title: Rate-dependent carbon and nitrogen kinetic isotope fractionation in hydrolysis of isoproturon.
abstract: Stable isotope fractionation permits quantifying contaminant degradation in the field when the transformation reaction is associated with a consistent isotope enrichment factor epsilon. When interpreted in conjunction with dual isotope plots, isotope fractionation is also particularly useful for elucidating reaction mechanisms. To assess the consistency of epsilon and dual isotope slopes in a two-step reaction, we investigated the abiotic hydrolysis of the herbicide isoproturon (3-(4-isopropylphenyl)-1,1-dimethylurea) using a fragmentation method that allows measuring isotope ratios in different parts of the molecule. Carbon and nitrogen position-specific isotope fractionation, as well as slopes in dual isotope plots, varied linearly with rate constants k(obs) depending on the presence of buffers that mediate the initial zwitterion formation. The correlation can be explained by two consecutive reaction steps (zwitterion formation followed by dimethylamine elimination) each of which has a different kinetic isotope effect and may be rate-limiting. Intrinsic isotope effects for both steps, extracted from our kinetic data using a novel theoretical treatment, agree well with values computed from density functional calculations. Our study therefore demonstrates that more variable isotope fractionation may be observed in simple chemical reactions than commonly thought, but that consistent epsilon or dual isotope slopes may nonetheless be encountered in certain molecular fragments.
authors.affiliation: Institute of Groundwater Ecology, Helmholtz Zentrum München, German Research Center for Environmental Health, Ingolstädter Landstr. 1, 85764 Neuherberg, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Penning
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Cramer
authors.forename: Martin
authors.initials: M
authors.lastname: Elsner
mesh_terms: D002247:Carbon Isotopes; D005591:Chemical Fractionation / Q000379:methods*; D006868:Hydrolysis; D007700:Kinetics; D008956:Models, Chemical; D009587:Nitrogen Isotopes; D010671:Phenylurea Compounds / Q000737:chemistry*; D012044:Regression Analysis; D013696:Temperature
chemical_list: D002247:Carbon Isotopes; D009587:Nitrogen Isotopes; D010671:Phenylurea Compounds; C028904:isoproturon