Kinetics of Cleavage of Paraoxon and Parathion by

Micellar cetyltrimethylammonium iodosobenzoate, (CTA)IBA, is highly reactive toward paraoxon (1) and parathion (2). In aqueous solution at pH 9, exces...
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Langmuir 2001, 17, 6108-6112

Kinetics of Cleavage of Paraoxon and Parathion by Cetyltrimethylammonium Iodosobenzoate Robert A. Moss,* Suseela Kanamathareddy, and Saketh Vijayaraghavan Department of Chemistry, Rutgers, The State University of New Jersey, New Brunswick, New Jersey 08903 Received March 26, 2001. In Final Form: June 12, 2001 Micellar cetyltrimethylammonium iodosobenzoate, (CTA)IBA, is highly reactive toward paraoxon (1) and parathion (2). In aqueous solution at pH 9, excess (CTA)IBA mediates their hydrolyses with kobs(max) ) 0.014 and 0.0030 s-1, respectively, corresponding to half-lives of 50 s and 3.8 min. Comparisons with other phosphorolytic reagents suggest that (CTA)IBA merits serious consideration for the remediation of paraoxon or parathion contamination.

Introduction Paraoxon (1) and parathion (2) are toxic, persistent pesticides with oral LD50 toxicities of 1.8 mg/kg (rat), 0.76 mg/kg (mouse) for 1 and 2.0 mg/kg (rat), 5.0 mg/kg (mouse) for 2.1 Their mode of action is analogous to that of the nerve agent sarin (3); i.e., phosphorylation and subsequent inactivation of the enzyme acetlylcholinesterase, which is essential for nerve impulse transmission.

The efficient chemical destruction of phosphonate nerve agents is an urgent and topical goal,1,2 and the remediation of paraoxon or parathion contamination is similarly important. As first noted in 1983,3 o-iodosobenzoate (4, IBA), in its preferred 1-oxido-1,2-benziodoxol-3(1H)-one form,4 is an effective catalyst for the rapid cleavage of “simulant” phosphates (e.g., p-nitrophenyl diphenyl phosphate, 5, PNPDPP),5 and also for nerve agents such as sarin and soman.6 To solubilize both IBA and the target substrate in the mildly basic aqueous solutions required for efficient IBA catalysis, the reagent is generally deployed in micellar solutions of cetyltrimethylammonium chloride (CTA)Cl.3,5

In 1986, we reported that the functional IBA surfactant 6, comicellized with 5-fold excess (CTA)Cl afforded a kinetic advantage of 43 600 in the cleavage of paraoxon (1) These data are from the National Toxicology Program website: http://ntp-server.niehs.nih.gov/. For 2, the human oral LD50 is 3 mg/kg. (2) (a) Yang, Y.-C. Acc. Chem. Res. 1999, 32, 109. (b) Yang, Y.-C. Chem. Ind. (London) 1995, 334. (c) Yang, Y.-C.; Baker J. A.; Ward, J. R. Chem. Rev. 1992, 92, 1729. (3) Moss, R. A.; Alwis, K.; Bizzigotti, G. O. J. Am. Chem. Soc. 1983, 105, 681. (4) Moss, R. A.; Vijayaraghavan, S.; Emge, T. J. Chem. Commun. 1998, 1559, and references cited therein. (5) Moss, R. A.; Alwis, K. W.; Shin, J.-S. J. Am. Chem. Soc. 1984, 106, 2651. (6) Hammond, P. S.; Forster, J. S.; Lieske, C. N.; Durst, H. D. J. Am. Chem. Soc. 1989, 111, 7860.

