Ferredoxin-mediated reduction of 2-nitrothiophene inhibits photosynthesis: mechanism and herbicidal potential

Searching for compounds that inhibit the growth of photosynthetic organisms highlighted a prominent effect at micromolar concentrations of the nitroheteroaromatic thioether, 2-nitrothiophene, applied in the light. Since similar effects were reminiscent to those obtained also by radicals produced under excessive illumination or by herbicides, and in light of its redox potential, we suspected that 2-nitrothiophene was reduced by ferredoxin, a major reducing compound in the light. In silico examination using docking and tunneling computing algorithms of the putative interaction between 2-nitrothiophene and cyanobacterial ferredoxin has suggested a site of interaction enabling robust electron transfer from the iron-sulfur cluster of ferredoxin to the nitro group of 2-nitrothiophene. ESR and oximetry analyses of cyanobacterial cells (Anabaena PCC7120) treated with 50 μM 2-nitrothiophene under illumination revealed accumulation of oxygen radicals and peroxides. Gas chromatography mass spectrometry analysis of 2-nitrothiophene-treated cells identified cytotoxic nitroso and non-toxic amino derivatives. These products of the degradation pathway of 2-nitrohiophene, which initializes with a single electron transfer that forms a short-live anion radical, are then decomposed to nitrate and thiophene, and may be further reduced to a nitroso hydroxylamine and amino derivatives. This mechanism of toxicity is similar to that of nitroimidazoles (e.g. ornidazole and metronidazole) reduced by ferredoxin in anaerobic bacteria and protozoa, but differs from that of ornidazole in planta.