Article pubs.acs.org/JPCB

Alkaline Hydrolysis of Organophosphorus Pesticides: The Dependence of the Reaction Mechanism on the Incoming Group Conformation Edyta Dyguda-Kazimierowicz,* Szczepan Roszak, and W. Andrzej Sokalski Department of Chemistry, Wrocław University of Technology Wyb. Wyspiańskiego 27, 50-370 Wrocław, Poland S Supporting Information *

ABSTRACT: The fundamental mechanism of organophosphate hydrolysis is the subject of a growing interest resulting from the need for safe disposal of phosphoroorganic pesticides. Herein, we present a detailed ab initio study of the gas-phase mechanisms of alkaline hydrolysis of P−O and P−S bonds in a number of organophosphorus pesticides, including paraoxon, methyl parathion, fenitrothion, demeton-S, acephate, phosalone, azinophos-ethyl, and malathion. Our main finding is that the incoming group conformation influences the mechanism of decomposition of organophosphate and organothiophosphate compounds. Depending on the orientation of the attacking nucleophile, hydrolysis reaction might follow either a multistep pathway characterized by the presence of a pentavalent intermediate or a one-step mechanism proceeding through a single transition state. Despite a widely accepted view of the phosphotriester P−O bonds being decomposed exclusively via a direct-displacement mechanism, the occurrence of alternative, qualitatively distinct reaction pathways was confirmed for alkaline hydrolysis of both P−O and P−S bonds. As the pesticides included in our quantum chemical analysis involve organophosphate, phosphorothioate, and phosphorodithioate compounds, the influence of oxygen to sulfur substitution on the structural and energetic characteristics of the hydrolysis pathway is also discussed.

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earlier computational results12 concerning mechanisms of decomposition of organophosphate neurotoxins have already been utilized in the design process of a mononuclear zinc metalloenzyme capable of performing the hydrolysis of a coumarinyl analogue of the nerve agent cyclosarin.13 Our present work is focused on the degradation mechanisms of organophosphorus pesticides. In contrast to P−O bond containing compounds, which are efficiently decomposed by phosphotriesterase, the rate of hydrolysis of P−S bond, present in many organothiophosphate pesticides, is significantly lower.8 As evidenced by the experimental kinetic isotope effects for alkaline and enzymatic hydrolysis of the P−O bond in paraoxon,14 both reactions fit to the characteristics of an SN2like concerted associative mechanism. The resemblance of enzymatic and nonenzymatic phosphotriester degradation suggests that the knowledge of the mechanism of alkaline hydrolysis could provide a valid starting point for computational approaches directed toward improvement of PTE properties. In the case of hydrolysis of phosphotriester compounds, nucleophilic substitution at the phosphorus center appears to follow an associative pathway, for which two limiting scenarios exist15,16 (Scheme 1). Addition−elimination mechanism involves the presence of a pentacoordinate phosphorane

INTRODUCTION Organophosphorus compounds are among the most widely used pesticides worldwide. The accumulation of these highly toxic agents in water supplies and food products, increasing the risk of human exposure, raises environmental and health concerns regarding the availability of safe and economically feasible methods of detoxification.1 The nonharmful removal of organophosphate is therefore a matter of urgent need, as it would be applicable for not only environment decontamination, but also postexposure treatment or prophylactics. A promising detoxification approach involves enzymatic biodegradation.2−4 Among the several organophosphate degrading enzymes, the best characterized is a bacterial enzyme phosphotriesterase (PTE),5,6 which makes it a prospective candidate due to its ability to cleave various phosphorus-ester bonds7,8 and to hydrolyze one of its substrates, paraoxon, at the rate approaching the diffusion-controlled limit.9 Noticeably, the PTE catalytic activity can be modified with the aim of tailoring enzyme properties toward specific targets.10 The introduction of the rational biocatalyst design into the process of mutant development would save a significant amount of timeconsuming and expensive laboratory work. In the case of de novo enzyme design or redesign allowing for the novel catalytic activity to be introduced, computational approaches are required to provide a reasonable starting scaffold structure, that could further be refined with experimental techniques.11 However, prior to the computational enzyme design, a molecular level understanding of the reaction is needed. Our © XXXX American Chemical Society

Received: April 6, 2014 Revised: June 8, 2014

A

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Scheme 1. Variants of SN2-like Nucleophilic Substitution at the Phosphorus Center

Chart 1. Structures of the Organophosphorus Pesticides Considered in This Study

intermediate resulting in the triple-well shape of the potential energy surface. It can then be described as a two-step process composed of intermediate formation and its further decomposition. The direct-displacement pathway leads through a single SN2-like transition state directly toward products, which

is described by a double-well energy profile. The approach of the nucleophilic hydroxide ion is accompanied by the leaving group expulsion. Independently of the number of steps along the hydrolysis pathway, the favorable mechanism involves the entering and leaving groups positioned on the opposite sides of B

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the plane formed by the three remaining atoms bonded to the phosphorus center.17 The multistep pathway was shown to occur in the case of P− F bond hydrolysis of phosphofluoridate compounds (e.g., sarin, O,O-diisopropyl phosphorofluoridate,) as well as acephate (P− S bond), O,S-dimethyl methylphosphonothiolate (VX model compound; P−S bond) and tabun (P−CN bond).12,17−19 The two-step addition−elimination process described for these compounds involves a trigonal bipyramidal intermediate, the formation of which constitutes the rate-determining step. In contrast, P−O bond-containing compounds (e.g., paraoxon, parathion, fenitrothion) along with demeton-S (P−S bond) appear to be hydrolyzed via a single-step mechanism.12,18,20 According to our previous results, alkaline hydrolysis of organophosphorus compounds may proceed along two alternative pathways that differ in the conformation of the attacking hydroxide ion.12 In particular, the lower energy barrier pathway corresponds to the organophosphate species with hydroxide proton pointing in the direction of a phosphoryl oxygen atom. The similar conclusion was reached in the study of model systems of nucleophilic substitution at phosphorus atom, where this effect was ascribed to an intramolecular hydrogen bond with the phosphoryl oxygen atom.21 In this work we extend our previous study of alkaline hydrolysis of paraoxon, acephate, and demeton-S hydrolysis12 by employing a more accurate description of the reaction mechanism. Higher level of theory for the analysis of the potential energy surface enabled identification of new stationary points, changing the overall characteristics of the reaction pathway. For further study of the possible mechanisms of P−S bond hydrolysis, we selected the following organothiophosphate pesticides: malathion, phosalone, and azinophos-ethyl (Chart 1). Similarly to acephate and demeton-S, these compounds are decomposed by phosphotriesterase at a lower catalytic rate compared to paraoxon and the related P−O bond containing pesticides.8 To the best of our knowledge, there is no experimental data concerning the PTE ability to hydrolyze fenitrothion, another commonly used pesticide.22 Since fenitrothion differs from experimentally verified PTE substrate methyl parathion,8 only by the presence of additional methyl group at the phenyl ring (Chart 1), it is likely that PTE is capable of cleaving the P−O bond in fenitrothion as well. Thus, both fenitrothion and methyl parathion were also included in our study. Overall, organophosphorus pesticides considered in this work include organophosphate (paraoxon), phosphoramidothioate (acephate), and organothiophosphate compounds. Considering the type of the leaving group, organothiophosphate pesticides can further be classified into phenyl (methyl parathion, fenitrothion), heterocyclic (phosalone, azinophosethyl), and aliphatic (demeton-S, malathion) compounds. These species also differ by the number of phosphorus-bonded sulfur atoms. In particular, phosalone, azinophos-ethyl, and malathion containing two sulfur atoms belong to phosphorodithioates, while methyl parathion, fenitrothion, and demetonS possess a single sulfur atom (phosphorothioates). Considering the sulfur substitution site, P−S bond hydrolysis occurs in the case of demeton-S, whereas methyl parathion and fenitrothion feature phosphoryl sulfur atom (Chart 1). Such a choice of organophosphorus pesticides enables analysis of the influence of both sulfur substitution and the type of leaving group moiety on the mechanism of phosphotriester hydrolysis.