in pH 8 Tris buffer, relative to micellar (CTA)Cl.7 This encouraging result stimulated subsequent investigations of the reactivity of IBA toward paraoxon. Some of the results are summarized in Table 1.6-10 There is some uncertainty in the “base” rate constant for paraoxon cleavage in micellar (CTA)Cl alone (Table 1, entries 8 and 9), but it is clear that a catalytic advantage of 700-2000 can be obtained upon inclusion of IBA together with the (CTA)Cl in phosphate buffers (entries 1, 3, 4). Of greater interest is the observation of a 48 000fold enhancement (entry 7) when (50 mM) equimolar IBA/ (CTA)Cl blends are employed in pH 8 buffers that do not contain added KCl or phosphate buffer anions that compete with IBA for binding sites near the micellar surface. Indeed, this catalytic system is as reactive toward paraoxon as functional IBA surfactant 6 (entries 7 and 5), and more reactive than a copper-hexadecyltrimethylethylenediamine complex (entry 6). Note, too, that the reactivity of functional micellar reagent 6 appears to be suppressed by the presence of 0.1 M phosphate; compare krel for entries 2 and 5. Not only is the equimolar IBA/(CTA)Cl blend much easier to prepare than the synthetically demanding functional IBA surfactant, but it is just as “efficient” in the sense of supplying one IBA moiety for every surfactant molecule. The IBA/(CTA)Cl blend of entry 7 is also very reactive toward parathion (2), affording kobs ) 1.5 × 10-3 s-1 for parathion cleavage under the conditions described in Table 1.8,11 In 1989, Bunton et al. prepared cetyltrimethylammonium iodosobenzoate, (CTA)IBA (7), and examined its reactivity toward PNPDPP (5).12 Under optimal micellar conditions (∼0.4 M at pH 9), (CTA)IBA gave kψ ∼ 4.5 s-1 for the cleavage of PNPDPP,12 reactivity similar to that of IBA surfactant 6 in micellar (CTA)Cl (kψ ) 1.14 s-1 at pH 8).7 Indeed, Bunton’s analysis demonstrates that both systems afford similar “micellar” rate constants, km ∼ 6 s-1.12 The results again demonstrate that IBA, in the form (7) Moss, R. A.; Kim, K. Y.; Swarup, S. J. Am. Chem. Soc. 1986, 108, 788. (8) Moss, R. A.; Morales-Rojas, H. Langmuir 2000, 16, 6485. (9) Moss, R. A.; Kotchevar, A. T.; Park, B. D.; Scrimin, P. Langmuir 1996, 12, 2200. (10) Scrimin, P.; Ghirlanda, G.; Tecilla, P.; Moss, R. A. Langmuir 1996, 12, 6235. (11) For a summary of parathion cleavage kinetics analogous to Table 1, see ref 8, Table 3. (12) Bunton, C. A.; Mhala, M. M.; Moffatt, J. R. J. Phys. Chem. 1989, 93, 854.

10.1021/la010453x CCC: $20.00 © 2001 American Chemical Society Published on Web 08/25/2001

Cleavage of Paraoxon and Parathion by (CTA)IBA

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Table 1. Comparative Kinetics for Some Hydrolytic Cleavages of Paraoxon (1) entry

reagent

1a

IBA/(CTA)Cl

2a 3b

6/(CTA)Cl IBA/(CTA)Cl

4c 5d

IBA/(CTA)Cl 6/(CTA)Cl

6e

Cu-C16TMEDf/ (CTA)NO3 IBA/(CTA)Cl (CTA)Cl (CTA)Cl

7b 8a 9d a

kobs (s-1)

conditions 10-5

10-4

10-2

[1] ) 6.5 × M, [IBA] ) 1 × M, [CTA(Cl)] ) 1 × M, pH 7.5, 0.1 M PO4, 25 °C as in entry 1, but 6, instead of IBA [1] ) 1 × 10-5 M, [IBA] ) 1 × 10-4 M, CTA(Cl)] ) 5 × 10-3 M, pH 8, 0.01 M PO4, 0.01 M KCl, 25 °C as in entry 3, but with 0.02 M PO4 [1] ) 1 × 10-5 M, [6] ) 4 × 10-3 M, [CTA(Cl)] ) 2 × 10-2 M, pH 8, 0.01 M Tris, 0.01 M KCl, 25 °C [1] ) 2 × 10-5 M, [Cu-C16TMED] ) 1 × 10-3 M, [(CTA)NO3] ) 1 × 10-2 M, pH 8, 0.05 M HEPES, 25 °C [1] ) 5 × 10-3 M, [IBA] ) [CTACl] ) 5 × 10-2 M, pH 8, 0.1 M Tris, 25 °C as in entry 1, but no IBA as in entry 5, but no 6, and [(CTA)Cl] ) 5 × 10-3 M

krel

10-5

7.4 × 102

5.0 × 10-5 1.7 × 10-4

6.3 × 102 2.1 × 103

1.35 × 10-4 3.46 × 10-3

1.7 × 103 4.4 × 104

3.3 × 10-4

4.2 × 103

3.8 × 10-3 4.53 × 10-6 7.94 × 10-8

4.8 × 104 5.7 1.0

5.9 ×

Reference 6. b Reference 8. c Reference 9. d Reference 7. e Reference 10. f Cu complex of N-n-hexadecyl-N,N′,N′-trimethylethylenediamine.

of a reactive counterion paired with a nonfunctional cationic surfactant, is just as effective and efficient as a more complicated functional IBA surfactant like 6.

