QSAR for polychlorinated organic compounds (PCOCs). I. Prediction of partition properties for PCOCs using quantum chemical parameters

Accumulation and fate processes of organochlorine pesticides (OCPs) in soil profiles in Mt. Shergyla, Tibetan Plateau: A comparison on different forest types

Previous work documented that forest plays an important role in the deposition of persistent organic pollutants (POPs) in the southeast Tibetan Plateau (TP) due to the "forest filter effect". However, forest types in the southeast TP are entirely different and the influence on POPs fate and forest filter effect by different forests remains unclear. This study focused on the distribution and transfer of organochlorine pesticides (OCPs) in soil of different forest types (quercus, birch, fir, and spruce dominated forests) in Mt. Shergyla, southeast TP under similar environmental and meteorological conditions. Total levels of ∑HCHs, ∑DDTs and HCB in soils ranged from < LOD to 2.25 ng/g dry weight (dw), < LOD-10.2 ng/g dw, and < LOD-0.95 ng/g dw, respectively. Concentrations of OCPs in humus layers were significantly higher than those in mineral layers in the four forest types. Relatively higher ∑DDTs concentrations were found in soil profile of broadleaved birch forest, while higher concentrations of ∑HCHs and HCB were found in soil profile of coniferous fir forest, and the same trend was observed in fresh leaf samples. Air-to-ground fluxes and mobility of OCPs in the four forest types were also evaluated. Relatively higher fluxes were found in fir forests than in other forest types, suggesting that fir forest could be more effective to transfer OCPs from the air into soil in the southeast TP. The findings in this study would be helpful for improving model simulation of POPs fate in different forest ecosystem.

Environmental risk assessment of selected organic chemicals based on TOC test and QSAR estimation models

Environmental risks of organic chemicals have been greatly determined by their persistence, bioaccumulation, and toxicity (PBT) and physicochemical properties. Major regulations in different countries and regions identify chemicals according to their bioconcentration factor (BCF) and octanol-water partition coefficient (Kow), which frequently displays a substantial correlation with the sediment sorption coefficient (Koc). Half-life or degradability is crucial for the persistence evaluation of chemicals. Quantitative structure activity relationship (QSAR) estimation models are indispensable for predicting environmental fate and health effects in the absence of field- or laboratory-based data. In this study, 39 chemicals of high concern were chosen for half-life testing based on total organic carbon (TOC) degradation, and two widely accepted and highly used QSAR estimation models (i.e., EPI Suite and PBT Profiler) were adopted for environmental risk evaluation. The experimental results and estimated data, as well as the two model-based results were compared, based on the water solubility, Kow, Koc, BCF and half-life. Environmental risk assessment of the selected compounds was achieved by combining experimental data and estimation models. It was concluded that both EPI Suite and PBT Profiler were fairly accurate in measuring the physicochemical properties and degradation half-lives for water, soil, and sediment. However, the half-lives between the experimental and the estimated results were still not absolutely consistent. This suggests deficiencies of the prediction models in some ways, and the necessity to combine the experimental data and predicted results for the evaluation of environmental fate and risks of pollutants.

Prediction of the Fate of Organic Compounds in the Environment From Their Molecular Properties: A Review

A comprehensive review of quantitative structure-activity relationships (QSAR) allowing the prediction of the fate of organic compounds in the environment from their molecular properties was done. The considered processes were water dissolution, dissociation, volatilization, retention on soils and sediments (mainly adsorption and desorption), degradation (biotic and abiotic), and absorption by plants. A total of 790 equations involving 686 structural molecular descriptors are reported to estimate 90 environmental parameters related to these processes. A significant number of equations was found for dissociation process (pKa), water dissolution or hydrophobic behavior (especially through the KOW parameter), adsorption to soils and biodegradation. A lack of QSAR was observed to estimate desorption or potential of transfer to water. Among the 686 molecular descriptors, five were found to be dominant in the 790 collected equations and the most generic ones: four quantum-chemical descriptors, the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital (ELUMO), polarizability (α) and dipole moment (μ), and one constitutional descriptor, the molecular weight. Keeping in mind that the combination of descriptors belonging to different categories (constitutional, topological, quantum-chemical) led to improve QSAR performances, these descriptors should be considered for the development of new QSAR, for further predictions of environmental parameters. This review also allows finding of the relevant QSAR equations to predict the fate of a wide diversity of compounds in the environment.

On the use of electronic descriptors for QSAR modelling of PCDDs, PCDFs and dioxin-like PCBs

The electronic properties of 29 polychlorinated dibenzo-p-dioxins and dibenzofurans and dioxin-like polychlorinated biphenyls that have been included in the toxic equivalency factor system have been investigated and used to derive quantum mechanical (QM) chemical descriptors for QSAR modelling. Their utility in this context was investigated alongside descriptors based on ultraviolet absorption data and traditional 2D descriptors including log K(ow), polarizability, molecular surface properties, van der Waals volume and selected connectivity indices. The QM descriptors were calculated using the semi-empirical AM1 method and the density functional theory method B3-LYP/6-31G**. Atom-specific and molecular quantum chemical descriptors were calculated to compare the electronic properties of dioxin-like compounds regardless of their chemical class, with particular emphasis on the lateral positions. Multivariate analysis revealed differences between the chemical classes in terms of their electronic properties and also highlighted differences between congeners. The results obtained demonstrated the importance of considering molecular orbital energies, but also indicated that the ratios of the coefficients of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at the lateral carbons were important. In addition, the digitalized UV spectra contained chemical information that provided crucial insights into dioxin-like activity.