Article

COMPUTATIONAL DETAILS

The gas-phase reaction profiles were studied applying the density functional theory (DFT) functional B3LYP23,24 with the 6−31+G(d) basis set.25−27 The nature of stationary points was verified by vibrational analysis. For selected first-order saddle points, intrinsic reaction coordinate (IRC) calculations28 were performed to reveal the geometries of the local minima associated with a given transition state. The reported energy values refer to MP2/6−311++G(2d,2p) model chemistry27,29 applied to DFT-optimized structures (in what follows denoted as the MP2/6-311++G(2d,2p)//B3LYP/6−31+G(d) level of theory). Additionally, the model of malathion was simplified by truncation of the terminal ethyl groups belonging to the leaving group (see Chart 1 for the full structure). It has been suggested that in malathion biodegradation performed by soil microorganisms, the P-S bond cleavage is preceded by the hydrolysis of the aforementioned ethyl groups.30 Thermodynamic properties (enthalpies and Gibbs free energies) were determined at the B3LYP/6-31+G(d) level of theory from vibrational frequencies computed at the fully optimized structures of stationary points along a reaction coordinate. Unless stated otherwise, energy values reported herein include zero-point vibrational energy (ZPE). To account for the influence of aqueous solvation, polarizable continuum model (PCM)31 was applied in single point MP2/6-311+ +G(2d,2p) calculations employing gas-phase B3LYP/631+G(d) geometries. The reaction profiles including transition states are characterized by structures differing significantly regarding distribution of electronic density as an effect of formation and breaking of chemical bonds as well as the presence of quasibonds in transition states. The proper reproduction of these effects requires the flexible configurational space for the construction of wave function. The performance of single reference methods applied in this work was tested against multireference approaches. The single point calculations of energy for DFT optimized structures were performed within the complete active space self-consistent field (CASSCF) method32 and as MP2-level energy correlation correction to the CASSCF energy (CASMP2).33 The standard 6-31G(d) atomic basis set25,26 was applied. Since the active space was selected in the restricted way, no diffuse functions were included to reduce complications due to the lack of the localization of canonical orbitals. The active space was constructed from molecular orbitals corresponding to four (three single and one double) chemical bonds of phosphorus and two highest occupied molecular orbitals of the OH− group. The virtual space includes additional, left after bonding, molecular orbitals corresponding to four d orbitals of phosphorus. Such a choice leads to the space of 11 orbitals with distributed 14 electrons. Paraoxon, methyl parathion, and azinophos-ethyl were selected as compounds containing the representing moieties of the pesticides studied herein. The single and multireference results are qualitatively similar (Table S1 in Supporting Information) and in both cases are very sensitive to the amount of correlation energy included. The correlation energy leads to the decreasing of activation energy as well as energy of reactions. Because the selection of active space was performed independently for each complex, it may lead to not systematic results, which has to be treated with caution. On the contrary, MP2 calculations, taking into account the whole reference space, lead to stable results, and the selection of this method as a main thermodynamic data C

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directly associated with the reaction coordinate, i.e., P···OH and P···OLG (LG = leaving group) separation, are summarized in Table 1. Relative energies of the geometries along the reaction coordinate are reported in Table 2. In the case of fenitrothion, two alternative leaving group conformations are considered. The difference in the geometry of the reactant complexes is shown in Figure 2, whereas all the structures associated with both conformations are given in the Supporting Information. For all three compounds decomposed via P−O bond hydrolysis, two alternative reaction pathways were identified, in agreement with our previous results.12 A path is characterized by the hydroxide proton pointing in the same direction as the phosphoryl oxygen (sulfur) atom, whereas in the case of B path, it is positioned in the opposite way (Figure 1). Both A and B paths start from the same substrate, in what follows referred to as INTS, which represents the hydrogenbonded complex of a phosphotriester and the nucleophilic hydroxide ion. The P···OH distance in INTS geometries is equal to 3.83, 3.81, and 3.81 Å for paraoxon, methyl parathion, and fenitrothion, respectively (Figure 1 and Table 1). The difference in the hydroxide orientation emerges in the next stationary point, TSa or TSb transition state, and it persists throughout the remaining reaction pathway. The interaction between the hydroxide proton and the phosphoryl oxygen (sulfur) atom in TSa seems to prevent the closer approach of the nuclephile, as described by slightly larger P···OH distance compared to TSb structure (Table 1). In the case of paraoxon, P···OH separation in TSb is shorter by 0.12 Å relative to the corresponding value in the TSa geometry. The larger INTS→ TSb energy difference results in the more advanced B path transition state, consistent with the Hammond−Leffler postulate.35,36 Remarkably, the influence of alternative incoming group conformations manifests itself not only quantitatively (by altering the geometry and the height of an activation energy barrier), but most of all qualitatively, as A and B paths appear to follow different types of an associative mechanism. In particular, the presence of a trigonal bipyramidal intermediate, INTa, has only been confirmed for the A path. Accordingly, the A path corresponds to the addition−elimination mechanism composed of the intermediate formation and decomposition steps. Transition state structures for both these steps, TSa and TS2a, were characterized confirming the multistep characteristics of the A path. As the second energy barrier is substantially lower compared to the first energy of activation (Table 3 and Figure 3), the rate-limiting step of the alkaline hydrolysis of the P−O bond seems to be associated with the formation of the intermediate. In contrast to the multistep description of the A path, the transition state TSb is immediately followed by the reaction product complex, INTP. Any attempt of the reoptimization of INTa geometry by the modification of hydroxide ion position resulted in the leaving group departure. Interestingly, relaxed scans of potential energy surface (PES) corresponding to the rotation around the HO−P bond also exhibited the P−O bond cleavage in the vicinity of the 180° dihedral angle formed by HO−PO(S) atoms. PES scans performed for the P−OLG bond stretching were characterized by the steady decrease in energy, with no local minimum or maximum that might indicate the presence of the intermediate or the second transition state. Overall, the energy barrier for the A path intermediate formation is about 1 kcal·mol−1 lower compared to the direct decomposition occurring in the B path (Table 3).

