A Possible Mechanism for Thiourea-Based Toxicities: Kinetics and Mechanism of Decomposition of Thiourea Dioxides in Alkaline Solutions

Abstract

The decomposition kinetics of a series of thiourea dioxides has been studied in alkaline media. In aerobic conditions the decomposition is characterized by an induction period, which is followed by the formation of dithionite, S2O42-. The rates of consumption of the thiourea dioxide and the formation of dithionite follow zero-order kinetics. No dithionite is formed in anaerobic conditions, although the thiourea dioxides can still rapidly decompose in the absence of oxygen to give sulfite and a urea as the decomposition products. No dithionite is formed until all the dioxygen in solution has been consumed, and hence the induction time is determined by the initial oxygen concentration in solution. A comprehensive mechanism that can adequately explain the decomposition is proposed in which the initial step is the cleavage of the C−S bond to give a urea residue and the sulfoxylate ion, SO22-. The sulfoxylate ion is next rapidly oxidized by oxygen to give the anion radical, SO2-, which is the precursor to the formation of dithionite via a rapid equilibrium. In aerobic environments the sufoxylate ion can produce the highly tissue-damaging series of reactive oxygen species superoxide, peroxide, and hydroxyl radical. These species could be responsible for the inherent toxicities associated with thioureas.