Products in light-mediated reactions of free methionine-riboflavin mixtures that are biocidal to microorganisms

Abstract

Solutions of methionine (1 mM) plus riboflavin (26.6 μM) (MR mixture) in the presence of light are toxigenic to a wide spectrum of microorganisms. Agrobacterium tumefaciens and Verticillium dahliae are representative of these microorganisms and were used in this study. The biocidal activity of the MR mixture was significantly enhanced when aerated or supplemented with Cu2+ or Fe2+ ions; the increased toxicity suggested the involvement of the O2−-driven Haber–Weiss reaction, from which the associated production and reactivity of O2−, H2O2, and 1O2 may have contributed to the biocidal activity. Although the presence of EDTA did not lessen the toxicity of the MR mixture, the addition of butylated hydroxyanisole effectively quenched its biocidal activity. Supplementation of MR mixture with antioxidants like α-tocopherol and β-carotene also was effective in reducing its biocidal activity and suggested further that the production of the above-mentioned oxygen species was critical for a photodynamic toxigenic reaction. In addition to the involvement of oxygen derivatives, the role of various methionine photodegradation products in the biocidal reaction was also evaluated. Methional and acrolein were among the methionine-derived components that were toxic. The highest amounts of methional and acrolein detected in the MR reaction system during an 8-h period by HPLC were approximately 46.5 and 10.6 μM, respectively. However, to achieve the toxicity equivalent to that of the MR mixture, the production of more than 5 mM methional or 100 μM acrolein would be necessary. Thus, the overall toxicity of the MR mixture appeared to be associated with the generation of highly reactive oxygen radicals. Key words: methionine–riboflavin mixture, photodynamic toxicity, methional, acrolein, activated oxygen derivatives, Agrobacterium tumefaciens, Verticillium dahliae.