source is reasonable. The contribution of excited configurations to the wave function is negligible, justifying the use of a single reference approach. All calculations were performed using the Gaussian 09 program.34

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RESULTS AND DISCUSSION Mechanisms of P−O Bond Hydrolysis. The reaction profiles obtained from our calculations for alkaline P−O bond hydrolysis refer to the structures of reaction stationary points presented in Figure 1 (paraoxon) and in the Supporting Information (methyl parathion and fenitrothion). The distances

Figure 1. B3LYP/6−31+G(d) geometries of the stationary points along a reaction coordinate for the alkaline hydrolysis of paraoxon (A and B paths). Distances indicated in Å. D

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Table 1. Selected Interatomic Distancesa Associated with B3LYP/6-31+G(d) Geometries Determined along a Reaction Coordinate for the Alkaline Hydrolysis of the Compounds Studied Herein paraoxon methyl parathion fenitrothion fenitrothionc demeton-S demeton-Sc acephate phosalone phosalonec azinophos-ethyl malathion a

P···OH P···OLG P···OH P···OLG P···OH P···OLG P···OH P···SLG P···OH P···SLG P···OH P···SLG P···OH P···SLG P···OH P···SLG P···OH P···SLG P···OH P···SLG P···OH P···SLG

INTS

TS

INT

TS2

3.83 1.64 3.81 1.64 3.81 1.64 3.92 1.64 4.27 2.12 4.18 2.11 3.76 2.17 4.26 2.13 4.01 2.14 4.29 2.13 4.12 2.14

3.08 (2.90)b 1.70 (1.71) 3.13 (2.96) 1.70 (1.71) 3.14 (2.96) 1.70 (1.71) 3.10 (2.95) 1.70 (1.71) 2.93 (2.75) 2.13 (2.19) 2.84 (2.71) 2.18 (2.19) 2.48 (2.39) 2.19 (2.21) 3.02 (2.88) 2.18 (2.19) 3.02 (2.96) 2.18 (2.19) 3.00 (2.89) 2.18 (2.19) 3.06 (2.94) 2.18 (2.19)

1.73 1.98 1.73 1.91 1.73 1.90 1.73 1.90 1.74 2.50 1.75 2.42 1.76 2.32 1.74 2.37 1.74 2.37 1.74 2.37 1.74 2.37

1.72 2.11 1.70 2.24 1.70 2.24 1.70 2.24 1.72 2.86 1.72 2.68 1.72 2.72 1.73 2.42 1.71 2.60 1.71 2.54 1.71 2.69

INTP 1.62 4.44 1.62 4.58 1.62 4.57 1.62 4.57 1.63 5.27 1.63 5.13 1.63 5.11 1.63 5.23 1.63 5.29 1.63 5.28 1.63 5.35

(1.62) (4.56) (1.62) (4.59) (1.62) (4.58) (1.62) (4.58) (1.62) (5.23) (1.62) (5.23) (1.63) (5.09) (1.62) (5.27) (1.62) (5.27) (1.62) (5.29) (1.62) (5.30)

In units of Å bIn parentheses given are the distances associated with B path structures. cA′, B′ path conformations.

Å, respectively. The corresponding values in the TS2a structure are equal to 2.11 and 2.24 Å. While the extent of P···OLG bond breakage in the TS2a is larger in phosphorothioates, intermediate geometry exhibits the opposite tendency described by larger P···OLG separation in phosphate compound, paraoxon. The greater similarity of INTa and TS2a geometries observed in the paraoxon hydrolysis is then consistent with the lower energy barrier for the intermediate decomposition. The analysis of the relative energy of consecutive stationary points for phosphate and phosphorothioates hydrolysis reveals little influence of sulfur substitution on the height of the energy barriers associated with TSa and TSb transition state (Table 2). Irrespectively of the phosphoryl atom type or the reaction mechanism (i.e., A or B path), all P-O bond hydrolyses considered in this work are exothermic with INTS→INTP energy difference exceeding −30 kcal·mol−1. Differences resulting from sulfur substitution occur in the intermediate decomposition step present in A path. In particular, phosphorothioates hydrolysis is associated with the more stable (relative to INTS) intermediate, INTa. The leaving group conformation of fenitrothion denoted as INTS in Figure 2 was selected based on the lower value of an activation energy barrier. The alternative conformation, distinguished by primes, differs by the placement of the phenyl ring-attached methyl group, which can either be positioned in a vicinity of the attacking nucleophile (A′ and B′ path geometries) or on the opposite side (A and B path geometries; see Figure 2 for comparison of A and A′ conformations). Accordingly, lower energy barrier A and B paths correspond to the methyl group positioned oppositely relative to the hydroxide ion. The gas-phase energy of such a conformation of substrate complex is slightly higher compared to the INT structure. Upon solvation, this energy difference becomes negligible (Table 4). Compared to the geometries obtained for methyl parathion hydrolysis, only the INTS′ and TSa′

Table 2. MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d) Relative Energiesa for the Hydrolysis of P−O Bond ΔEb paraoxon

methyl parathion

fenitrothion

INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP INTS′→TS′ INTS′→INT′ INT′→TS2′ INT′→INTP′ INTS′→INTP′

ΔGc

A path

B path

A path

B path

8.7 −16.5 0.4 −16.9 −33.5 8.7 −21.0 2.5 −12.7 −33.8 9.0 −20.9 2.5 −12.7 −33.6 10.1 −19.4 2.6 −12.7 −32.0

