Autoinhibition by Iodide Ion in the Methionine–Iodine Reaction

Abstract

The methionine–iodine reaction was reinvestigated spectrophotometrically in detail monitoring the absorbance belonging to the isosbestic point of iodine at 468 nm, at T = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered acidic medium. The stoichiometric ratio of the reactants was determined to be 1:1 producing methionine sulfoxide as the lone sulfur-containing product. The direct reaction between methionine and iodine was found to be relatively rapid in the absence of initially added iodide ion, and it can conveniently be followed by the stopped-flow technique. Reduction of iodine eventually leads to the formation of iodide ion that inhibits the reaction making the whole system autoinhibitory with respect to the halide ion. We have also shown that this inhibitory effect appears quite prominently, and addition of iodide ion in the millimole concentration range may result in a rate law where the formal kinetic order of this species becomes −2. In contrast to this, hydrogen ion has just a mildly inhibitory effect giving rise to the fact that iodine is the kinetically active species in the system but not hypoiodous acid. The surprisingly complex kinetics of this simple reaction may readily be interpreted via the initiating rapidly established iodonium-transfer process between the reactants followed by the subsequent hydrolytic decomposition of the short-lived iodinated methionine. A seven-step kinetic model to be able to describe the most important characteristics of the measured kinetic curves is established and discussed in detail.

Mediating Oxidation of Thioethers with Iodine—A Mild and Versatile Pathway to Trigger the Formation of Peptide Hydrogels

“…The largest signal at 54 % can be attributed to a di‐oxidized species (either disulfoxide or monosulfone), while a second smaller peak at 4 % with the same mass to charge ratio corresponding to a likewise di‐oxidized species is found at shorter retention times ( t =27.8 min). In accordance with the literature [31] and our previous analysis on the auxiliary thiodiglycol, we assume that the disulfoxide is the major oxidation product and the monosulfone is formed to a negligible extent. No signals corresponding to a sulfone‐sulfoxide (triple‐oxidized) or disulfone (quadruple‐oxidized) peptide could be detected.…”

Investigating the Complexation and Release Behaviors of Iodine in Poly(vinylpyrrolidone)-Iodine Systems through Experimental and Computational Approaches

“…It is widely accepted that the antimicrobial action of iodine is rapid, even in low concentrations, but the exact mode of action is unknown . Some researchers have proposed that iodine penetrates into microorganisms and attacks key groups of proteins, nucleotides, and fatty acids, with the attack culminating in cell death. − The redox effect of iodine on sulfur compounds (e.g., methionine and thiourea dioxide) has been thoroughly studied by Horváth et al − The reduction product, iodide (I – ), shows an autoinhibitory effect due to the equilibrium: I – + I 2 ↔ I 3 – . Therefore, in order to make better use of the antibacterial properties of iodine, it is of great importance to study the complexation and release of iodine.…”

Vermittelnde Oxidation von Thioethern mit Iod – Eine vielseitige und milde Methode zur Bildung von Peptidhydrogelen

“…Das intensivste Signal ist mit 54 % einer dioxidierten Spezies (entweder Disulfoxid oder Monosulfon) zuzuordnen, während ein kleineres Signal mit 4 % und einem Masse-zu-Ladungsverhältnis ebenfalls entsprechend einer dioxidierten Spezies bei kürzeren Retentionszeiten (t = 27.8 min) gefunden wird. In Übereinstimmung mit der Literatur [31]…”

“…Das intensivste Signal ist mit 54 % einer dioxidierten Spezies (entweder Disulfoxid oder Monosulfon) zuzuordnen, während ein kleineres Signal mit 4 % und einem Masse‐zu‐Ladungsverhältnis ebenfalls entsprechend einer dioxidierten Spezies bei kürzeren Retentionszeiten ( t =27.8 min) gefunden wird. In Übereinstimmung mit der Literatur [31] sowie unseren vorherigen Analysen zum Hilfsstoff Thiodiglycol vermuten wir, dass das Disulfoxid als dominantes Oxidationsprodukt gebildet wird und das Monosulfon lediglich zu einem vernachlässigbaren Anteil vorliegt. Derweil wurden keine Hinweise auf die Bildung eines Sulfoxid‐Sulfons (dreifach oxidiert) oder eines Disulfons (vierfach oxidiert) gefunden.…”