First steps in the oxidation of sulfur-containing amino acids by hypohalogenation: Very fast generation of intermediate sulfenyl halides and halosulfonium cations

Sulfur-containing amino acids show an extraordinary binding towards HOCl/Cl O-. During the process, the C1 is transferred from the O to the S of the am ino acid. Met reacts with HOCl one order of magnitude faster than the non-S containing amino acids (k((Met+HOCl))=8.7.10(8) mol(-1) dm(3) s(-1)). Inst ead, Cys reacts as its thiolate (RS-), two orders-of-magnitude faster (k((R S-+HOCl))=1.2.10(9) mol(-1) dm(3) s(-1)). Cys reacts also with ClO- (k((RS- +ClO-))=1.9.10(5) mol(-1) dm(3) s(-1)). Such processes take place much more readily than the corresponding N-halogenation of the non-sulfur containing amino acids. To our knowledge, these are dw first kinetic measurements of the rate of formation of sulfenyl halides and halosulfonium cations in, aqu eous solution. Sulfenyl chlorides and chlorosulfonium ions derived from ami no acids are elusive, and sulfide-type amino acids (Met) eventually yield s ulfoxides (MetO), while thiol-type amino acids (Cys) lead to disulfides (Cy s(boolean AND)Cys) and sulfonic acids (Cya). The fate of sulfur-containing amino acids upon oxidation with HOCl/ClO- seems to be related to their muta gen-inactivation ability. (C) 2000 Elsevier Science Ltd. All rights reserve d.