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Bimolecular rate constants for the hydroxyl radical, ·OH, reaction with domoic acid, kainic acid, and several model compounds were determined using e...
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Environ. Sci. Technol. 2009, 43, 6764–6768

Bimolecular Rate Constant Determination for the Reaction of Hydroxyl Radicals with Domoic and Kainic Acid in Aqueous Solution KIMBERLY G. JONES,† WILLIAM J. COOPER,‡ AND S T E P H E N P . M E Z Y K * ,§ Marine, Earth, and Atmospheric Science Department, North Carolina State University, Jordan Hall, Raleigh, North Carolina 27695, Director and Professor, Urban Water Research Center, Department of Civil and Environmental Engineering, University of California Irvine, Irvine, California 92697-2175, and Department of Chemistry and Biochemistry, California State University Long Beach, Long Beach, California 90840

Received April 14, 2009. Revised manuscript received July 11, 2009. Accepted July 16, 2009.

Bimolecular rate constants for the hydroxyl radical, · OH, reaction with domoic acid, kainic acid, and several model compounds were determined using electron pulse radiolysis and transient absorption spectroscopy. These oxidation rate constants were determined as (9.22 ( 0.60) × 109 M-1 s-1 and (2.46 ( 0.19) × 109 M-1 s-1, for domoic acid and kainic acid,respectively.Modelcompoundrateconstantmeasurements suggested that the conjugated double bond of the C4 substituent of domoic acid, and the double bond in kainic acid, were the preferential initial site of · OH reaction, which would destroy the conjugation and/or molecular conformation of these toxins. These reaction rate constants were also used to determine potential persistence in natural water systems, however the calculated half-life for domoic acid of ∼34 days implies that significant photodegradation via · OH reaction mechanisms is unlikely.

Introduction In the 1950s, both (-)-domoic acid and (-)-kainic acid were isolated from the red marine algae of the same family (Rhodomelaceae), Chondria armata and Digenea simplex, respectively (1, 2). These algae, possessing the compounds known as kainoids, have been used in anthelmintic (roundworm disease) folk medicine in Japan for centuries (3) but have more recently received notoriety as the potent neurotoxins and excitatory amino acids responsible for amnesic shellfish poisoning (ASP). Domoic acid, produced by the pinnate diatoms Pseudo-nitzschia spp. and sequestered by filter-feeding shellfish, surfaced as the causative toxin in a deadly seafood poisoning incident in Canada in 1987 (4). Both domoic and kainic acid are water-soluble members of the kainoids, a group of neurologically active amino acids. The mechanism of domoic acid toxicity, and kainic acid’s * Corresponding author phone: 562-985-4649; fax: 562-985-8557; e-mail: [email protected]. † North Carolina State University. ‡ University of California Irvine. § California State University Long Beach. 6764