9.9

8.6 −16.8 0.7 −20.4 −37.1 8.8 −21.0 1.5 −17.2 −38.1 9.3 −20.7 1.1 −16.5 −37.1 10.1 −19.1 2.0 −17.1 −36.2

10.6

−33.8 10.2

−33.6 10.1

−33.4 11.7

−31.9

−37.3 10.7

−37.7 10.6

−36.7 12.2

−35.9

In units of kcal·mol−1. bΔE corresponds to the sum of electronic and zero-point energies. cΔG stands for the Gibbs free energy.

a

Sulfur substitution leads to the slight elongation of the P··· OH distance in both TSa and TSb geometries determined for phosphorothioate hydrolysis (Table 1). More pronounced differences occur in the case of P···OLG in the A path intermediate and the following transition state, i.e., INTa and TS2a structures. For instance, P···OLG separation in paraoxon and methyl parathion INTa geometry amounts to 1.98 and 1.91 E

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Figure 2. B3LYP/6-31+G(d) geometries of the alternative reactant conformations for the alkaline hydrolysis of (a) fenitrothion, (b) demeton-S, (c) phosalone. Selected interatomic distances (Å) are indicated.

structures along the reaction coordinate). Additionally, the alternative leaving group conformations of demeton-S and phosalone reactant complexes, INTS′, are compared to the corresponding INTS structures in Figure 2. Unless stated otherwise, A and B paths of demeton-S and phosalone hydrolysis are discussed in what follows. The overall description of the mechanisms presented herein is analogous to the results obtained for the P-O bond decomposition. Both A and B paths were identified, featuring either multistep addition−elimination characteristics with the presence of a pentacoordinate intermediate, INTa (A path) or a one-step mechanism proceeding via a single transition state, TSb (B path). Considering the energy barriers associated with these paths (Table 7 and Figures 3, 4), gas-phase B paths appear to be the most costly, whereas the rate-limiting step of A path, i.e., intermediate formation, is characterized by an activation energy value lower by about 1 kcal·mol−1. The occurrence of two qualitatively distinct reaction pathways for the alkaline hydrolysis of compounds studied here was further confirmed by demeton-S and phosalone IRC trajectories developed from TSa and TSb geometries (Figure 5). In particular, energy profiles corresponding to TSa→INTa reaction pathway exhibit little curvature in the vicinity of INTa geometry. Optimization of the last points of these IRC trajectories resulted in the actual intermediate geometry. While the energy gradient also changes significantly for the similar values of the TSb→INTPb reaction coordinate (i.e., for the structure resembling intermediate geometry), the PES curvature is still sufficient to promote further electronic and geometrical changes leading to final leaving group departure. It can be noticed in the A path energy profiles, that P−S bond stretching advances slowly until the INTa geometry is reached. In the case of B paths, the increase in P−S distance progresses continously until the product complex is reached. The differences in structures of organothiophosphates considered here are not systematic, which prevents direct assessment and generalization of the impact of sulfur substitution. Nevertheless, some conclusions can still be drawn upon careful comparison of selected compounds. The consequences of introducing sulfur atom in the phosphoester

Table 3. 6-311++G(2d,2p)//B3LYP/6-31+G(d) activation Energy Barriersa for the Hydrolysis of P−O Bond A path/Ib paraoxon

methyl parathion

fenitrothion

fenitrothiong

ΔEa ΔGae f ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a d

8.7 8.6 6.3 8.7 8.8 6.3 9.0 9.3 6.5 10.1 10.1 6.5

A path/IIc 0.4 0.7 0.1 2.5 1.5 2.5 2.5 1.1 2.6 2.6 2.0 2.6

B path 9.9 10.6 6.5 10.2 10.7 6.7 10.1 10.6 6.9 11.7 12.2 6.9

In units of kcal·mol−1. bIntermediate formation. cIntermediate decomposition. dΔEa corresponds to the sum of electronic and zero -point energies. eΔGa stands for the Gibbs free energy. fΔEPCM a represents electronic energy in solution. gA′, B′ path conformations. a

structures exhibit some differences regarding the P···OH distance, indicating the minor influence of the methyl substitution on the overall mechanism of hydrolysis reaction. Taking into account the ability of phosphotriesterase to decompose methyl parathion and other compounds with considerably larger leaving groups,7,8 it can be inferred from the similarity of gas-phase mechanisms of methyl parathion and fenitrothion alkaline hydrolysis that fenitrothion is also likely to be accepted by PTE as a substrate. In summary, B paths obtained here feature higher energy barriers compared to A paths (Table 3 and Figure 3). However, the difference in energy barrier height is not significant, and it actually disappears in the solution represented within the PCM approach. Mechanisms of P−S Bond Hydrolysis. The results obtained for the hydrolysis of pesticides containing the P−S bond, i.e., demeton-S, acephate, phosalone, azinophos-ethyl, and malathion are summarized in Table 1 (selected distances only; see Supporting Information for stationary point geometries) and Tables 5 and 6 (relative energy values of the F

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Figure 3. Potential energy surface along a reaction coordinate for the hydrolysis of (a) paraoxon, (b) methyl parathion, (c) fenitrothion, and (d) demeton-S. MP2/6−311++G(2d,2p) relative energies (normalized with respect to the reactants, INTS; ZPE included) were evaluated for structures fully optimized at the B3LYP/6−31+G(d) level of theory.

bond might be evaluated based on the results for demeton-S and paraoxon that differ in the leaving group structure (Chart 1). Compared to paraoxon, the structure of demeton-S reactant complex, INTS, is described by an elongated P···OH distance equal to 4.27 Å (Table 1). On the contrary, the corresponding values in demeton-S TSa and TSb geometries are decreased by 0.15 Å. Noticeably, the latter distances are slightly increased upon phosphoryl sulfur substitution. Assuming that the presence of P-S bond is the main reason for the observed structural differences between demeton-S and paraoxon, phosphoester and phosphoryl sulfur substitutions appear to exert the opposite effects in terms of the P···OH separation characterizing the TSa and TSb structures. Contrary to expectations, the larger structural differences between demeton-S INTS and TSa (TSb) geometries do not result in the greater height of the activation energy barriers. Apparently, the distance to be traveled by the incoming group while forming the transition state structure is not the only determinant of the corresponding energy barrier. Another compound featuring phosphoryl oxygen and P−S bond, i.e., acephate, was not included in this comparison due to more pronounced structural diversity including different protonation state of the reactant complex, which will be discussed in what follows. In the case of phosphorodithioates containing both the phosphoester and phosphoryl sulfur atoms, P···OH distances in TSa and TSb transition state structures resemble the analogous

