47 - Synergism between Tetrachlorocatechol and NaN3: Detection and Identification of Two Novel Azido-Substituted Chloro-O-Semiquinone Radicals

Abstract

We have shown previously that exposing bacteria to tetrachlorocatechol (TCC) and 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 quinoids were identified as the major and minor reaction products, respectively; and two unusual N3-substituted Cl-O-semiquinone radicals were detected and characterized as the major radicals by complementary application of direct ESR and HPLC/MS with N-15 isotope-labeled NaN3. We proposed a novel molecular mechanism for the reaction between TCC and NaN3: N3– may attack on Cl4-O-semiquinone radical, forming two transient 4-N3-3,5,6-Cl3- and 4,5-(N3)2-3,6-Cl2-O-semiquinone radicals consecutively. The 2nd radical may either undergo an unusual zwitt-azido cleavage to form the less-toxic 3(2H)-furanone product, or further oxidize to form the more toxic 4-NH2-5-N3-3,6-Cl2-O-benzoquinone. A good correlation was observed between the biphasic formation of this toxic quinone due to the two competing pathways of the radical intermediate and the biphasic synergism. This is the first report of detection and identification of two novel N3-substituted Cl-O-semiquinone radicals, and an unprecedented ring-contraction mechanism via an unusually facile zwitt-azido rearrangement.