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neuroexcitatory properties, is explained (5) by its structural similarity and much stronger receptor affinity to the excitatory neurotransmitter glutamic acid (see Figure 1). Domoic acid is three times more potent than kainic acid, and up to 100 times more potent than glutamic acid itself (6). The strength of binding at the kainate receptor, one of three types of the ionotropic (ion channel) class of glutamate receptors, determines the potency of neuroexcitation (7). It has been determined that the domoic acid C4 stereochemistry, substitution, and molecular conformation strongly influence the binding. The nature of the C4 substituent is particularly critical: the Z-configuration of a C1′ alkene is more active than the E-configuration, and compounds with unsaturated substituents have much higher activities than those with saturated moieties. The potency of domoic acid has become an issue of serious concern, as the incidents of Pseudo-nitzschia spp. blooms have increased in recent times. When the algae is consumed, domoic acid becomes concentrated in the visceral hepatopancreas or gill structures of animals such as oysters, mussels, sardines, scallops, clams, crabs, and anchovies (8). Thus, like humans, when animals then consume this concentrated domoic acid, they may become disoriented, and the result is often death (9, 10). Moreover, this problem could be exacerbated through the increased use of seawater desalination plants that are being installed on a global basis (11), which discharge brine back into the sea (12). If a bloom should occur near the plant water intake, then biotoxins such as domoic and kainic acids could be significantly concentrated in the discharge. Therefore, active toxin removal strategies, such as the use of advanced oxidative processes (AOPs) that incorporate oxidizing hydroxyl radicals, may also be required in some situations. In general, the environmental fate of domoic acid is poorly understood. Studies of the biodegradation of domoic acid by a variety of bacteria isolated from the marine environment indicated that it was generally inhibitory to resting cells or growing cultures (13, 14). Bates et al. (15), suggested instead that photodegradation was the dominant pathway, and that the bacteria may even protect the domoic acid by scavenging any superoxide produced by incident light. Direct and indirect, sensitized, photochemical degradation pathways are also a potentially important removal mechanism of these acids in seawater. Studies have shown that domoic acid in seawater is rapidly degraded upon exposure to simulated sunlight (16), with exposure to fullspectrum light for 22 h resulting in ∼40% degradation. Bouillon et al. (17) reported similar results, with domoic acid concentrations decreasing exponentially as a function of irradiation time (10 h incubation, 84-18 nmol L-1); in contrast to no loss observed for dark controls. Using a multivariate, microscale, high-throughput experimental approach, Fisher et al. (18), showed that Fe(III) and dissolved organic matter (DOM) are significant promoters of domoic acid photooxidation but that the phosphate ion interacted with Fe(III) to inhibit the photooxidation of domoic acid. Domoic acid in these matrices had half-lives ranging over 12-36 h. Similar studies have also been conducted for kainic acid in seawater (19). Another potential degradation pathway for domoic and kainic acids is through reaction with the photochemically generated hydroxyl radical. However, the importance of this · OH radical reaction will depend upon its rate constant and efficiency in destroying the activity of these toxins. These same considerations will apply to any AOP treatments that also generate this radical. While investigation of hydroxyl 10.1021/es901128c CCC: $40.75

 2009 American Chemical Society

Published on Web 08/03/2009

FIGURE 1. Structures of domoic acid (A), kainic acid (B) and model compounds (C-G) investigated in this study. radical reaction kinetics and mechanisms with freshwater toxins such as microcystin-LR has recently been reported (20), no analogous data exists for these marine toxins. To allow for the quantitative evaluation of these radical reactions under real-world conditions, where interfering reactions from water and biological matrix species will also occur, the specific rate constants for the hydroxyl radical reaction with domoic acid and kainic acid have been determined. Further, we have also measured reaction rate constants for some model compounds, in order to elucidate the dominant reaction sites in these two toxins. These kinetic data were also used to determine potential persistence in natural water systems as a result of indirect photolysis.

Experimental Section Domoic acid (Fluka, >97%; Biomedicals, Inc., >98%) and (-)(a)-kainic acid (hydrate) (Cayman Chem., >98%) with their stated purity verified by the manufacturers were used as received. Other model chemicals; trans-4-hydroxy-L-proline, 5-(hydroxymethyl)-2-pyrrolinone, L-pyroglutamic acid, and proline were obtained and used at the highest purity available. All chemical solutions were made using high quality Millipore Milli-Q charcoal-filtered (TOC < 13 µg L-1) deionized (>18.0 MΩ) water that was saturated by sparging with high purity N2O to remove dissolved oxygen immediately before irradiation. Solutions were buffered using 5 mM phosphate to pH 7.5, ensuring that domoic acid (pKa ) 1.85, 4.47, 4.75, and 10.60 (21)) and kainic acid (pKa ) 1.96, 6.02, and 10.80 (19)) were in their ionized forms predominately found in natural waters. Kinetic measurements on these solutions were performed using the linear accelerator/ absorption spectroscopy system at the Department of Energy Radiation Laboratory, University of Notre Dame. Details of the equipment and data analysis have been given previously (22-24). Solution flow rates in the kinetic experiments were adjusted so that each pulse irradiation was performed on a fresh sample, and multiple traces (5-15) were averaged to produce a single kinetic trace. Typically, 2-5 ns pulses of 8 MeV electrons, generating radical concentrations of 1-3 µM per pulse, were used in these

