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Photolytic degradation of methylmercury enhanced by binding to natural organic ligands.

Zhang T, Hsu-Kim H - Nat Geosci (2010)

Bottom Line: In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive.Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition.Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

View Article: PubMed Central - PubMed

Affiliation: Duke University, Department of Civil & Environmental Engineering, 121 Hudson Hall, Durham, NC 27708 USA.

ABSTRACT
Monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs. Sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters. Rates of photodecomposition, however, can vary drastically between surface waters2-5 for reasons that are largely unknown. Here, we show that photodegradation occurs through singlet oxygen, a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter. The kinetics of degradation, however, depended on water constituents that bind methylmercury cations. Relatively fast degradation rates (similar to observations in freshwater lakes) applied only to methylmercury species bound to organic sulfur-containing thiol ligands such as glutathione, mercaptoacetate, and humics. In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive. Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition. Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

No MeSH data available.


Related in: MedlinePlus

MeHg degradation via generation of 1O2 from photosensitized humic acidMeHg was degraded by UV-A in simulated water containing MeHg (0.5 nM), SRHA (2.8 mg-C/L), and phosphate (10 mM, pH 7.3). a) Replicate samples were amended with either D2O (a singlet oxygen enhancer), NaN3 or β-carotene (singlet oxygen quenchers); b) No difference was observed in replicates amended with isoprene (a quencher for triplet state dissolved NOM) or isopropyl alcohol (quencher for •OH). Error bars represent ±1 s.d. for replicate measurements (n=2–3).
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Figure 2: MeHg degradation via generation of 1O2 from photosensitized humic acidMeHg was degraded by UV-A in simulated water containing MeHg (0.5 nM), SRHA (2.8 mg-C/L), and phosphate (10 mM, pH 7.3). a) Replicate samples were amended with either D2O (a singlet oxygen enhancer), NaN3 or β-carotene (singlet oxygen quenchers); b) No difference was observed in replicates amended with isoprene (a quencher for triplet state dissolved NOM) or isopropyl alcohol (quencher for •OH). Error bars represent ±1 s.d. for replicate measurements (n=2–3).

Mentions: Sunlight irradiation of chromophoric NOM is known to generate reactive intermediates such as •OH, superoxide, and 1O221. In additional photodegradation experiments, we utilized selective probes to identify the reactive intermediate that was responsible for MeHg decomposition (Figure 2). The addition of β-carotene and sodium azide (NaN3), which are probes for 1O2, resulted in slower MeHg degradation kinetics (Figure 2a). Experiments were also repeated in 98.5% deuterated water (D2O), a solvent that is ~13 times slower than H2O at quenching singlet oxygen19,22 and thus, increases steady state 1O2 levels. The use of D2O appeared to increase the rate of decomposition; however, this enhancement was smaller than expected. The addition of isopropyl alcohol (a probe for •OH23) and isoprene (a probe for photosensitized triplet state NOM23–24) did not appreciably change photodegradation rates (Figure 2b). MeHg degradation was not observed in dark controls: the recovery of MeHg was 92% (± 8.3%) after 120 hr. Additional dark experiments with enzyme-generated superoxide indicated that superoxide was not capable of decomposing MeHg within this time frame (Supplementary Figure S2). Collectively these results suggested that 1O2 was the reactive intermediate responsible for photodegradation in our experiments and not other types of intermediates such as •OH, photosensitized NOM, and superoxide.


Photolytic degradation of methylmercury enhanced by binding to natural organic ligands.

Zhang T, Hsu-Kim H - Nat Geosci (2010)

MeHg degradation via generation of 1O2 from photosensitized humic acidMeHg was degraded by UV-A in simulated water containing MeHg (0.5 nM), SRHA (2.8 mg-C/L), and phosphate (10 mM, pH 7.3). a) Replicate samples were amended with either D2O (a singlet oxygen enhancer), NaN3 or β-carotene (singlet oxygen quenchers); b) No difference was observed in replicates amended with isoprene (a quencher for triplet state dissolved NOM) or isopropyl alcohol (quencher for •OH). Error bars represent ±1 s.d. for replicate measurements (n=2–3).
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2902198&req=5

Figure 2: MeHg degradation via generation of 1O2 from photosensitized humic acidMeHg was degraded by UV-A in simulated water containing MeHg (0.5 nM), SRHA (2.8 mg-C/L), and phosphate (10 mM, pH 7.3). a) Replicate samples were amended with either D2O (a singlet oxygen enhancer), NaN3 or β-carotene (singlet oxygen quenchers); b) No difference was observed in replicates amended with isoprene (a quencher for triplet state dissolved NOM) or isopropyl alcohol (quencher for •OH). Error bars represent ±1 s.d. for replicate measurements (n=2–3).
Mentions: Sunlight irradiation of chromophoric NOM is known to generate reactive intermediates such as •OH, superoxide, and 1O221. In additional photodegradation experiments, we utilized selective probes to identify the reactive intermediate that was responsible for MeHg decomposition (Figure 2). The addition of β-carotene and sodium azide (NaN3), which are probes for 1O2, resulted in slower MeHg degradation kinetics (Figure 2a). Experiments were also repeated in 98.5% deuterated water (D2O), a solvent that is ~13 times slower than H2O at quenching singlet oxygen19,22 and thus, increases steady state 1O2 levels. The use of D2O appeared to increase the rate of decomposition; however, this enhancement was smaller than expected. The addition of isopropyl alcohol (a probe for •OH23) and isoprene (a probe for photosensitized triplet state NOM23–24) did not appreciably change photodegradation rates (Figure 2b). MeHg degradation was not observed in dark controls: the recovery of MeHg was 92% (± 8.3%) after 120 hr. Additional dark experiments with enzyme-generated superoxide indicated that superoxide was not capable of decomposing MeHg within this time frame (Supplementary Figure S2). Collectively these results suggested that 1O2 was the reactive intermediate responsible for photodegradation in our experiments and not other types of intermediates such as •OH, photosensitized NOM, and superoxide.

Bottom Line: In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive.Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition.Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

View Article: PubMed Central - PubMed

Affiliation: Duke University, Department of Civil & Environmental Engineering, 121 Hudson Hall, Durham, NC 27708 USA.

ABSTRACT
Monomethylmercury is a neurotoxin that poses significant risks to human health1 due to its bioaccumulation in food webs. Sunlight degradation to inorganic mercury is an important component of the mercury cycle that maintains methylmercury at low concentrations in natural waters. Rates of photodecomposition, however, can vary drastically between surface waters2-5 for reasons that are largely unknown. Here, we show that photodegradation occurs through singlet oxygen, a highly reactive form of dissolved oxygen generated by sunlight irradiation of dissolved natural organic matter. The kinetics of degradation, however, depended on water constituents that bind methylmercury cations. Relatively fast degradation rates (similar to observations in freshwater lakes) applied only to methylmercury species bound to organic sulfur-containing thiol ligands such as glutathione, mercaptoacetate, and humics. In contrast, methylmercury-chloride complexes, which are dominant in marine systems, were unreactive. Binding by thiols lowered the excitation energy of the carbon-mercury bond on the methylmercury molecule6-7 and subsequently increased reactivity towards bond breakage and decomposition. Our results explain methylmercury photodecomposition rates that are relatively rapid in freshwater lakes2-4 and slow in marine waters5.

No MeSH data available.


Related in: MedlinePlus