two thiotyrosine derivatives, (S)-4’-mercaptophenylalanine (Tty) and 3-N-(4’-mercaptophenyl)-(S)-2,3-diaminoproprionate (Aty), have been prepared and incorporated into polyalanine peptides. The ease of synthesis of protected forms of Tty and Aty amenable for solid phase synthesis, in four steps with 30% overall yield and six steps in 40% overall yield, respectively, make these attractive candidates as precursors of the optical trigger. CD and IR spectroscopy showed that the cyclic disulfide cross-linked peptides are much less helical than their linear counterparts. Following laser flash photolysis, peptide 7, which incorporates Tty, showed total recombination of the thiyl radicals within 1 ns. Peptide 16, which incorporates Aty, showed a significant amount of long-lived thiyl radicals from nanosecond to microsecond time scale. The process of recombination is hypothesized to be governed by the peptide conformation. Because of the significant amount of long-lived thiyl radicals generated from cyclic peptide 16, Aty should prove to be of general utility in the studies of protein folding on a time scale of sub-picoseconds and greater.