property characterizing organophosphate with the phosphoester and phosphoryl oxygen atoms, i.e., paraoxon. Presumably, contrary effects of oxygen to sulfur exchange that depend on the substitution site, manifesting themselves in varied nucleophile-phosphorus center separation in TSa (TSb) geometry, balance each other out when both oxygen atoms are replaced with sulfur. Despite apparent differences in geometrical parameters pertaining to P-S bond containing compounds, the respective activation energy barriers do not seem to be affected by the phosphoryl sulfur substitution. Interestingly, rather than the sulfur substitution, it is the leaving group conformation that seems to exert the most pronounced effect on the energy barrier height. It can be seen in the case of alternative conformations of demeton-S leaving group, that a certain geometry of the latter might double the values of the activation energy barriers for both A′ and B′ paths (Table 7). As evidenced by IRC results obtained for A, A′ and B, B′ paths of demeton-S hydrolysis (Figure 5), leaving group conformation affects only the relative energies of the structures along the reaction coordinate, while the qualitative description of the multistep and one-step reaction pathways remains the same. Considering that the lower energy barrier pathway corresponds to the higher energy leaving group conformation (Table 4), the question remains whether the energetically more feasible pathway is, in fact, accessible to the reacting system. While the energy differences between alternative INTS G

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Table 4. 6-311++G(2d,2p)//B3LYP/6-31+G(d) Relative Energiesa for the Alternative Conformations of the Stationary Points along Reaction Coordinate of the Hydrolysis of Fenitrothion, Demeton-S, and Phosalone ΔE fenitrothion

demeton-S

phosalone

ΔG

b

−1.6 −0.4 −0.1 −0.0 0.1 0.0 −0.1 −10.7 −3.9 −4.0 −1.1 −1.3 −1.9 −1.1 −1.5 0.0 1.7 1.4 2.7 0.0 0.0

INTS′−INTS TSa′−TSa TSb′−TSb INTa′−INTa TS2a′−TS2a INTPa′−INTPa INTPb′−INTPb INTS′-INTS TSa′−TSa TSb′−TSb INTa′−INTa TS2a′−TS2a INTPa′−INTPa INTPb′−INTPb INTS′−INTS TSa′−TSa TSb′−TSb INTa′−INTa TS2a′−TS2a INTPa′−INTPa INTPb′−INTPb

Table 6. 6-311++G(2d,2p)//B3LYP/6-31+G(d) Relative Energiesa for the Hydrolysis of P−S Bond in Phosalone, Azinophos-Ethyl, and Malathion ΔEb

ΔE

c

PCMd

−1.6 −0.8 0.0 0.0 0.9 −0.6 −0.7 −8.2 −4.4 −3.2 −0.7 −0.6 −2.7 −0.4 −0.9 0.0 1.5 1.1 1.0 0.5 0.0

0.1 0.1 0.1 0.0 0.0 0.0 −0.1 −2.0 0.7 −2.2 −0.5 −0.9 −0.9 −1.0 2.4 0.0 −1.5 0.9 1.4 1.4 0.0

phosalone

azinophos-ethyl

malathion

ΔE demeton-S

acephated

INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP INTS′→TS′ INTS′→INT′ INT′→TS2′ INT′→INTP′ INTS′→INTP′ INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP

5.7 −17.9 0.4 −14.5 −32.3 12.5 −8.2 0.2 −15.4 −23.6 36.3 21.0 2.1 −5.5 15.5

ΔG

B path 6.5

−34.2 13.2

−24.6 40.2

15.0

A path 8.2 −17.3 1.1 −17.0 −34.3 12.0 −9.8 1.1 −19.1 −28.9 38.7 23.1 1.9 −8.6 14.5

B path

A path

B path

4.8 −24.7 0.2 −12.4 −37.1 6.3 −21.7 1.5 −13.8 −35.5 5.2 −24.1 −1.4 −15.5 −39.6 1.5 −28.4 3.7 −10.5 −39.0

6.3

6.7 −23.7 1.4 −16.6 −40.3 7.6 −21.7 1.3 −17.2 −38.9 6.2 −23.3 −0.3 −18.4 −41.7 2.9 −26.9 2.9 −14.2 −41.1

7.8

−37.3 9.5

−35.7 5.6

−40.0 2.6 −22.6 −16.1 −38.7

−39.7 10.2

−38.9 7.1

−41.4 4.6 −21.2 −19.6 −40.8

In units of kcal·mol−1. bΔE corresponds to the sum of electronic and zero-point energies. cΔG stands for the Gibbs free energy.

Table 7. 6-311++G(2d,2p)//B3LYP/6-31+G(d) activation energy barriersa for the hydrolysis of P-S bond.

Table 5. 6-311++G(2d,2p)//B3LYP/6-31+G(d) Relative Energiesa for the Hydrolysis of P−S Bond in Demeton-S and Acephate A path

A path

a

In units of kcal·mol−1. bΔE corresponds to the sum of electronic and zero-point energies. cΔG stands for the Gibbs free energy. dΔEPCM represents the electronic energy in solution. a

b

INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP INTS′→TS′ INTS′→INT′ INT′→TS2′ INT′→INTP′ INTS′→INTP′ INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP INTS→TS INTS→INT INT→TS2 INT→INTP INTS→INTP

ΔGc

A path/Ib demeton-S

c

B path

demeton-Sg

8.2 acephateh −36.7 13.2

phosalone

phosaloneg −29.0 42.5 azinophos-ethyl

13.2

malathion

In units of kcal·mol−1. bΔE corresponds to the sum of electronic and zero-point energies. cΔG stands for the Gibbs free energy. dResults for the first step of acephate hydrolysis correspond to the reaction between a water molecule and a deprotonated acephate. a

ΔEa ΔGae f ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a ΔEa ΔGa ΔEPCM a d

5.7 8.2 4.7 12.5 12.0 7.4 36.3 38.7 35.1 4.8 6.7 7.0 6.3 7.6 4.6 5.2 6.2 7.4 1.5 2.9 4.0

A path/IIc 0.4 1.1 −0.4 0.2 1.1 −0.8 2.1 1.9 2.1 0.2 1.4 0.5 1.5 1.3 1.0 −1.4 −0.3 −0.0 3.7 2.9 2.8