Benzylidene/2-chlorobenzylidene hydrazides: synthesis, antimicrobial activity, QSAR studies and antiviral evaluation

A series of benzylidene hydrazides (1–20) was synthesized and tested, in vitro, for antibacterial, antifungal and antiviral activities. The microbial screening results indicated that compounds having chloro and nitro substituents were the most active ones. The antiviral evaluation depicted that compounds 9 and 19 were active against Vesicular stomatitis virus (VSV) in HeLa cell cultures. QSAR investigations indicated that the multi-target QSAR model was effective in describing the antimicrobial (antibacterial and antifungal) activity over the one-target QSAR models. Further the mt-QSAR model indicated that the topological parameters, second order molecular connectivity index (²χ) and third order Kier's alpha shape index (κα₃) are effective in describing the antimicrobial activity of synthesized hydrazides.

Prediction of environmental partition coefficients and the Henry's law constants for 135 congeners of chlorodibenzothiophene

Polychlorinated dibenzothiophenes (PCDTs) could be classified as persistent organic pollutants (POPs) in the environment and are particularly interesting due to their structural resemblance to highly toxic dioxins. We show here some basic environmental properties such as n-octanol water (K(OW)), n-octanol/air (K(OA)) and air/water (K(AW)) partition coefficients as well as Henry's law constants (K(H)) for all 135 congeners of chlorodibenzothiophene. Predictions were made by regression of principal components (PCR) and with aid of a set of standard chemicals, for which physical-chemical properties are well featured. Computed K(OW), K(OA), K(AW) and K(H) values for mono-CDTs ranged, respectively, between 4.66 and 4.71, 7.48 and 7.55, -2.84 and -2.82, 3.56 and 3.74; for di-CDTs between 5.02 and 5.28, 8.03 and 8.29, -3.01 and -2.95, 2.42 and 2.75; for tri-CDTs between 5.53 and 5.70, 8.65 and 8.87, -3.2 and -3.11, 1.58 and 1.92; for tetra-CDTs between 5.95 and 6.13, 9.27 and 9.50, -3.39 and -3.27, 1.02 and 1.33; for penta-CDTs between 6.38 and 6.51, 9.88 and 10.05, -3.54 and -3.45, 0.72 and 0.88; for hexa-CDTs between 6.83 and 6.97, 10.54 and 10.66, -3.72 and -3.64, 0.47 and 0.56; for hepta-CDTs between 7.28 and 7.35, 11.12 and 11.20, -3.81 and -3.87, 0.33 and 0.38; for octa-CDT 7.74, 11.78, -4.04 and 0.23. An estimated value of the three types of partition coefficient and Henry's law constants suggest that polychlorinated dibenzothiophenes are lipophilic and semi-volatile persistent organic pollutants. Their mobility in the environment seems to be very similar to that of some well-known POPs such as polychlorinated dibenzofurans, -dibenzo-p-dioxins, and -biphenyls or organochlorine pesticides.

Physico-chemical properties of PCDD/PCDFs and phthalate esters

QSPR models for water solubility (S), n-octanol/water partition coefficient (K(OW)), and Henry's law constant (H) for polychlorinated dibenzo-p-dioxins (PCDDs) and dibenzo-p-furans (PCDFs) and phthalates have been established based on two different sets of parameters. Those parameters were topology based characteristic root index (CRI) and three semi-empirical molecular descriptors, namely--energies of the highest occupied and the lowest unoccupied molecular orbital (E(HOMO) and E(LUMO)), and dipole moment (mu). The best fit equation found by "forward multiple linear regression" showed that the topology based CRI was the most important parameter for the modelling of solubility and n-octanol/water partition coefficient. For n-octanol/water partition coefficient a two-parameter equation including the CRI and E(HOMO) with a correlation coefficient of r = 0.992 was obtained whereas a three-parameter equation for solubility and Henry's law constant including the CRI, E(LUMO) and mu with a correlation coefficient of r = 0.986 and r = 0.933 was obtained, respectively. E(HOMO) and mu didn't appear in the same model because of the collinearity. The results of modified jackknife tests indicated that the three models were statistically robust. Mean deviation of calculated values from experimental data amounted to 0.27, 0.17, and 0.28 log units for the three properties mentioned. The developed models have been used to predict the S, K(OW) and H of compounds not included in the training sets.

Quantum vs. topological descriptors in the development of molecular models of groundwater pollution by pesticides

Using monitoring observations from two, independent studies of US groundwater comprising a total of 61 pesticide compounds, this study has shown that those compounds found in groundwater can be distinguished from those that cannot be found in groundwater on the basis of semi-empirical, quantum chemical and empirical molecular descriptors. For the semi-empirical descriptors, logistic regression models have been developed and validated against the dataset based on the semi-empirical and quantum chemical descriptors. Logistic regression models, based on the Debye dipole moment (mu), the hydration energy (DeltaHhyd), and van der Waals volume (VvdW), resulted in a maximal explained variation in the data of 74%. When topological indices were also included the explained variance in data increased to 91%, with 86% of the variation being explained by the rule that a compound will be found in groundwater if: 0.28mu < 6chip(v) where 6chip(v) is the sixth-order molecular path connectivity and mu is the dipole moment of the compound. The significance of the dipole moment and hydration energy (or van der Waals volume) indicates that it is water solubility that controls mobility, with the inclusion of topological descriptors representing structural factors limiting the solubility. The dependence of leaching potential on the descriptors that control solubility indicates that predictions of environmental fate based on this approach may represent a strong alternative to the use of adsorption and degradation parameters.