Reactivity, Pathways, and Iodinated Disinfection Byproduct Formation during Chlorination of Iodotyrosines Derived from Edible Seaweed

Iodine derived from edible seaweed significantly enhances the formation of iodinated disinfection byproducts (I-DBPs) during household cooking. Reactions of chlorine with monoiodotyrosine (MIT) and diiodotyrosine (DIT) derived from seaweed were investigated. Species-specific second-order rate constants (25 degrees C) for the reaction of hypochlorous acid with neutral and anionic MIT were calculated to be 23.87 +/- 5.01 and 634.65 +/- 75.70 M-1 s(-1), respectively, while the corresponding rate constants for that with neutral and anionic DIT were determined to be 12.51 +/- 19.67 and 199.12 +/- 8.64 M-1 s(-1), respectively. Increasing temperature facilitated the reaction of chlorine with MIT and DIT. Based on the identification of 59 transformation products/DBPs from iodotyrosines by HPLC/Q-Orbitrap HRMS, three dominant reaction pathways were proposed. Thermodynamic results of computational modeling using density functional theory revealed that halogen exchange reaction follows a stepwise addition-elimination pathway. Among these DBPs, 3,5-diiodo-4-hydroxy-benzaldehyde and 3,5-diiodo-4-hydroxy-benzacetonitrle exhibited high toxic risk. During chlorination of MIT and DIT, iodinated trihalomethanes and haloacetic acids became dominant species at common cooking temperature (80 degrees C). These results provide insight into the mechanisms of halogen exchange reaction and imply important implications for the toxic risk associated with the exposure of I-DBPs from household cooking with iodine-containing food.