Given the desirability of simple, efficient reagents for the remediation of areas contaminated with paraoxon or parathion, we undertook a careful study of their cleavage by (CTA)IBA. We also compared the reactivity of this micellar reagent to that of IBA in (CTA)Cl or (CTA)Br, to measure any rate depressing effect of the surfactant’s Cl or Br counterions. Results and Discussion (CTA)IBA12 was readily prepared from “off-the-shelf” precursors by neutralizing a commercially available 25 wt % solution of (CTA)OH in methanol (ACROS Organics) with an equivalent of o-iodosobenzoic acid (Aldrich). Lyophilization of the resulting solution afforded (CTA)IBA as a clean, dry white powder. Material prepared in this manner afforded appropriate IR and NMR spectra and C, H, N analyses. The microanalytical iodine content was 1-2% “light” (cf., Experimental Section). Rate constants for the cleavage of 1 and 2 by (CTA)IBA were determined in 0.01 M Bis-Tris buffer at 25 °C and pH 9.0, where the IBA (pKa ∼ 7.2),3,5 was >98% in its reactive anionic form. Released p-nitrophenylate ion was monitored by UV spectroscopy at 400 nm as a function of time. In Figure 1, we present rate constant/[surfactant] profiles for the cleavages of 1 × 10-5 M paraoxon by (CTA)IBA, equimolar [CTA)Cl + NaIBA], and equimolar [(CTA)Br + NaIBA]. For (CTA)IBA, the surfactant concentration was varied from 1 × 10-3 M to 0.20 M (see inset, Figure 1). The profile shows a steep rise in kobs with increasing [(CTA)IBA]. At concentrations J0.05 M, a saturation plateau is observed and kobs becomes effectively independent of concentration. This behavior is typical of reactive counterion or functional surfactants: kobs approaches a maximum value at surfactant concentrations where “all” of the substrate is bound to the micelles. Analogous behavior was found in the cleavage of 5 by functional micellar IBA surfactant 6 or (CTA)IBA.12 From the profile in Figure 1, kψ(max) ∼ 0.014 - 0.015 s-1.

Figure 1. Observed rate constants (k, s-1) for the cleavage of paraoxon (1) as a function of surfactant concentration (M) in Bis-Tris buffer at pH 9. The surfactants are (CTA)IBA (9), 1:1 (CTA)Cl + NaIBA (b), and 1:1 (CTA)Br + NaIBA (2). The solid lines are generated via eq 1; see text. Inset: Extension of the (CTA)IBA profile to 0.2 M surfactant.

The experimental points in the profile are well-fitted by the 1:1 binding isotherm corresponding to13

kobs )

P1P2[(CTA)IBA] 1 + P2[(CTA)IBA]

(1)

where P1 ) km, the micellar rate constant, and P2 ) K/N, the ratio of the binding constant of the substrate to the micelle (K), and the micellar aggregation number (N). The fit generated the solid line in Figure 1, affording km ) 0.017 s-1 and K/N ) 62 M-1. Assuming N ∼ 50-100 gives estimated bounds of (3.1-6.2) × 103 M-1 for the binding constant of paraoxon to (CTA)IBA. A more conventional treatment of the (CTA)IBA/ paraoxon data can be carried out according to14

(

)(

1 1 1 ) + (ko - kobs) (ko - km) (ko - km)

)

N K(CD - cmc) (2)

where kobs is the observed rate constant at a given surfactant concentration, CD, km is the micellar rate constant, ko is the rate constant for the cleavage of (13) Connors, K. A. Binding Constants: The Measurement of Molecular Complex Stability; Wiley: New York, 1987; pp 59, 219. (14) Fendler, J. H.; Fendler, E. J. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975; pp 86f. When kobs . ko and CD . cmc, eq 2 reduces to eq 1. However, eq 2 is more appropriate at surfactant concentrations close to the cmc.