B path 6.5 8.2 7.6 13.2 13.2 7.4 40.2 42.5 36.4 6.3 7.8 7.4 9.5 10.2 3.5 5.6 7.1 6.6 2.6 4.6 3.9

In units of kcal·mol−1. bIntermediate formation. cIntermediate decomposition. dΔEa corresponds to the sum of electronic and zero -point energies. eΔGa stands for the Gibbs free energy. fΔEaPCM represents electronic energy in solution. gA′, B′ path conformations h Results for the first step of acephate hydrolysis correspond to the reaction between a water molecule and a deprotonated acephate. a

geometries of fenitrothion and phosalone are insignificant, the reactant complex of demeton-S exhibiting the bent-chain conformation of the aliphatic leaving group (INTS′; Figure 2) is more stable relative to the straight-chain geometry, INTS, by as much as 10.7 kcal·mol−1 (Table 4). This energy difference is considerably decreased upon solvation, which suggests that H

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Figure 4. Potential energy surface along a reaction coordinate for the hydrolysis of (a) phosalone, (b) azinophos-ethyl, (c) malathion and (d) acephate. MP2/6-311++G(2d,2p) relative energies (normalized with respect to the reactants, INTS; ZPE included) were evaluated for structures fully optimized at the B3LYP/6-31+G(d) level of theory. Results for the first step of acephate hydrolysis correspond to the reaction between water and deprotonated acephate.

corresponding relaxed PES scan starting from TSb structure revealed the absence of any local energy minimum or maximum upon stretching the P−S bond (Supporting Information). In the case of the A path, the analysis of an analogous PES scan provided the approximate structures of INTa and TS2a points very similar to the separately optimized ones. Thus, the INTb intermediate seems to be decomposed via a barrierless step. The current analysis of acephate hydrolysis, performed at a higher level of theory compared to the previous results,12 essentially confirmed our initial findings with respect to the substrate protonation state. In particular, despite identification of the transition state corresponding to the hydroxide to phosphorus attachment, TS1, the reactant complex, INTS, involves the water molecule interacting with a protonated acephate. However, in addition to the previously identified A path, which follows a multistep addition−elimination mechanism, our current results revealed the presence of a single-step B path. The IRC trajectories tracing the entire A and B path

one way to facilitate the reaction might be by designing the environment that would stabilize certain reactant complex conformation. It has been noted repeatedly that intermediate formation constitutes the rate-limiting step of the addition−elimination mechanism. Malathion hydrolysis constitutes an exception to this generalization, as the energy barrier for the intermediate decomposition step exceeds that for its formation. Of all the compounds studied here, the malathion degradation is described by the lowest energy barrier for the intermediate formation step of both the A path and the B path. Upon the solvent-induced change in the barrier heights, their values become almost equal. Another exception encountered during the analysis of malathion hydrolysis is the presence of a phosphorane intermediate in the B path. Despite our best efforts, no structure of the second transition state related to the intermediate decomposition was found. The reason for this failure might be associated with the shape of the PES, as the I

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Figure 5. Energy profiles for alkaline hydrolysis of demeton-S and phosalone: (a) A, A′ paths (demeton-S); (b) B, B′ paths (demeton-S); (c) A path (phosalone); (d) B path (phosalone). Potential energy changes (dotted lines; ZPE not included) with respect to the optimized reactants structure, INTS (INTS′ in the case of demeton-S) were generated by B3LYP/6-31+G(d) IRC calculations starting with (a) TSa, TSa′, (b) TSb, TSb′, (c) TSa, and (d) TSb geometries. Reaction coordinate “0” corresponds to the transition state structures. In the case of B (B′) paths negative and positive values of the reaction coordinate indicate the directions toward reactants (INTS) and products (INTPb), respectively. A (A′) path trajectories develop in the direction of the reaction intermediate, INT. Given in parentheses are the energy differences (in units of kcal·mol−1) of the last point of IRC trajectory relative to the nearest stationary point. Variation in the interatomic distances is given for a phosphorus−hydroxide oxygen as well as phosphorus−sulfur atoms separation.

the results already discussed). The geometrical and energetic properties of the corresponding reaction pathways are described in detail in the Supporting Information. Overall, it has been confirmed that it is the value of the activation energy barrier that is influenced by the change in the leaving group geometry, while the reaction mechanism remains essentially the same. The extent, to which the energy barrier height is affected, might be very different. In the case of acephate, the change in the leaving group geometry does not seem to exert any effect on the values of the energy barriers. Compared to the A and B reaction pathways of malathion hydrolysis, increased values of the energy barriers were found for the A′, B′ paths with different leaving group conformation. These values could be further increased if yet another conformation of the malathion departing group is considered (see the results for A″, B″ pathways in the Supporting Information). On the other hand, a substantial decrease in the energy barrier values was obtained for paraoxon and methyl parathion gas-phase hydrolysis encompassing a nonsymmetrical

reaction coordinates (Figure 6) confirm that both TSa and TSb transition state structures are obtained starting from water nucleophile and a protonated phosphotriester molecule. It is, however, impossible to directly determine an energy barrier associated with the formation of a hydroxide−acephate adduct. Our estimates based on the analysis of distances describing the proton position resulted in the values of 3.5 and 3.2·kcal·mol−1 (for A and B path, respectively), which is equal to the half of a barrier height obtained previously at the HF/6−31+G(d) level of theory.12 It has already been pointed out that both the incoming and leaving group could adapt various conformations. Different incoming or leaving group conformations might, in turn, affect the reaction energy barriers or reaction energy values to various extents. To further explore this issue, alternative conformations of the departing group were determined for paraoxon, methyl parathion, acephate, azinophos-ethyl, and malathion substrates. In the case of azinophos-ethyl and malathion, two different leaving group conformations were characterized (in addition to J

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Figure 6. Energy profile for the acephate hydrolysis. Potential energy changes (black dotted line; ZPE not included) with respect to the optimized reactants structure, INTS, were generated by B3LYP/6−31+G(d) IRC calculations starting with (a) TSa, (b) TS2a, and (c) TSb structures (reaction coordinate “0”). Variation in the interatomic distances is given for a phosphorus−hydroxide oxygen, phosphorus−sulfur, water molecule hydrogen− hydroxide oxygen, as well as water molecule hydrogen−nitrogen atoms separation. The values of 3.5 (3.2) kcal·mol−1 reflect the difference between the TS1a (TSb) energy and the energy associated with a reaction coordinate, where the curves describing O−H and N−H distances intersect each other.

could also assist the rational modification of the enzyme catalytic activity aimed at improvement of the enzyme efficiency or substrate specificity.