experiments, ensuring pseudo-first-order kinetic reaction conditions were constantly maintained. The electron radiolysis of pure water (or dilute solutions) forms hydroxyl radicals, hydrated electrons, and hydrogen atoms according to the established stoichiometry (25, 26) radiolysis

H2O 98 0.28• OH + 0.27 eaq + 0.06 •H + 0.05 H2 +

0.07 H2O2 + 0.27 H+ (1) where the numbers in eq 1 are G-values (yields) in units of µmol J-1. Absolute radical concentrations (dosimetry) was based on the transient absorption of (SCN)2•- at 475 nm, using 10-2 M potassium thiocyanate (KSCN) in N2O-saturated solution with Gε ) 5.2 × 10-4 m2 J-1 (27). To isolate the reaction of only hydroxyl radicals with these compounds, solutions were presaturated with N2O, which quantitatively converts hydrated electrons, eaq-, and some hydrogen atoms, · H, to · OH (25): eaq + N2O + H2O f N2 + OH- + •OH k2 )

9.1 × 109M-1s-1 (2) •

H + N2O f •OH + N2 k3 ) 2.1 × 106M-1s-1

(3)

The rate constant errors reported in this paper are the combination of measurement precision (using the standard deviation of the multiple kinetic or corrected intensity fits multiplied by the appropriate t-distribution value based on a 90% confidence interval), plus the uncertainty in the given compound purity. All measurements were performed at 21-23 °C.

Results and Discussion The reaction of the hydroxyl radical with all of the chemicals only gave very weak transient intensities, which were too small to allow accurate direct measurements of their intermediate growth kinetics. Therefore, the hydroxyl radical oxidation rate constants were obtained using SCN- comVOL. 43, NO. 17, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Measured Hydroxyl Radical Rate Constants (M-1s-1) of Domoic Acid, Kainic Acid, And Model Compounds. Values of This Study in Bold · OH rate constant (M-1s-1)

compound

FIGURE 2. (a) Transient absorption kinetics observed at 475 nm for 61.5 µM KSCN in N2O-saturated solution containing zero (0), 35.7 (O), 67.4 (∆), and 113.4 (3) µM domoic acid. Solid red lines are kinetics corrected for transient decay, as described in text. (b) Transformed competition kinetics plots of decay-corrected data for domoic acid (0) and kainic acid (O). Error bars are calculated as described in text. Solid lines are weighted linear fits, corresponding to hydroxyl radical reaction rate constants of (9.22 ( 0.60) × 109 M-1 s-1, and (2.46 ( 0.19) × 109 M-1 s-1, respectively. petition kinetics; by monitoring the changes in absorption of the (SCN)2• - transient absorption at 475 nm with different amounts of chemical (for example, domoic acid s DA) present. There was no significant absorption from the hydroxyl-radical-induced oxidation of either domoic or kainic acid at this wavelength. The hydroxyl radical competition was •

•

OH + DA f products

(4)

OH + SCN-(+SCN-) f OH- + (SCN)•2

(5)

which can be solved to give the expression (25): Abs(SCN)•-o 2 Abs(SCN)•2

)1+

k4[DA] k5[SCN-]

