Oxyhalogen–sulfur chemistry — Kinetics and mechanism of oxidation of methionine by aqueous iodine and acidified iodate

Abstract

The oxidation of methionine (Met) by acidic iodate and aqueous iodine was studied. Though the reaction is a simple two-electron oxidation to give methionine sulfoxide (Met–S=O), the dynamics of the reaction are, however, very complex, characterized by clock reaction characteristics and transient formation of iodine. In excess methionine conditions, the stoichiometry of the reaction was deduced to be IO3– + 3Met → I– + 3Met–S=O. In excess iodate, the iodide product reacts with iodate to give a final product of molecular iodine and a 2:5 stoichiometry: 2IO3– + 5Met + 2H+ → I2 + 5Met–S=O + H2O. The direct reaction of iodine and methionine is slow and mildly autoinhibitory, which explains the transient formation of iodine, even in conditions of excess methionine in which iodine is not a final product. The whole reaction scheme could be simulated by a simple network of 11 reactions.

Autoinhibition by Iodide Ion in the Methionine–Iodine Reaction

“…For the sake of completeness, it should be mentioned that the most intense peak of the S-methyl group of methionine sulfoxide and methionine sulfone may be observed at around 2.75 and 3.15 ppm, respectively. 32 As it is seen, the NMR-spectrum of the reaction mixture does not contain any observable peak in the 2.8–3.9 ppm region meaning that methionine sulfone cannot be the end-product of the title reaction. At the same time around 2.7 ppm, a singlet peak appears belonging to the methyl group of methionine sulfoxide.…”

“…In the case of iodide, the result presented here contradicts with the formal kinetic order of iodide reported previously. 32 As we shall later show, it is the consequence of the limited concentration range of iodide ion used in Chikwana’s experiments. It also means that the autoinhibitory feature of iodide ion is not as mild as mentioned in Chikwana’s report.…”

“…It is interesting to note that such a complex effect of hydrogen ion is not yet recognized previously. 32 …”

Hybrid Li-Ion and Li-O2 Battery Enabled by Oxyhalogen-Sulfur Electrochemistry

“…In the typical oxyhalogen-sulfur chemical reaction, the organic sulfur compounds react with iodates (e.g., HIO, HIO 2 , and HIO 3 ) and are oxidized to form sulfoxides (Reaction 1). 32,33 HIO + :…”

Synthesis and characterization of a ZnMn2O4nanostructure as a chemical nanosensor: a facile and new approach for colorimetric determination of omeprazole and lansoprazole drugs

“…36,57,58 The oxidation of drugs containing sulde groups can be also performed using iodate in an acidic medium and by following the liberated iodine spectrophotometrically, the concentration of the drugs can be determined. 59,60 However, these methods are multistep and tedious procedures. Consequently, we have developed a simple and accurate method for determination of such type of drugs based on their oxidation using semiconducting nanomaterials.…”