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Figure 2. Cmc determination for (CTA)IBA: observed rate constants, k (s-1) for the cleavage of paraoxon in pH 9 Bis-Tris buffer vs [(CTA)IBA], 5 × 10-4 to 1.3 × 10-3 M. The break-point at ∼8 × 10-4 M (CTA)IBA is taken as its cmc.

Moss et al.

Figure 3. Analysis of the paraoxon (CTA)IBA kinetic data (Figure 1) according to eq 2: 1/(ko - kobs) vs 1/(CD - cmc) (see text). From the slope and intercept, km ) 0.016 s-1 and K/N ) 79 M-1.

paraoxon at pH 9 in the absence of (CTA)IBA, K and N are as previously defined, and cmc is the critical micelle concentration of (CTA)IBA under the experimental conditions. We examined a range of values for ko: 7.5 × 10-7 s-1,15 7.5 × 10-8 s-1 (similar to the pH 8 value for paraoxon cleavage in (CTA)Cl; Table 1, entry 9),7 and 1 × 10-8 s-1 as a lower bound. In fact, because even our lowest kobs values were at least 100 times greater than ko, all three estimates gave excellent (r > 0.999) fits in eq 2 and led to very similar values of km and K/N. The cmc of (CTA)IBA was determined in 0.01 M pH 9 Bis-Tris buffer, containing 1 × 10-5 M paraoxon, from a graph of kobs vs [surfactant] in the surfactant concentration range 5.0 × 10-4 M to 1.3 × 10-3 M; cf., Figure 2. Taking the“break-point” of the correlation as indicative of the onset of micellization affords the cmc of (CTA)IBA as ∼8 × 10-4 M under these reaction conditions. With this cmc value, and ko ) 7.5 × 10-8 s-1, correlation of the kinetic data according to eq 2 gives Figure 3. From the slope and intercept of the correlation, we obtain km ) 0.016 s-1 and K/N ) 79 M-1, in good agreement with the corresponding values (0.017 s-1 and 62 M-1) obtained via eq 1. The upper limit estimate for K is therefore ∼7900 M-1 (with N ) 100). For comparison to (CTA)IBA, the cleavage of paraoxon was examined with 1:1 blends of NaIBA and either (CTA)Cl or (CTA)Br. Rate constant/[surfactant] profiles for these cases also appear in Figure 1, but solubility problems limited the concentrations of (CTA)Cl and (CTA)Br to maxima of 0.05 M. These systems turn out to be slightly less reactive than (CTA)IBA; kψ(max) values are 0.0090.010 s-1 for NaIBA with CTA(Br) or (CTA)Cl, compared to 0.014-0.015 s-1 for (CTA)IBA. The decrease in kψ(max) is likely due to competition between the Cl- or Brcounterions and IBA for binding sites near the cationic CTA headgroups within the micellar Stern layer, where the cleavage of the bound paraoxon occurs. Analogous kinetic studies were carried out with 1 × 10-5 M parathion (2); cf., Figure 4. Fits of the parathion experimental data to eq 1 are not as good as in the paraoxon

cases (Figure 1), but results are similar: the (CTA)IBA micellar reagent is somewhat more reactive than the NaIBA + (CTA)X systems. Thus, kψ(max) for parathion cleavage is 0.0030 s-1 for (CTA)IBA, 0.0025 for (CTA)Cl + NaIBA, and 0.0020 s-1 for (CTA)Br + NaIBA. ko for the cleavage of parathion at pH 9 can be estimated as ∼1 × 10-9 s-1 from the reported second-order rate constant for OH cleavage (∼1 × 10-4 M-1 s-1).16 The cmc of (CTA)IBA in 0.01 M pH 9 Bis-Tris buffer, containing 1 × 10-5 M parathion, is ∼6.8 × 10-4 M, as determined from the break-point of a correlation of kobs vs [surfactant], where the (CTA)IBA concentration ranged from 5 × 10-4 to 1 × 10-3 M. The correlation (not shown) closely resembles that of Figure 2 for the cmc of (CTA)IBA in the presence of paraoxon. With these parameters for ko and cmc, analysis of the (CTA)IBA/parathion profile of Figure 4 according to eq 2, produces the correlation of Figure 5, where r ) 0.979. From the slope and intercept of the correlation km ) 0.003 15 s-1 and K/N ) 566 M-1. Table 2 collects kinetic data for the cleavages of paraoxon and parathion.

