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Lim K.S.,Massachusetts Institute of Technology | Cui L.,Massachusetts Institute of Technology | Cui L.,Singapore MIT Alliance on Research and Technology | Taghizadeh K.,Massachusetts Institute of Technology | And 7 more authors.
Journal of the American Chemical Society | Year: 2012

Guanine is a major target for oxidation in DNA, with 8-oxo-7,8-dihydro- 2′-deoxyguanosine (8-oxodG) as a major product. 8-oxodG is itself significantly more susceptible to oxidation than guanine, with the resulting damage consisting of more than 10 different products. This complexity has hampered efforts to understand the determinants of biologically relevant DNA oxidation chemistry. To address this problem, we have developed a high mass accuracy mass spectrometric method to quantify oxidation products arising site specifically in DNA. We applied this method to quantify the role of sequence context in defining the spectrum of damage products arising from oxidation of 8-oxodG by two oxidants: nitrosoperoxycarbonate (ONOOCO2 -), a macrophage-derived chemical mediator of inflammation, and the classical one-electron oxidant, riboflavin-mediated photooxidation. The results reveal the predominance of dehydroguanidinohydantoin (DGh) in 8-oxodG oxidation by both oxidants. While the relative quantities of 8-oxodG oxidation products arising from ONOOCO2 - did not vary as a function of sequence context, products of riboflavin-mediated photooxidation of 8-oxodG were highly sequence dependent. Several of the 8-oxodG oxidation products underwent hydrolytic conversion to new products with half-lives of 2-7 h. The results have implications for understanding the chemistry of DNA oxidation and the biological response to the damage, with DNA damage recognition and repair systems faced with a complex and dynamic set of damage targets. © 2012 American Chemical Society.

Chen M.,Nanyang Technological University | Chen M.,Singapore MIT Alliance on Research and Technology | Lee J.-M.,Massachusetts Institute of Technology | Nurhati I.S.,Singapore MIT Alliance on Research and Technology | And 4 more authors.
Marine Chemistry | Year: 2015

The spatial and temporal variations of Pb were reconstructed from three corals along an east-west transect of the Singapore Straits - in proximity to one of the largest emerging Asian cities and one of the world's busiest ports. In addition to establish the variation in seawater Pb concentration, sources of Pb were also investigated via Pb isotopes in seawater and corals and compared with the isotopes in local aerosols and other potential Pb sources. The annual variation of Pb in Singapore corals seems to be related to local annual precipitation while the decadal-scale variation may be related to the long-term development of the region. The decrease of Pb/Ca in coral after 2005 coincided with the regional phasing out of leaded gasoline. The 206Pb/207Pb ratios in Singapore corals rose from as low as ~1.166 during the 1960s to ~1.189 by the late 1990s, and fluctuated between 1.180-1.198 from the late 1990s to 2010. 206Pb/207Pb in recent Singapore corals was elevated relative to Singapore atmospheric aerosols (~1.147, which is typical of urban aerosols throughout the southeastern Asian and Indonesian region). The linear distribution of 206Pb/207Pb vs. 208Pb/207Pb in triple isotope plots suggests two end-member mixing between the aerosol-Pb as a low 206Pb/207Pb end-member and an unidentified source with higher 206Pb/207Pb. Possible high 206Pb/207Pb sources include: surface runoff from imported south Chinese Pb, crustal materials from riverine inputs (sediments and suspended matter) or land reclamation, and vessels/dockyard activities. © 2014 Elsevier B.V.

Chen M.,Nanyang Technological University | Chen M.,Singapore MIT Alliance on Research and Technology | Boyle E.A.,Singapore MIT Alliance on Research and Technology | Boyle E.A.,Massachusetts Institute of Technology | And 3 more authors.
Environmental Pollution | Year: 2016

Reconstructing the history of metal deposition in Singapore lake sediments contributes to understanding the anthropogenic and natural metal deposition in the data-sparse Southeast Asia. To this end, we present a sedimentary record of Pb, Pb isotopes and eleven other metals (Ag, As, Ba, Cd, Co, Cr, Cu, Ni, Tl, U and Zn) from a well-dated sediment core collected near the depocenter of MacRitchie Reservoir in central Singapore. Before the 1900s, the sedimentary Pb concentration was less than 2 mg/kg for both soil and sediment, with a corresponding 206Pb/207Pb of ∼1.20. The Pb concentration increased to 55 mg/kg in the 1990s, and correspondingly the 206Pb/207Pb decreased to less than 1.14. The 206Pb/207Pb in the core top sediment is concordant with the 206Pb/207Pb signal of aerosols in Singapore and other Southeast Asian cities, suggesting that Pb in the reservoir sediment was mainly from atmospheric deposition. Using the Pb concentration in the topmost layer of sediment, the estimated atmospheric Pb flux in Singapore today is ∼1.6 × 10-2 g/m2 yr. The concentrations of eleven other metals preserved in the sediment were also determined. A principal component analysis showed that most of the metals exhibit an increasing trend towards 1990s with a local concentration peak in the mid-20th century. © 2016 Published by Elsevier Ltd.

Chen M.,Singapore MIT Alliance on Research and Technology | Chen M.,Nanyang Technological University | Boyle E.A.,Singapore MIT Alliance on Research and Technology | Boyle E.A.,Massachusetts Institute of Technology | And 7 more authors.
Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | Year: 2016

Atmospheric aerosols are the dominant source of Pb to the modern marine environment, and as a result, in most regions of the ocean the Pb isotopic composition of dissolved Pb in the surface ocean (and in corals) matches that of the regional aerosols. In the Singapore Strait, however, there is a large offset between seawater dissolved and coral Pb isotopes and that of the regional aerosols. We propose that this difference results from isotope exchange between dissolved Pb supplied by anthropogenic aerosol deposition and adsorbed natural crustal Pb on weathered particles delivered to the ocean by coastal rivers. To investigate this issue, Pb isotope exchange was assessed through a closed-system exchange experiment using estuarine waters collected at the Johor River mouth (which discharges to the Singapore Strait). During the experiment, a known amount of dissolved Pb with the isotopic composition of NBS-981 (206Pb/207Pb = 1.093) was spiked into the unfiltered Johor water (dissolved and particulate 206Pb/207Pb = 1.199) and the changing isotopic composition of the dissolved Pb was monitored. The mixing ratio of the estuarine and spike Pb should have produced a dissolved 206Pb/207Pb isotopic composition of 1.161, but within a week, the 206Pb/207Pb in the water increased to 1.190 and continued to increase to 1.197 during the next two months without significant changes of the dissolved Pb concentration. The kinetics of isotope exchange was assessed using a simple Kd model, which assumes multiple sub-reservoirs within the particulate matter with different exchange rate constants. The Kd model reproduced 56% of the observed Pb isotope variance. Both the closedsystem experiment and field measurements imply that isotope exchange can be an important mechanism for controlling Pb and Pb isotopes in coastal waters. A similar process may occur for other trace elements. This article is part of the themed issue 'Biological and climatic impacts of ocean trace element chemistry'. © 2016 The Author(s) Published by the Royal Society. All rights reserved.

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