conformation of the leaving group. In general, these energy barrier differences are less pronounced upon solvation. Nevertheless, promoting a certain leaving group geometry might provide a way of controlling the energetic feasibility of the decomposition reaction. There has been a general agreement on the greater similarity between the enzymatic and gas-phase transition states as opposed to the relevant process in the solution.37,38 Recently published results encompassing explicit determination of the protein dielectric constants confirm that the electrostatic properties of the protein interior resemble the gas-phase rather than the solvent environment.39 In line with the theoretical study of PTE-catalyzed hydrolysis of paraoxon, showing the presence of pentavalent intermediate,40 our present results confirmed that a similar reaction scenario might also occur under the gas-phase conditions. As evidenced by application of our earlier results12 in the protocol of de novo enzyme design,13 the molecular-level understanding of the nonenzymatic reaction mechanism constitutes an important step in the development of a novel biocatalyst. The detailed knowledge of the factors affecting the reaction scenario including its energetic feasibility

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CONCLUSIONS The quantum chemical study of the gas-phase alkaline hydrolysis of P−O and P−S bonds in eight organophosphate pesticides has led us to the following conclusions: • All the reaction mechanisms reported herein comprise variants of an associative SN2-like mechanism. • The conformation of the attacking nucleophile (i.e., the entering group) triggers the reaction pathway that will be followed, since different reaction mechanisms are associated with hydroxide proton pointing in the same or in the opposite direction relative to the phosphoryl oxygen (sulfur) atom. • Multistep addition−elimination mechanism is observed for the nucleophile conformation in which the hydroxide proton points in the same direction as the phosphoryl oxygen (sulfur) atom. The rate-limiting step encompasses formation of the intermediate. K

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• The single-step direct-displacement mechanism is followed by organophosphate species that feature hydroxide proton pointing in the opposite direction relative to the phosphoryl oxygen (sulfur) atom. • The most energetically feasible reaction pathway corresponds to the addition−elimination mechanism occurring for hydroxide proton positioned in the vicinity of phosphoryl oxygen (sulfur) atom. Presumably, stabilization of the attached hydroxyl group resulting from the short contact with the phosphoryl atom favors this particular reaction course. • Alternative leaving group conformations influence the height of an energy barrier, while the reaction mechanism remains unchanged. • Considering multiple leaving group conformations, lower energy conformers appear to be hydrolyzed via the higher energy barrier. In the solution, however, the energy difference between alternative conformers decreases significantly, so that the lower energy barrier path becomes more accessible.

(3) Zhang, J. L.; Qiao, C. L. Novel Approaches for Remediation of Pesticide Pollutants. Int. J. Environ. Pollut. 2002, 18, 423−433. (4) Raushel, F. M. Bacterial Detoxification of Organophosphate Nerve Agents. Curr. Opin. Microbiol. 2002, 5, 288−295. (5) Dumas, D. P.; Caldwell, S. R.; Wild, J. R.; Raushel, F. M. Purification and Properties of the Phosphotriesterase from Pseudomonas diminuta. J. Biol. Chem. 1989, 264, 19659−19665. (6) Ghanem, E.; Raushel, F. M. Detoxification of Organophosphate Nerve Agents by Bacterial Phosphotriesterase. Toxicol. Appl. Pharmacol. 2005, 207, S459−S470. (7) Dumas, D. P.; Durst, H. D.; Landis, W. G.; Raushel, F. M.; Wild, J. R. Inactivation of Organophosphorus Nerve Agents by the Phosphotriesterase from Pseudomonas Diminuta. Arch. Biochem. Biophys. 1990, 277, 155−159. (8) Lai, K.; Stolowich, N. J.; Wild, J. R. Characterization of P−S Bond Hydrolysis in Organophosphorothioate Pesticides by Organophosphorus Hydrolase. Arch. Biochem. Biophys. 1995, 318, 59−64. (9) Caldwell, S. R.; Newcomb, J. R.; Schlecht, K. A.; Raushel, F. M. Limits of Diffusion in the Hydrolysis of Substrates by the Phosphotriesterase from Pseudomonas Diminuta. Biochemistry 1991, 30, 7438−7444. (10) Hill, C. M.; Li, W.-S.; Thoden, J. B.; Holden, H. M.; Raushel, F. M. Enhanced Degradation of Chemical Warfare Agents through Molecular Engineering of the Phosphotriesterase Active Site. J. Am. Chem. Soc. 2003, 125, 8990−8991. (11) Röthlisberger, D.; Khersonsky, O.; Wollacott, A. M.; Jiang, L.; DeChancie, J.; Betker, J.; Gallaher, J. L.; Althoff, E. A.; Zanghellini, A.; Dym, O.; Albeck, S.; Houk, K. N.; Tawfik, D. S.; Baker, D. Kemp Elimination Catalysts by Computational Enzyme Design. Nature 2008, 453, 190−195. (12) Dyguda-Kazimierowicz, E.; Sokalski, W. A.; Leszczyski, J. GasPhase Mechanisms of Degradation of Hazardous Organophosphorus Compounds: Do They Follow a Common Pattern of Alkaline Hydrolysis Reaction as in Phosphotriesterase? J. Phys. Chem. B 2008, 112, 9982−9991. (13) Khare, S. D.; Kipnis, Y.; Greisen, P. J.; Takeuchi, R.; Ashani, Y.; Goldsmith, M.; Song, Y. F.; Gallaher, J. L.; Silman, I.; Leader, H.; Sussman, J. L.; Stoddard, B. L.; Tawfik, D. S.; Baker, D. Computational Redesign of a Mononuclear Zinc Metalloenzyme for Organophosphate Hydrolysis. Nat. Chem. Biol. 2012, 8, 294−300. (14) Caldwell, S. R.; Raushel, F. M.; Weiss, P. M.; Cleland, W. W. Transition-State Structures for Enzymic and Alkaline Phosphotriester Hydrolysis. Biochemistry 1991, 30, 7444−7450. (15) Knowles, J. R. Enzyme-Catalyzed Phosphoryl Transfer Reactions. Annu. Rev. Biochem. 1980, 49, 877−919. (16) Cox, J. R., Jr.; Ramsay, O. B. Mechanisms of Nucleophilic Substitution in Phosphate Esters. Chem. Rev. 1964, 64, 317−352. (17) Xiong, Y.; Zhan, C.-G. Reaction Pathways and Free Energy Barriers for Alkaline Hydrolysis of Insecticide 2-Trimethylammonioethyl Methylphosphonofluoridate and Related Organophosphorus Compounds: Electrostatic and Steric Effects. J. Org. Chem. 2004, 69, 8451−8458. (18) Zheng, F.; Zhan, C.-G.; Ornstein, R. L. Theoretical Studies of Reaction Pathways and Energy Barriers for Alkaline Hydrolysis of Phosphotriesterase Substrates Paraoxon and Related Toxic Phosphofluoridate Nerve Agents. J. Chem. Soc., Perkin Trans. 2001, 2, 2355− 2363. (19) Šeckute, J.; Menke, J. L.; Emnett, R. J.; Patterson, E. V.; Cramer, C. J. Ab Initio Molecular Orbital and Density Functional Studies on the Solvolysis of Sarin and O,S-Dimethyl Methylphosphonothiolate, a VX-like Compound. J. Org. Chem. 2005, 70, 8649−8660. (20) Mandal, D.; Mondal, B.; Das, A. K. Nucleophilic Degradation of Fenitrothion Insecticide and Performance of Nucleophiles: A Computational Study. J. Phys. Chem. A 2012, 116, 2536−2546. (21) van Bochove, M. A.; Swart, M.; Bickelhaupt, F. M. Nucleophilic Substitution at Phosphorus Centers (SN2@P). ChemPhysChem 2007, 8, 2452−2463.