(15) From k2 ) 7.5 × 10-2 M-1 s-1 for the cleavage of paraoxon by KOH, adjusted to [OH-] ) 10-5 M (pH 9): Dumas, D. P.; Caldwell, S. R.; Wild, J. R. Raushell, F. M. J. Biol. Chem. 1989, 264, 19659.

(16) Kazankov, G. M.; Sergeeva, V. S.; Efremenko, E. N.; Alexandrova, L.; Varfolomeev, S. D.; Ryabov, A. D. Angew. Chem., Int. Ed. 2000, 39, 3117.

Figure 4. Observed rate constants (k, s-1) for the cleavage of parathion (2) as a function of surfactant concentration (M) in Bis-Tris buffer at pH 9. The surfactants are (CTA)IBA (9), 1:1 (CTA)Cl + NaIBA (b), and 1:1 (CTA)Br + NaIBA (2). The solid lines are generated via eq 1; see text. Inset: Extension of the (CTA)IBA profile to 0.2 M surfactant.

Cleavage of Paraoxon and Parathion by (CTA)IBA

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It is necessary, however, to maintain an excess of IBA over the substrate. Although micellar iodosocarboxylates rapidly turn over in the cleavage of PNPDPP (5),5-7 they are less efficient with paraoxon. For example, iodosonaphthoate (INA, 8) in (CTA)Cl turns over more slowly with excess paraoxon than with PNPDPP,8 presumably because the phosphorylated intermediate18 (9) is not as easily cleaved by OH- attack at I18 when R ) Et as when R ) Ph; i.e., (PhO)2P(O)O- is a better leaving group than (EtO)2P(O)O-. With parathion as the substrate, no turnover occurs with iodosocarboxylate reagents because a subsequent redox reaction occurs between the iodosocarboxylate and the diethyl thiophosphate fragment (10) released during the parathion cleavage. The iodosocarboxylate is reduced to the inactive iodocarboxylate, while 10 is converted to diethyl phosphate (11).8

Figure 5. Analysis of the parathion-(CTA)IBA kinetic data (Figure 4) according to eq 2: 1/(ko - kobs) vs 1/(CD - cmc). From the slope and intercept, km ) 0.0031 s-1 and K/N ) 566 M-1. Table 2. Kinetic Data for Micellar Cleavages of Paraoxon and Parathiona kψ(max)b,c (s-1) substrate

(CTA) IBA

(CTA)Cl + NaIBAd

(CTA)Br + NaIBAd

kme (s-1)

paraoxon (1) 0.014 (80) 0.010 (30) 0.0093 (40) 0.016 parathion (2) 0.0030 (15) 0.0025 (10) 0.0020 (15) 0.0031

K/Nf (M-1) 79 566

a Conditions: [substrate] ) 1 × 10-5 M, 0.01 M Bis-Tris buffer, pH 9.0, 25 °C. b Maximum observed pseudo-first-order rate constant for substrate cleavage. c Values in parentheses are the concentrations (mM) of surfactant at which kψ(max) is observed. d A 1:1 ratio of surfactant and NaIBA was maintained throughout. e ”Micellar” rate constant for substrate cleavage by (CTA)IBA obtained from eq 2; see text. f Ratio of binding constant to aggregation number for (CTA)IBA cleavage of substrate, obtained from eq 2; see text.

