Development of a colorimetric method for the dosage of OI- anions and I2 in aqueous media.

Lactoperoxidase catalyzes the oxidation of thiocyanate (SCN-) and iodide (I-) in presence of hydrogen peroxide in hypothiocyanite (OSCN-) ions and, depending on the pH, in hypoiodite (OI-) ions or in iode (I2). Oxidized SCN- and I- are part of the lactoperoxidase system, which is a natural biological protection in cow milk, and are described as having inhibitory properties against pathogenic human bacteria, fungi and viruses. We have developed an aqueous solution containing only OSCN- and OI- ions (without the enzyme) and we tested it successfully against plant pathogens. In order to characterize this new soft chemical control against plant pathogens we had to determine the concentration of OSCN- and OI- ions. The dosage of OSCN- consists in a well referenced colorimetric method but no procedure is described for the determination of OI- ions. We have thus developed an easy method, based on the oxidation of the amine moiety of 3,3',5,5'- tetramethylbenzidine (TMB) by OI- or I2 in a strongly absorbing blue product for the detection and dosage of both molecules. Interestingly the OSCN- ions are not able to oxidize TMB and render this method specific to enzymatic oxidized iodide. We have calculated its sensitivity, repeatability and linearity. This method could also be used for the determination of OCI- and OBr- ions produced during the enzymatic oxidation of chloride and bromide by mammalian's peroxidases.