The critical role of unique azido-substituted chloro-O-semiquinone radical intermediates in the synergistic toxicity between sodium azide and chlorocatecholic carcinogens

We have shown previously that exposing bacteria to tetrachlorocatechol (TCC) and sodium azide (NaN3) together causes synergistic cytotoxicity in a biphasic mode. However, the underlying chemical mechanism remains unclear. In this study, an unexpected ring-contraction 3(2H)-furanone and two quinoid-compounds were identified as the major and minor reaction products, respectively; and two unusual azido-substituted chloro-O-semiquinone radicals were detected and characterized as the major radical intermediates by complementary applications of direct ESR, HPLC/ESI-Q-TOF and high-resolution MS studies with nitrogen-15 isotope-labeled NaN3. Taken together, we proposed a novel molecular mechanism for the reaction of TCC/NaN3: N3- may attack on tetrachloro-O-semiquinone radical, forming two transient 4-azido-3,5,6-trichloro- and 4,5-diazido-3,6-dichloro-O-semiquinone radicals, consecutively. The second-radical intermediate may either undergo an unusual zwitt-azido cleavage to form the less-toxic ring-contraction 3(2H)-furanone product, or further oxidize to form the more toxic quinoid-product 4-amino-5-azido-3,6-dichloro-O-benzoquinone. A good correlation was observed between the biphasic formation of this toxic quinone due to the two competing decomposition pathways of the radical intermediate and the biphasic synergism between TCC and NaN3, which are dependent on their molar-ratios. This is the first report of detection and identification of two unique azido-substituted chloro-O-semiquinone radicals, and an unprecedented ring-contraction mechanism via an unusually mild and facile zwitt-azido rearrangement.