The rate constant/[sufactant] profiles (Figures 1 and 4) and the kobs and km values of Table 2 indicate that paraoxon is 4-5 times more reactive than parathion toward micellar IBA. This is a reflection of intrinsic reactivity differences: the second-order rate constants for their reactions with OH- are 7.5 × 10-2 M-1 s-1 (paraoxon)15 and 9.5 × 10-5 M-1 s-1 (parathion).16 Parathion is somewhat more hydrophobic than paraoxon and, consequently, binds more strongly to (CTA) micelles. This can be seen qualitatively in Figures 4 and 1, where the rise of kobs with increasing surfactant concentration is sharper for parathion than paraoxon. A quantitative comparison emerges from the K/N values computed from eq 2 and displayed in Table 2. Although one cannot be certain that the micellar aggregation number (N) remains constant for (CTA)IBA micelles as the substrate changes from paraoxon to parathion, it is very likely that K is at least 7 times (566/79) larger for parathion.17 Another consequence of the stronger binding of parathion is that kψ(max) is obtained at a lower surfactant concentration for parathion relative to paraoxon (Table 2). Under the excess (CTA)IBA conditions described in Table 2, half-lives for the cleavages of paraoxon and parathion are 50 s and 3.8 min, respectively. Therefore, practical decontamination protocols for these persistent, toxic materials could be developed with micellar (CTA)IBA. (17) If N does vary with the substrate, it is likely to be larger with the more hydrophobic parathion, which leads to an even larger difference between K(parathion) and K(paraoxon).

A brief 31P NMR product study was conducted for the cleavages of paraoxon and parathion by (CTA)IBA. The reaction conditions were [substrate] ) 5 × 10-3 M, [(CTA)IBA] ) 5 × 10-2 M, pH 9, 0.1 M Bis-Tris buffer in 20/80 D2O/H2O at 25 °C. With parathion, the initial substrate 31P resonance (δ 62.5) decreased with the appearance of product signals at δ 55.7 (10) and δ 1.25 (11).19 With time, the δ 55.7 signal decreased in favor of δ 1.25, until all of the thiophosphate was converted to diethyl phosphate. Under the conditions described here, the overall reaction is very clean; 11 is the only P-containing product. The cleavage of paraoxon by (CTA)IBA was followed under identical conditions. The substrate (31P δ -6.21)19 afforded only 11 (δ 1.25) upon IBA-mediated hydrolysis. In Table 1 we summarized the comparative kinetics of paraoxon cleavage by various iodosocarboxylates under several micellar regimes. Kinetic advantages up to 48 000 were obtainable with the inclusion of IBA. From Table 2, the best kinetic result with (CTA)IBA vs paraoxon is kψ ) 0.014 s-1 which, on the krel scale of Table 1, translates into a kinetic advantage of ∼176 000, relative to (CTA)Cl. (CTA)IBA is therefore the most reactive micellar IBA preparation yet deployed against paraoxon. Toward parathion, with a kinetic advantage of ∼3 × 106 relative to ko,16 (CTA)IBA also claims pride of place compared to previous IBA preparations.8 How does (CTA)IBA compare with other highy reactive reagents? There are three competing systems of interest: the phosphotriesterase derived from Pseudomonas diminuta,15 the platinum aryloxime metallacycles, 12,16 and micellar (deprotonated) 1-cetyl-3-(2-oximopropyl)imidazolium chloride, 13.20

(18) Moss, R. A.; Zhang, H. J. Am. Chem. Soc. 1994, 116, 4471. (19) For signal assignments, see ref 8. 31P NMR chemical shifts are relative to external 85% H3PO4. (20) Simanenko, Y. S.; Karpichev, E. A.; Prokop’eva, T. M.; Panchenko, B. V.; Bunton, C. A. Langmuir 2001, 17, 581.

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The purified phosphotriesterase is extraordinarily reactive toward paraoxon (kcat. ) 2100 s-1) and parathion (kcat. ∼ 600 s-1) at pH 9.15 The kinetic advantage of the enzyme against paraoxon is ∼3 × 1011, relative to unassisted hydrolysis at pH 7. No other reported reagent kinetically competes with the enzyme. However, complications of availability, purification, and stability could limit its utility. Platinum reagents 12 (R ) H or MeO) are also very reactive toward paraoxon and parathion. Second-order rate constants for the cleavage of paraoxon at pH 8.5 are 914 M-1 s-1 (12, R ) MeO) or 310 M-1 s-1 (R ) H), where k2 ) 10 M-1 s-1 (R ) H) for parathion.16 These reactivities exceed those of (CTA)IBA, although the cost of the platinum reagents would need to be considered in practical applications. Finally, the cleavage of paraoxon by 50 mM micellar oximate 13 occurs with kobs ∼ 0.015 s-1 at pH 12.9.20 The reactivity is comparable to that of (CTA)IBA, but the operating pH is nearly four units more basic for reagent 13. From a practical standpoint, (CTA)IBA, which is readily prepared from commercially available reagents merits serious consideration for the remediation of paraoxon or parathion contamination. Although excess reagent would be required, cleavage half-lives of several minutes or less promise efficient decontaminations of these toxic materials. Experimental Section Materials and Methods. Paraoxon (1) and parathion (2) were purchased from Supelco. For kinetics studies they were dispensed as 1 mM solutions in MeCN. Cetyltrimethylammonium chloride (Eastman) and cetyltrimethylammonium bromide (Aldrich) were used as received. The sodium salt of o-iodosobenzoic acid, NaIBA, was prepared from molar equivalents of IBA (Aldrich) and 0.096 N aqueous NaOH, followed by lyophilization to dryness. Routine 1H NMR spectra were obtained at 300 MHz. 31P NMR spectra were measured at 121 MHz. 1H NMR chemical shifts are referenced to DSS; 31P NMR chemical shifts refer to external 85% H3PO4. UV-vis kinetics were recorded on an HP 8453 diodearray spectrometer equipped with a thermostated 3 mL cuvette, path length 1 cm. Solutions for kinetics were prepared with

