Oxygen Radical Irradiation Transforms an Organic Fertilizer L-Tryptophan into an Environment and Human-Friendly Bactericide

Organic fertilizers are useful in hydroponic agriculture to bring about a sustainable society with minimal pollution. Irradiation of L-tryptophan (L-Trp) solutions with electrically neutral atomic oxygen produces immediate bactericidal activity. After oxygen radical irradiation of an L-Trp- containing Escherichia coli suspension at pH 6.3 for 5 min, a colony-forming unit assay showed a 6-log reduction in the number of surviving E. coli. Kynurenine and N'-formylkynurenine were identified as irradiation products in the solution using liquid chromatography-mass spectroscopy and nuclear magnetic resonance spectroscopy. Tryptophan radicals (Trp center dot) are precursors of kynurenine and N'-formylkynurenine and are detected using spin -trap electron spin resonance. The bactericidal effect is mediated by central carbon metabolism via enzyme deactivation in E. coli. Enzymatic deactivation via the oxygen radical irradiation of a solution containing the amino acid L-Trp eventually resulted in bacterial death.