Kinetics and Mechanism of the Oxidation of N-Acetyl Homocysteine Thiolactone with Aqueous Iodine and Iodate

Abstract

The kinetics of N-acetyl homocysteine thiolactone (NAHT) oxidation by aqueous iodine and iodate were studied by spectrophotometric techniques. The iodate-NAHT reaction is slow and results in the formation of N-acetyl homocysteine thiolactone sulfoxide as the sole product (NAHTSO). The stoichiometry of the reaction was deduced as: IO3(-) + 3NAHT → I(-) + 3NAHTSO (S1). In excess iodate conditions, the iodide produced in S1 is oxidized to give iodine: IO3(-) + 5I(-) + 6H(+) → 3I2 + 3H2O (S2). Thus in excess iodate conditions the overall stoichiometry of the reaction is a linear combination of S1 and S2 that eliminates iodide, 5S1 + S2: 2IO3(-)+ 5NAHT+ 2H(+) → I2 + 5NAHTSO + H2O. There was a 1:1 stoichiometry for the NAHT - I2 reaction: NAHT+ I2 + H2O → NAHTSO +2I(-) + 2H(+) (S3). All reactions, S1, S2 and S3 occur simultaneously and since they are all comparable in rate; complex dynamics were observed. Iodide catalyzes S1 and S2 but inhibits S3. Iodide is a product of both S1 and S3. It has the most profound effect on the overall global dynamics observed. The overall reaction scheme which involved S1, S2 and S3 was modeled by a simple 12-reaction mechanistic scheme which gave a very good fit to experimental data.