Murray State University
Mechanistic Studies of the Photodegradation of β-Carotene in Chloromethane Solvents
Institution
Murray State University
Faculty Advisor/ Mentor
Mark Masthay
Abstract
Light-activated herbicides used to kill broadleaf weeds in wheat and soybean agriculture mediate their toxicity by destroying antioxidant carotenes via an herbicide-to-carotene photoinduced electron transfer (PET) process. We find that orange solutions of β-carotene (βC) dissolved in carbon tetrachloride (CCl4), chloroform (CHCl3), and dichloromethane (CH2Cl2) solvents become colorless upon exposure to ultraviolet and visible light from mercury lamps. The color loss is induced by wavelengths that are absorbed by solvent (e.g., the 254 nm mercury line, which is absorbed by CCl4), as well as by wavelengths at which βC absorbs but the solvents are transparent (e.g., the 281.7, 313, and 436 nm mercury lines). Free radical quenchers slow the reaction rates significantly. In combination, these results suggest that the photodegradation proceeds via two competing mechanisms: (i) a βC-to-solvent PET mechanism in which the solvents dissociate to yield •CHxCl3-x radicals and Cl, which subsequently add across the double bonds of βC, and (ii) a solvent photolysis mechanism in which solvent molecules absorb light and dissociate to yield •Cl and •CHxCl3-x which add across the double bonds. In our poster, we detail these two proposed mechanisms, and discuss their significance with regard to the mode of action as well as the design of light-activated herbicides.
Mechanistic Studies of the Photodegradation of β-Carotene in Chloromethane Solvents
Light-activated herbicides used to kill broadleaf weeds in wheat and soybean agriculture mediate their toxicity by destroying antioxidant carotenes via an herbicide-to-carotene photoinduced electron transfer (PET) process. We find that orange solutions of β-carotene (βC) dissolved in carbon tetrachloride (CCl4), chloroform (CHCl3), and dichloromethane (CH2Cl2) solvents become colorless upon exposure to ultraviolet and visible light from mercury lamps. The color loss is induced by wavelengths that are absorbed by solvent (e.g., the 254 nm mercury line, which is absorbed by CCl4), as well as by wavelengths at which βC absorbs but the solvents are transparent (e.g., the 281.7, 313, and 436 nm mercury lines). Free radical quenchers slow the reaction rates significantly. In combination, these results suggest that the photodegradation proceeds via two competing mechanisms: (i) a βC-to-solvent PET mechanism in which the solvents dissociate to yield •CHxCl3-x radicals and Cl, which subsequently add across the double bonds of βC, and (ii) a solvent photolysis mechanism in which solvent molecules absorb light and dissociate to yield •Cl and •CHxCl3-x which add across the double bonds. In our poster, we detail these two proposed mechanisms, and discuss their significance with regard to the mode of action as well as the design of light-activated herbicides.