Benzoxazinone-mediated triazine degradation: A proposed reaction mechanism

Authors: WILLETT, CAMMY - University Of Missouri; Lerch, Robert; LIN, CHUNG-HO - University Of Missouri; GOYNE, KEITH - University Of Missouri; LEIGH, NATHAN - University Of Missouri; ROBERTS, CRAIG - University Of Missouri

Submitted to: Journal of Agricultural and Food Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/2016
Publication Date: 5/23/2016
Citation: Willett, C.D., Lerch, R.N., Lin, C., Goyne, K.W., Leigh, N.D., Roberts, C.A. 2016. Benzoxazinone-mediated triazine degradation: A proposed reaction mechanism. Journal of Agricultural and Food Chemistry. 64(24):4858-4865. doi: 10.1021/acs.jafc.6b01017.

Interpretive Summary: Long-term and widespread use of the corn herbicide atrazine has led to significant contamination of surface and ground water resources across the U. S. Corn Belt. New strategies are needed to remove atrazine residues from soil and prevent its off-site transport. Forage grasses have been identified as potential sources of chemicals that can enhance the breakdown of atrazine in soils. The use of plants to rid the environment of a contaminant is called phytoremediation and this strategy has been applied to other point and non-point source pollutants. In previous work, a systematic assessment of phytochemicals in the atrazine tolerant forage, Eastern gamagrass (EG), was conducted and led to the discovery of a novel herbicide degrading chemical called DIBOA-Glc (DBG). DBG belongs to a class of well known plant defense chemicals called benzoxazinones (Bx). For over 60 years, researchers have speculated about the nature of the reaction between triazine herbicides, like atrazine, and Bx compounds. Here, we present for the first time a detailed description of the reaction between atrazine and DBG. Data from this investigation confirmed that DBG was degraded in the reaction, and definitively refutes the long-standing hypothesis that Bx compounds act as catalysts and are preserved in the reaction. In addition, we offer a new evidence-based hypothesis for the reaction mechanism that is more comprehensive than anything currently presented in the literature. The detailed explanation of this reaction indicated a slow reaction rate, suggesting that Bx compounds likely need to be used in combination with existing phytoremediation or agricultural management practices to further enhance atrazine degradation in soils and minimize contamination of water resources. Those benefitting include other researchers interested in mitigating herbicide transport from cropland or interested in the chemical behavior of Bx compounds but could ultimately benefit the public by leading to new strategies for reducing herbicide contamination in streams and drinking water sources.

Technical Abstract: The role of benzoxazinones (Bx, 2-hydroxy-2H-1,4-benzoxazin-3(4H)-one)) in triazine degradation and resistance has been studied for over half a century. In this research, the fundamental parameters of the reaction between DIBOA-Glc (2-ß-D-glucopyranosyloxy-4-hydroxy-1,4-benzoxazin-3-one) and atrazine (ATR, 6-chloro-N-ethyl-N’-(1-methylethyl)-1,3,5-triazine-2,4-diamine) were examined. Through a series of experiments employing a variety of chromatographic and spectroscopic techniques, the DIBOA-Glc-/ATR reaction was characterized in terms of reactant and product kinetics, stoichiometry, identification of a reaction intermediate, reaction products formed, and pH effects. Results of these experiments demonstrated that the reaction mechanism proceeds via nucleophilic attack of the hydroxamic acid moiety of DIBOA-Glc at the C-2 position of the triazine ring, forming hydroxyatrazine (HA, 2-hydroxy-4-ethylamino-6-isopropylamino-s-triazine), with associated degradation of DIBOA-Glc. The degradation of reactants followed 1st-order kinetics with a non-catalytic role of DIBOA-Glc, and the reaction rate increased with decreasing solution pH. A reaction intermediate was identified as a DIBOA-Glc-HA conjugate, indicating a 1:1 DIBOA-Glc:ATR stoichiometry. Reaction products included HA and Cl-, but definitive identification of DIBOA-Glc reaction product(s) was not attained. With these reaction parameters elucidated, DIBOA-Glc can be evaluated in terms of its potential for a myriad of applications, including its use to address the problem of widespread ATR contamination of water resources.