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Lower chlorinated polychlorinated biphenyls (PCBs) are readily metabolized via hydroxylated metabolites to reactive PCB quinones. Although these PCB metabolites elicit biochemical changes by mechanisms involving cellular target molecules, such as the aryl hydrocarbon receptor, and toxicity by interacting with enzymes like topoisomerases, only few PCB quinones have been synthesized and their conformational properties investigated. Similar to the parent compounds, knowledge of the three-dimensional structure of PCB quinones may therefore be important to assess their fate and risk. To address this gap in our knowledge, the gas phase molecular structure of a series of PCB quinones was predicted using HF/3-21G, B3LYP/6-31G?? and UB3LYP/6-311G?? calculations and compared to the respective solid state structure. All three methods overestimated the Cl-C bond length, but otherwise provided a reasonable approximation of the solid state bond angles and bond lengths. Overall, the UB3LYP/6-311G?? level of theory yielded the best approximation of the molecular structure of PCB quinones in the solid state. Chlorine addition at the ortho position of both rings was found to increase the dihedral angle of the resulting quinone compound, which may have important implications for their interaction with cellular targets and, thus, their toxicity. 相似文献
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Alison H. Purcell David W. Bressler Michael J. Paul Michael T. Barbour Ed T. Rankin James L. Carter Vincent H. Resh 《Journal of the American Water Resources Association》2009,45(2):306-319
Abstract: Biological indicators, particularly benthic macroinvertebrates, are widely used and effective measures of the impact of urbanization on stream ecosystems. A multimetric biological index of urbanization was developed using a large benthic macroinvertebrate dataset (n = 1,835) from the Baltimore, Maryland, metropolitan area and then validated with datasets from Cleveland, Ohio (n = 79); San Jose, California (n = 85); and a different subset of the Baltimore data (n = 85). The biological metrics used to develop the multimetric index were selected using several criteria and were required to represent ecological attributes of macroinvertebrate assemblages including taxonomic composition and richness (number of taxa in the insect orders of Ephemeroptera, Plecoptera, and Trichoptera), functional feeding group (number of taxa designated as filterers), and habit (percent of individuals which cling to the substrate). Quantile regression was used to select metrics and characterize the relationship between the final biological index and an urban gradient (composed of population density, road density, and urban land use). Although more complex biological indices exist, this simplified multimetric index showed a consistent relationship between biological indicators and urban conditions (as measured by quantile regression) in three climatic regions of the United States and can serve as an assessment tool for environmental managers to prioritize urban stream sites for restoration and protection. 相似文献
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The aim of this experiment was to investigate the relationship between perceived annoyance and perceived loudness for complex community sounds. The two perceptual attributes were scaled by the method of magnitude estimation. In all, forty subjects scaled ten different tape recorded sounds presented at two different Zwicker loudness values. The results show that for physically loud sounds only the perceived annoyance may be exchanged for perceived loudness, while for physically soft sounds, the perceived annoyance deviates greatly from perceived loudness. Thus, for physically soft sounds, more attention is paid to other aspects of the sound, such as the perceptual counterpart of the physical parameter sharpness, than to the loudness-related sound pressure level. This means that the physically determined Zwicker loudness is inappropriate to use for predicting the perceived annoyance of complex sounds. 相似文献
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Rankin Shannon Oedekoven Cornelia Archer Frederick 《Environmental and Ecological Statistics》2020,27(2):233-251
Environmental and Ecological Statistics - Cetacean abundance estimation often relies on distance sampling methods using shipboard visual line-transect surveys, which assumes that all animals on the... 相似文献
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