Moss et al. “steam-distilled” water (Culligan Bottled Water Co., East Orange, NJ). Elemental analyses were performed by Quantitative Technologies, Inc., Whitehouse, NJ. Cetyltrimethylammonium Iodosobenzoate ((CTA)IBA, 7). (CTA)IBA was prepared by neutralizing solid o-iodosobenzoic acid (Aldrich) with commercially available 25% (w/w) (CTA)OH in methanol (ACROS Organics, Fisher Scientific). The resulting solution was lyophilized to dryness, affording a dry white powder of (CTA)IBA. IR (KBr pellet): 1617 cm-1 (carbonyl).21 NMR (δ, D2O): 7.86, 7.84 (d, 1 H); 7.64-7.52 (m, 2H), 7.29, 7.26, 7.24 (t, 1 H) [total: 4 aromatic protons], 2.89-3.21 (m + s, 11 H, Me3N+ and CH2N+), 0.93-1.40 (m, 28 H, (CH2)14CH2N+), 0.80-0.82 (“t”, 3H, term. CH3). Anal.Calcd. for C26H46NIO3: C, 57.04; H, 8.40; N, 2.61; I, 23.21. Found: C, 57.32; H, 8.30; N, 2.61; I, 21.50, 20.92 (duplicate). Kinetics. For rate constant/[surfactant-catalyst] profiles, Figures 1 and 4, solutions were buffered with 0.01 M Bis-Tris (bis[2-hydroxyethyl]iminotris[hydroxymethyl]methane) and adjusted to pH 9.0 with dilute KOH or 1.2% aqueous HF. Stock solutions of 0.2 M (CTA)IBA or [0.05 M (CTA)Cl + 0.05 M NaIBA] or [0.05 M (CTA)Br +0.05 M NaIBA] in steam-distilled water were freshly prepared for each profile. Final concentrations for the kinetics runs were obtained by taking appropriate volumes of the stock solutions in the UV cuvette, and diluting with 0.01 M Bis-Tris buffer to 2970 µL. Kinetics were initiated by syringe addition of 30 µL of a 1 mM stock solution of substrate in MeCN. Kinetics were monitored by following the absorbance of the released p-nitrophenylate ion at 400 nm. Results appear in Figures 1 and 4, and Table 2. Individual reactions were followed to at least five half-lives. Reproducibilities in these kinetic runs are typically (5%.8 For cmc experiments, the kinetics were ascertained with 5 × 10-4 M to 1.3 × 10-3 M (CTA)IBA (for paraoxon) or 5 × 10-4 to 1.0 × 10-3 M (CTA)IBA (for parathion) with 1 × 10-5 M substrates; cf., Figure 2. The “break-point”, at which the slope of kobs vs [(CTA)IBA] abruptly changes, was taken as the cmc of (CTA)IBA under the reaction conditions.

Acknowledgment. We are grateful to the U.S. Army Research Office for financial support. We thank Mr. Hugo Morales-Rojas for helpful discussions. LA010453X (21) This absorption is at the low-frequency end of the carboxyl region for benziodoxolones as described by: Baker, G. P.; Mann, F. G.; Sheppard, N.; Tetlow, A. J. J. Chem. Soc. 1965, 3721.