(6)

where Abs(SCN)2•-o is the maximum transient radical anion concentration for only the pure thiocyanate solution, Abs(SCN)2•- is the reduced absorbance when domoic acid is also present, and [DA] and [SCN-] are the added concentrations of domoic acid and thiocyanate, respectively. A plot of the intensity ratio Abs(SCN)2•-o/Abs(SCN)2•- against the concentration ratio [DA]/[SCN-] gives a straight line of slope k4/k5. Based on the rate constant for hydroxyl radical reaction with SCN-, k5 ) 1.1 × 1010 M-1 s-1 (25), the k4 rate constant is readily determined. Typical kinetic data for the (SCN)2•- intermediate at 61.5 µM SCN- with different amounts of domoic acid present are shown in Figure 2a. On the time scale of these reactions, a small amount of intermediate decay was observed. This decay follows overall second-order kinetics, and was attributed to reactions of (SCN)2•- reaction with other radicals present (collectively grouped as R•): • (SCN)•2 + R f products

(7)

This kinetic profile of the (SCN)2•- transient has no analytical integrated solution. Therefore, to account for this slight decay, thus improving our fitting statistics, the initial small portion of this second-order decay was modeled by 6766

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domoic acid kainic acid trans-4-hydroxy-L-proline 5-(hydroxymethyl)2-pyrrolidinone L-pyroglutamic acid proline glutamic acid pyrrolidinone butadiene 1,3-cyclohexadiene 2-methyl-propene

literature value (M-1s-1)

(9.22 ( 0.60) × 109 (2.46 ( 0.19) × 109 (4.40 ( 0.18) × 108 (1.93 ( 0.13) × 109 (1.05 ( 0.10) × 109 (2.78 ( 0.21) × 108 3.1 × 108 (35) 2.3 × 108 (34) 2.2 × 109 (36) 7.0 × 109 (39) 9.9 × 109 (40) 5.4 × 109 (39)

first-order kinetics, following the method of Westerhoff et al. (28). This gives the reaction sequence: •

OH + SCN-(+SCN-) f (SCN)•2

(8)

(SCN)•2 f products

(9)

where, the observed transient absorbance data is for the intermediate (SCN)2•-. This consecutive reaction mechanism has an analytical solution of the form: Abs(SCN)-• 2

)

[•OH]oε(SCN)2-•k8 k 9 - k8

{e-k8t - e-k9t}

(10)

where Abs(SCN)2-• is the time-dependent thiocyanate radical anion absorbance, [ · OH]o is the initial hydroxyl radical concentration, ε(SCN)2-• ) 8100 M-1 cm-1 (29) is the determined absorption coefficient for this radical, and k8 and k9 are the rate constants as given above. Fitting this equation to the measured absorption allowed us to correct for the observed decay. These corrections are seen as the solid lines in Figure 2a. The transformed data for domoic and kainic acids are shown in Figure 2b, with corresponding hydroxyl radical rate constants of (9.22 ( 0.60) × 109 M-1 s-1 and (2.46 ( 0.19) × 109 M-1 s-1, respectively. It is recognized that this approximation of fitting the multiple second-order decays by a single pseudo-first-order decay could also lead to some error, so as a further test of this approach, we also analyzed these data using noncorrected kinetic trace peak values. Derived rate constants for the latter were less than 6% different from the corrected data values. The most common pathway for the oxidation of amino acids, such as glutamic acid, involves the hydroxyl radicalmediated abstraction of a hydrogen atom to form a carboncentered radical at the R-position of the nitrogen function (30, 31). The radical formation is facilitated and stabilized by the complementary electron-donating and electron-withdrawing effects of the substituents, to delocalize charge and unpaired spin density that develop in reaction transition states (32, 33). To obtain further insight into the dominant site of hydroxyl radical reaction with these two toxins, model compound studies were also conducted. These model compounds were chosen to isolate mimics of specific substructures of the two toxins, and their measured rate constants allowed the determination of the preferred initial hydroxyl radical reaction site. All the rate constants of this study are summarized in Table 1, along with literature data for other relevant

