LAUSR

ABSTRACT We have recently shown that the pyridinium aldoximes, best‐known as therapeutic antidotes for chemical warfare nerve‐agents, could markedly detoxify the carcinogenic tetrachloro‐1,4‐benzoquinone (TCBQ) via an unusual double Beckmann fragmentation mechanism. However, it is still not clear why pralidoxime (2‐PAM) cannot provide full protection against TCBQ‐induced biological damages even when 2‐PAM was in excess. Here we show, unexpectedly, that TCBQ can also activate pralidoxime to generate a reactive iminyl radical intermediate in two‐consecutive steps, which was detected and unequivocally characterized by the complementary application of ESR spin‐trapping, HPLC/MS and nitrogen‐15 isotope‐labeling studies. The same iminyl radical was observed when TCBQ was substituted by other halogenated quinones. The end product of iminyl radical was isolated and identified as its corresponding reactive and toxic aldehyde. Based on these data, we proposed that the reaction of 2‐PAM and TCBQ might be through the following two competing pathways: a nucleophilic attack of 2‐PAM on TCBQ forms an unstable transient intermediate, which can decompose not only heterolytically to form 2‐CMP via double Beckmann fragmentation, but also homolytically leading to the formation of a reactive iminyl radical in double‐steps, which then via H abstraction and further hydrolyzation to form its corresponding more toxic aldehyde. Analogous radical homolysis mechanism was observed with other halogenated quinones and pyridinium aldoximes. This study represents the first detection and identification of reactive iminyl radical intermediates produced under normal physiological conditions, which provides direct experimental evidence to explain only the partial protection by 2‐PAM against TCBQ‐induced biological damages, and also the potential side‐toxic effects induced by 2‐PAM and other pyridinium aldoxime nerve‐agent antidotes. Graphic abstract Figure. No Caption available. HighlightsTCBQ activated 2‐PAM to generate reactive iminyl radicals in two‐consecutive steps.Analogous radical homolysis mechanism was observed with other haloquinones & aldoximes.This is the 1st detection of iminyl radical under normal physiological condition.The finding can explain partial protection by 2‐PAM against TCBQ‐induced damages.It may also explain some of the side‐toxic effects induced by pyridinium aldoximes.