Strong Base Anion Exchange Selectivity of Nine Perfluoroalkyl Chemicals Relevant to Drinking Water

Selectivity with respect to chloride (K PFAS ∕ C ) was determined for nine drinking water relevant perfluoroalkyl and polyfluoroalkyl substances (PFAS): perfluoro-2-propoxypropanoic acid (GenX), five perfluoroalkyl carboxylic acids (PFCAs), and three perfluoroalkyl sulfonic acids (PFSAs). Three single-use strong base anion exchange gel resins were investigated, targeting drinking water relevant equilibrium PFAS liquid concentrations (≤500 ng/L). Except for the longest carbon chain PFCA (perfluorodecanoic acid) and PFSA (perfluorooctanesulfonic acid) studied, PFAS followed traditional ion exchange theory (law of mass action), including increasing equilibrium PFAS liquid concentrations with increasing equilibrium chloride liquid concentrations. Overall, K PFAS ∕ C values were (i) similar among resins for a given PFAS, (ii) 1-5 orders of magnitude greater than the selectivity of inorganic anions (e.g., nitrate) previously studied, (iii) 2 orders of magnitude greater for the same carbon chain length PFSA versus PFCA, (iv) found to proportionally increase with carbon chain length for both PFSAs and PFCAs, and (v) similar for GenX and perfluorohexanoic acid (six-carbon PFCA). A multisolute competition experiment demonstrated binary isotherm-determined K PFAS ∕ C values could be applied to simulate a multisolute system, extending work previously done with only inorganic anions to PFAS. Ultimately, estimated K PFAS ∕ C values allow future extension and validation of an open-source anion exchange column model to PFAS.

The Impacts of Potassium Permanganate and Powdered Activated Carbon on Cyanotoxin Release

Bench-scale trials were performed to: (1) Expose Microcystis aeruginosa cells to potassium permanganate doses of 1, 3 and 5 mg/L, at contact times of 15, 30 and 90 minutes, pH levels of 7 and 9, and turbidities of 0.1, 5 and 20 NTU; (2) Compare the impacts of oxidation alone and oxidation plus powdered activated carbon for the final 60 minutes of contact time and (3) evaluate the impact of these treatment conditions on extracellular microcystins, extra- plus intracellular (combined) microcystins, cell membrane integrity and chlorophyll-a concentrations. Toxin releases from the cells were observed at both pH levels. The greatest toxin releases were observed at the lowest KMnO₄ doses. The toxin releases were accompanied by relatively stable total cell counts, increases in membrane compromised cells and decreases in chlorophyll-a. The application of 10 mg/L PAC resulted in extracellular toxin concentrations that were markedly lower than those observed in oxidant-only situations.

Mechanistic Insights into the Hydrolysis of Organophosphorus Compounds by Paraoxonase-1: Exploring the Limits of Substrate Tolerance in a Promiscuous Enzyme

We designed, synthesized and screened a library of analogs of the organophosphate pesticide metabolite paraoxon against a recombinant variant of human serum paraoxonase-1. Alterations of both the aryloxy leaving group and the retained alkyl chains of paraoxon analogs resulted in substantial changes to binding and hydrolysis, as measured directly by spectrophotometric methods or in competition experiments with paraoxon. Increases or decreases in the steric bulk of the retained groups generally reduced the rate of hydrolysis, while modifications of the leaving group modulated both binding and turnover. Studies on the hydrolysis of phosphoryl azide analogs as well as amino-modified paraoxon analogs, the former being developed as photo-affinity labels, found enhanced tolerance of structural modifications, when compared with O-alkyl substituted molecules. Results from computational modeling predict a predominant active site binding mode for these molecules which is consistent with several proposed catalytic mechanisms in the literature, and from which a molecular-level explanation of the experimental trends is attempted. Overall, the results of this study suggest that while paraoxonase-1 is a promiscuous enzyme, there are substantial constraints in the active site pocket, which may relate to both the leaving group and the retained portion of paraoxon analogs.

Oxidative dehalogenation of perhalogenated benzenes by cytochrome P450 compound I

Resolution of the identity PBE (RI-PBE) and B3LYP density functional theory calculations are used to understand the cytochrome P450-catalyzed, Compound I-mediated oxidation of perchlorobenzenes, perfluorobenzenes, their phenols, and mixed chlorofluorobenzenes to form benzoquinones. Addition of Compound I to the chlorine-bearing carbon of perchlorobenzenes and perchlorophenols results in an apparently barrierless 1,2-shift of the chlorine atom to form hexachlorocyclohexadienones and hydroxypentachlorocyclohexadienones, respectively. Hexachlorocyclohexadienone has a significant electron affinity, and its radical anion expels chloride in a facile manner to give the pentachlorophenoxyl radical. Deprotonation of hydroxypentachlorocyclohexadienones results in the expulsion of chloride and provides a direct route to the production of tetrachloroquinones. Barrier heights for Compound I addition to fluorine-bearing carbons of hexafluorobenzene and pentafluorophenol are comparable to those computed for oxidation of benzene via an analogous reaction path. In contrast to the chlorinated cases, fluorine migration to cyclohexadienones occurs with a moderate barrier. Additionally, gas-phase elimination of fluoride from the hexafluorocyclohexadienone radical anion and deprotonated hydroxypentafluorocyclohexadienone are not facile. Rather, consideration of implicit and explicit solvent is required to achieve favorable thermochemistry for fluoride elimination and generation of the experimentally observed products. Finally, the theoretical approach described herein is predictive of the experimentally observed preferential elimination of fluorine from chloropentafluorobenzene and 1,3,5-trichloro-2,4,6-trifluorobenzene. These studies illustrate the effectiveness of P450 Compound I as an oxidant of halogenated aromatic hydrocarbons, which are persistent environmental contaminants, and the potential utility of such computational methods for predicting P450 metabolism.

Simultaneous and stereoselective formation of planar and axial chiralities in enantiopure sulfinyl iron diene complexes

[reaction: see text] Enantiopure (1Z,3E)-1-sulfinyl dienes bearing an o-dithianylphenyl group can be prepared and complexed with (bda)Fe(CO)(3) to afford the corresponding sulfinyl diene iron(0) tricarbonyl complexes. This diastereoselective complexation introduces planar and axial chirality simultaneously, with a high degree of facial selectivity as well as atropselectivity. Dynamic kinetic resolution is likely to be the origin of the atropselectivity.