Photochemically mediated processes, both primary and secondary, have important ramifications with respect to modifications of pollutants, regulation of the redox properties of natural waters, and the decomposition of humic substances (43-45). Sunlight-induced photochemical processes have been determined to be a primary degradation mechanism for domoic acid (15, 17, 18). While direct sunlight photolysis of hydrogen peroxide is one possible mechanism for the formation of · OH in surface waters, it is not considered as a primary mechanism (45-47). Significant sources of · OH radicals may come from secondary (indirect or sensitized) Photo-Fenton reactions, the oxidation of Cu(I) and Fe(II) by H2O2 and/or by nitrate ion photolysis (43, 48-51). Based on a measured photoreactant [ · OH] steady-state concentration of 2.5 × 10-17 M (Midday, June, sunlight 1 kW m-2 with DOM (43)), the loss of domoic acid by this mechanism would have a half-life of ∼34 days in a natural water system. This is considerably longer than the photochemical half-life for these toxins in a marine environment, which has been determined (19) as 12-34 h and 40-100 h for domoic and kainic acids, respectively. It is therefore unlikely that significant destruction of these kainoid-like compounds would occur by this pathway naturally. However, these large rate constants suggest that AOP type treatments of domoic or kainic acid-contaminated waters could be effective to remediate concentrate before disposal, thus reducing environmental impacts. FIGURE 3. Proposed initial hydroxyl radical reaction at the double bonds in both domoic acid (A) and kainic acid (B). compounds. The relatively slow hydroxyl radical reaction rate constants for glutamic acid (2.3 × 108 M-1 s-1 (34)), proline (2.78 ( 0.21) × 108 M-1 s-1 in this study (in good agreement with the previously reported value of 3.1 × 108 M-1 s-1 (35)), and trans-4-hydroxy-L-proline ((4.40 ( 0.18) × 108 M-1 s-1) suggests that hydroxyl radical induced oxidation at this moiety in domoic and kainic acids will not occur to any significant extent. These structures need to be activated in order to have competing reactivity, for example, by inclusion of a carbonyl bond (see significantly faster reaction rate constants for pyrrolidinone (2.2 × 109 M-1 s-1 (36)), 5-(hydroxymethyl)-2-pyrrolidinone ((1.93 ( 0.13) × 109 M-1 s-1) or pyroglutamic acid ((1.05 ( 0.10) × 109 M-1 s-1). The rate constant for hydroxyl radical reaction with 2-methylpropionic acid has not been reported, however, the corresponding value for isopropanol of 1.9 × 109 M-1 s-1 (37) which would be expected to be similar (38) is significantly slower than observed for domoic acid. However, the measured rate constants for butadiene (7.0 × 109 (39)) and 1,3cyclohexadiene (9.9 × 109 M-1 s-1 (40)) are in similar magnitude to our measured value of k4, suggesting that the hydroxyl radical oxidation occurs predominantly at this conjugated double-bond system within the molecule. Our proposed reaction mechanism is shown in Figure 3, and it is believed that the radical oxidation would destroy the conjugation and molecular conformation, resulting in free rotation around the carbons of the C4 substituent (41, 42) and the formation of a thermodynamically favored tertiary carbon-centered radical. Similarly for kainic acid, we believe that significant hydroxyl radical reaction occurs at the single terminal double bond (see also Figure 3), in agreement with previous determinations (19). These results are consistent with the determined dominant pathways for the hydroxyl radical addition to the unsaturated moieties of microcystin-LR (20). The larger rate constant for this freshwater toxin, k · OH ) 2.3 × 1010 M-1 s-1, reflects the additional (∼40%) hydroxyl radical reaction at the unsaturated ring of this species.

Acknowledgments Most of the kinetics measurements described herein were performed at the Radiation Laboratory, University of Notre Dame, which is supported by the Office of Basic Energy Sciences of the U.S. Department of Energy. We would also like to acknowledge the assistance of Julie Peller who performed some of the early domoic acid transient spectral measurements in this study. This is contribution 42 from the UCI Urban Water Research Center.

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