ASSOCIATED CONTENT

S Supporting Information *

Table containing selected calibration data from HF, CASSCF, MP2, CASMP2, and DFT calculations. Illustrations of stationary point geometries along A′ and B′ paths of fenitrothion, demeton-S, and phosalone hydrolysis and relaxed potential energy surface scans corresponding to the malathion intermediate decomposition. Detailed structural and energetic characteristics of the reaction pathways encompassing alternative leaving group conformations of paraoxon, methyl parathion, acephate, azinophos-ethyl, and malathion. This material is available free of charge via the Internet at http:// pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Wrocław Research Centre EIT+ under the project BIOMED “Biotechnologies and advanced medical technologies” (POIG 01.01.02-02-003/08) financed from the European Regional Development Fund Operational Programme Innovative Economy 1.1.2. We also thank Wrocław University of Technology for support. The project was financed in part by a statutory activity subsidy from the Polish Ministry of Science and Higher Education for the Faculty of Chemistry of Wroclaw University of Technology. Calculations were performed at the Wrocław Supercomputer and Networking Center (WCSS), Poznań Supercomputer and Networking Center (PCSS), and the Interdisciplinary Center for Modeling (ICM) in Warsaw.

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REFERENCES

(1) Raushel, F. M. Chemical Biology: Catalytic Detoxification. Nature 2011, 469, 310−311. (2) Singh, B. K.; Walker, A. Microbial Degradation of Organophosphorus Compounds. FEMS Microbiol. Rev. 2006, 30, 428−571. L

dx.doi.org/10.1021/jp503382j | J. Phys. Chem. B XXXX, XXX, XXX−XXX

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Article

(22) Badach, H.; Nazimek, T.; Kamiñska, I. A. Pesticide Content in Drinking Water Samples Collected from Orchard Areas in Central Poland. Ann. Agric. Environ. Med. 2007, 14, 109−114. (23) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (24) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields. J. Phys. Chem. 1994, 98, 11623−11627. (25) Hariharan, P. C.; Pople, J. A. The Influence of Polarization Functions on Molecular-Orbital Hydrogenation Energies. Theoret. Chimica Acta 1973, 28, 213−222. (26) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; DeFrees, D. J.; Pople, J. A.; Gordon, M. S. Self-Consistent Molecular Orbital Methods. XXIII. A Polarization-type Basis Set for Second-Row Elements. J. Chem. Phys. 1982, 77, 3654−3665. (27) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. SelfConsistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phys. 1980, 72, 650−654. (28) Gonzalez, C.; Schlegel, H. B. An Improved Algorithm for Reaction Path Following. J. Chem. Phys. 1989, 90, 2154−2161. (29) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; v. R. Schleyer, P. Efficient Diffuse Function-Augmented Basis-Sets for Anion Calculations. III. The 3-21+G Basis Set for 1st-Row Elements, Li−F. J. Comput. Chem. 1983, 4, 294−301. (30) Karpouzas, D. G.; Singh, B. K. Microbial Degradation of Organophosphorus Xenobiotics: Metabolic Pathways and Molecular Basis. Adv. Microb. Physiol. 2006, 51, 119−185. (31) Cancés, E.; Mennucci, B.; Tomasi, J. A New Integral Equation Formalism for the Polarizable Continuum Model: Theoretical Background and Applications to Isotropic and Anistropic Dielectrics. J. Chem. Phys. 1997, 107, 3032−3041. (32) Eade, R. H. A.; Robb, M. A. Direct Minimization in MC SCF Theory - The Quasi-Newton Method. Chem. Phys. Lett. 1981, 83, 362−368. (33) McDouall, J. J. W.; Peasley, K.; Robb, M. A. A Simple MC-SCF Perturbation Theory: Orthogonal Valence Bond Møller-Plesset 2 (OVB-MP2). Chem. Phys. Lett. 1988, 148, 183−189. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F. et al. Gaussian 09, revision B.01; Gaussian Inc.: Wallingford, CT, 2009. (35) Hammond, G. S. A Correlation of Reaction Rates. J. Am. Chem. Soc. 1955, 77, 334−338. (36) Leffler, J. E.; Grunwald, E. Rates and Equilibria of Organic Reactions, 1st ed.; Wiley: New York, 1963. (37) Dewar, M. J.; Storch, D. M. Alternative View of Enzyme Reactions. Proc. Natl. Acad. Sci. U.S.A. 1985, 82, 2225−2229. (38) Devi-Kesavan, L. S.; Gao, J. Combined QM/MM Study of the Mechanism and Kinetic Isotope Effect of the Nucleophilic Substitution Reaction in Haloalkane Dehalogenase. J. Am. Chem. Soc. 2003, 125, 1532−1540. (39) Kukic, P.; Farrell, D.; McIntosh, L. P.; García-Moreno, B.; Jensen, K. S.; Toleikis, Z.; Teilum, K.; Nielsen, J. E. Protein Dielectric Constants Determined from NMR Chemical Shift Perturbations. J. Am. Chem. Soc. 2013, 135, 16968−16976. (40) Chen, S.-L.; Fang, W.-H.; Himo, F. Theoretical Study of the Phosphotriesterase Reaction Mechanism. J. Phys. Chem. B 2007, 111, 1253−1255.

M

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