Reactivity and transformation of antibacterial N-oxides in the presence of manganese oxide.

Organic N-oxides are an important structural class in many pharmaceutical and industrial chemicals. Little is known aboutthe potential transformation of organic N-oxides at the sediment-water interface. Veterinary antibacterial agents carbadox and olaquindox are examples of commonly used heterocyclic N-oxides. Investigation with various N-oxides including carbadox, olaquindox, quinoline N-oxide, and quindoxin revealed surprisingly high reactivity toward MnO2 for all of the compounds except olaquindox. Desoxycarbadox and quinoxaline, two structurally related compounds that lack an N-oxide functional group, showed much lower or no reactivity toward MnO2. Comparisons among the previous compounds indicate that N-oxide moiety is the primary reactive site to MnO2, and substitution at the alpha-C adjacent to the N-oxide group is critical in determining the overall reactivity. Reactions of N-oxides with MnO2 appeared to be oxidation, with generation of Mn2+ parallel to degradation of the parent organics. Product characterization confirmed that quinoline N-oxide and quindoxin transformed into 2-hydroxyquinoline and quinoxaline 2,3-diol, respectively, in reactions with MnO2. The transformation involves separate steps of N-oxide moiety deoxygenation and neighboring alpha-C hydroxylation as elucidated by 18O isotope experiments. All of the experimental results pointed to a mechanism that involves an N-oxide radical intermediate. This is the first study to report such transformation reactivity of organic N-oxides toward manganese oxide, offering a new degradation pathway that could be important for the fate of this group of compounds in the aquatic environment.