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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

Effect of ligand complexation on degradation of MeHga) Singlet oxygen-induced degradation of MeHg (350 nM) complexed with GSH, MA, Cl−, or OH−/HPO42− in water containing 2-chorophenol (40 μM) and phosphate (10 mM, pH=7.3). 1O2 was generated by UV-A (λ =365 nm) irradiation of rose bengal. Lines indicate model predictions from rate constants. Error bars represent ±1 s.d. for replicate measurements (n=2–3); b) Direct UV-C (λ =254 nm) degradation of MeHg at varying initial ratios of MeHg and GSH (4 μM initial MeHg, 1 mM phosphate, pH 7.4). Solid line indicates a linear regression of data for [GSH]0:[MeHg]0 ≤ 1.
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Figure 3: Effect of ligand complexation on degradation of MeHga) Singlet oxygen-induced degradation of MeHg (350 nM) complexed with GSH, MA, Cl−, or OH−/HPO42− in water containing 2-chorophenol (40 μM) and phosphate (10 mM, pH=7.3). 1O2 was generated by UV-A (λ =365 nm) irradiation of rose bengal. Lines indicate model predictions from rate constants. Error bars represent ±1 s.d. for replicate measurements (n=2–3); b) Direct UV-C (λ =254 nm) degradation of MeHg at varying initial ratios of MeHg and GSH (4 μM initial MeHg, 1 mM phosphate, pH 7.4). Solid line indicates a linear regression of data for [GSH]0:[MeHg]0 ≤ 1.

Mentions: In water containing GSH and MA, we observed faster MeHg decomposition by 1O2 than in water containing Cl− or the phosphate buffer alone (Figure 3a). We observed similar trends in experiments with seawater (Supplementary Figure S5). These results indicated that 1O2–induced degradation was faster for CH3Hg-GSH and CH3Hg-MA complexes than for CH3HgCl and CH3HgOH/CH3HgHPO4− complexes (corresponding to PO4-buffered water with no additional ligands).


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

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

Effect of ligand complexation on degradation of MeHga) Singlet oxygen-induced degradation of MeHg (350 nM) complexed with GSH, MA, Cl−, or OH−/HPO42− in water containing 2-chorophenol (40 μM) and phosphate (10 mM, pH=7.3). 1O2 was generated by UV-A (λ =365 nm) irradiation of rose bengal. Lines indicate model predictions from rate constants. Error bars represent ±1 s.d. for replicate measurements (n=2–3); b) Direct UV-C (λ =254 nm) degradation of MeHg at varying initial ratios of MeHg and GSH (4 μM initial MeHg, 1 mM phosphate, pH 7.4). Solid line indicates a linear regression of data for [GSH]0:[MeHg]0 ≤ 1.
© Copyright Policy
Related In: Results  -  Collection

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

Figure 3: Effect of ligand complexation on degradation of MeHga) Singlet oxygen-induced degradation of MeHg (350 nM) complexed with GSH, MA, Cl−, or OH−/HPO42− in water containing 2-chorophenol (40 μM) and phosphate (10 mM, pH=7.3). 1O2 was generated by UV-A (λ =365 nm) irradiation of rose bengal. Lines indicate model predictions from rate constants. Error bars represent ±1 s.d. for replicate measurements (n=2–3); b) Direct UV-C (λ =254 nm) degradation of MeHg at varying initial ratios of MeHg and GSH (4 μM initial MeHg, 1 mM phosphate, pH 7.4). Solid line indicates a linear regression of data for [GSH]0:[MeHg]0 ≤ 1.
Mentions: In water containing GSH and MA, we observed faster MeHg decomposition by 1O2 than in water containing Cl− or the phosphate buffer alone (Figure 3a). We observed similar trends in experiments with seawater (Supplementary Figure S5). These results indicated that 1O2–induced degradation was faster for CH3Hg-GSH and CH3Hg-MA complexes than for CH3HgCl and CH3HgOH/CH3HgHPO4− complexes (corresponding to PO4-buffered water with no additional ligands).

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