Dataset Information

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

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.

SUBMITTER: Zhang T

PROVIDER: S-EPMC2902198 | biostudies-literature | 2010 Jul

REPOSITORIES: biostudies-literature

ACCESS DATA

Publications

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

Zhang Tong T Hsu-Kim Heileen H

Nature geoscience 20100701 7

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 diss ...[more]

PMID: 20634995

Similar Datasets

Project description:The photodissociation of small molecules has been extensively studied because of the increase in environmental problems related to the atmosphere of the Earth. In this work, the photodissociation mechanisms of hydroxylamine (NH2OH) as a model molecule in its lowest singlet-excited (S1) state were systematically studied using the complete active space second-order perturbation theory (CASPT2) and transition state theory (TST). In particular, this study focused on nonradiative relaxation processes that convert the S0 → S1 excited-state molecule to its products in their respective electronic ground states. The potential energy curves obtained from relaxed scans suggest that O-H dissociation is the preferred process in the S1 state. For the N-O and N-H dissociation pathways, thermally excited precursors were hypothesized to form in the S0 state to circumvent O-H dissociation. Thus, S0 → S1 vertical excitations lead to transition structures in the S1 state, which fragment to their respective electronic-ground-state products. The thermodynamic and kinetic results confirmed the precursor hypothesis, showing that the exothermic energy caused by the formation of HNO and H2 is sufficient to generate such precursors in the S0 state. Additionally, the TST confirmed that unimolecular isomerization-dissociation is a two-step process that generates products effectively by direct photolysis of the corresponding covalent bonds. In particular, the process consists of O-H bond dissociation, followed by spontaneous isomerization and formation of H2 in its electronic ground state, resulting in the high quantum yield observed in the UV absorption experiments in the preferential formation of HNO and H2. The configuration interaction coefficients of the characteristic structures on the potential energy curves revealed considerable changes in the multiconfigurational character of the wavefunctions, especially for the transition structures. These are characterized by the development of Rydberg orbitals, being produced at the intersection of the S0 and S1 states. The present study highlights the effects of thermal selectivity and the multiconfigurational character of the wavefunctions on photodissociation. Because detailed information on the photolytic mechanisms of isolated NH2OH is limited both theoretically and experimentally, these results provide fundamental insight into unimolecular photodissociation, posing ground for future studies on related systems.

Dataset Information

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

Publications

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

Similar Datasets

OmicsDI is part of the ELIXIR infrastructure

Tweets