Estimation of n-Octanol/Water Partition Coefficients (log ) of Polychlorinated Biphenyls by Using Quantum Chemical Descriptors and Partial Least Squares

Hindawi Publishing Corporation Journal of Chemistry Volume 2013, Article ID 740548, 8 pages http://dx.doi.org/10.1155/2013/740548 Research Article Estimation of n-Octanol/Water Partition Coefficients (logKOW) of Polychlorinated Biphenyls by Using Quantum Chemical Descriptors and Partial Least Squares Fang-Li Zhang,1 Xing-Jian Yang,1 Xiu-Ling Xue,2 Xue-Qin Tao,3 Gui-Ning Lu,1,4 and Zhi Dang1,5 1 School of Environment and Energy, South China University of Technology, Guangzhou 510006, China College of Chemical Engineering, Huaqiao University, Xiamen 361021, China 3 School of Environmental Science and Engineering, Zhongkai University of Agriculture and Engineering, Guangzhou 510225, China 4 Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, Guangzhou 510006, China 5 The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters, Ministry of Education, Guangzhou 510006, China 2 Correspondence should be addressed to Gui-Ning Lu; Received 15 May 2013; Revised 29 July 2013; Accepted 22 August 2013 Academic Editor: Enrico Veschetti Copyright © 2013 Fang-Li Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The n-octanol/water partition coefficient (log 𝐾OW ) is a useful parameter for the assessment of the environmental fate and impact of xenobiotic trace contaminants. Quantitative structure-activity relationship (QSAR) model for log 𝐾OW of polychlorinated biphenyls (PCBs) was analyzed by using the density functional theory at B3LYP/6-31G(d) level and the partial least squares (PLS) method with an optimizing procedure. A PLS model with reasonably good coefficient (𝑅2 = 0.992) and cross-validation test (𝑄2 cum = 0.988) values was obtained. All the predicted values are within the range of ±0.3 log unit from the observed values. The log 𝐾OW values of 7 PCBs in the test set predicted by the model are very close to those observed, indicating that this model has high fitting precision and good predictability. The PLS analysis showed that PCBs with larger electronic spatial extent and lower molecular total energy values tend to be more hydrophobic and lipophilic. 1. Introduction Polychlorinated biphenyls (PCBs) are highly stable industrial chemical products that have been used as industrial fluids, flame retardants, diluents, hydraulic fluids, and dielectric fluids. Due to careless disposal practices and chemical stability, PCBs are some of the most prevalent pollutants in the total environment. Generally, a sum of 209 theoretical PCBs exist, and approximately 150 PCBs are found in the environment [1]. Despite the fact that the production and the use of PCBs have long been banned, they are still considered environmental contaminants as they are among the most widespread and persistent man-made products in the ecosystem. It is well known that these chemicals are toxic and lipophilic and tend to be bioaccumulated. Some interfere with mammalian and avian reproduction, and others disturb human’s germ cell development or are promoters of cancer. Some PCBs are included in the increasing list of environmental pollutants that are able to mimic estrogen action and are termed “environmental estrogens” [2]. PCBs are categorized as persistent organic pollutants (POPs) [3]. Individual PCB congeners exhibit different physicochemical properties and biological activities that result in different environmental distributions and toxicity profiles. The variable composition of PCB residues in environmental matrices and their different mechanisms of toxicity complicate the development of scientifically based regulations for the risk assessment [4]. The n-octanol/water distribution coefficient (𝐾OW ) is an important physicochemical 2 property suitable for the characterization of the lipophilicity of the compounds such as PCBs. 𝐾OW has been shown to correlate with aquatic solubility, toxicity, and bioaccumulation [5–7], and as such it is a useful parameter for the assessment of the environmental fate and impact of xenobiotic trace contaminants [8]. During the last 50 years, 𝐾OW has proven to be a valuable tool in many areas of natural sciences: chemistry, biochemistry, biology, pharmacology, environmental sciences, and so forth. During that period, many hundreds or even thousands of scientific publications have been made in which the successful application of 𝐾OW in correlation with many physical, chemical, or biological properties has been demonstrated for a large variety of organic chemicals. Unfortunately, many correlations have been also published in which the processes studied have nothing in common with the process of partition between water and n-octanol phases. It must be emphasized that in order to correlate, two properties do not have to be related; both of them may simply be related to a third property. The shake flask is the classical method for measuring 𝐾OW , and this procedure will give reproducible and accurate results for chemicals with log 𝐾OW . Accurate results can also be obtained for more hydrophobic chemicals, but this will need a very precise and skillful handling and the use of very pure n-octanol. The problem with this procedure is that the complete phase separation is difficult to achieve, and this may lead to the underestimation of 𝐾OW , particularly for the more hydrophobic chemicals [9]. It is impractical to measure the 𝐾OW of all the PCBs directly in the laboratory because of the extremely low water solubility of some high-chlorinated PCBs. The lack of complete, reliable, and comparable data has led to the development of different 𝐾OW estimation methods. With the advent of inexpensive and rapid computation, there has been a remarkable growth in the area of quantitative structureactivity relationships (QSAR), which correlate the properties of pollutants with relevant properties and molecular descriptors [11]. A large number of calculation methods have been presently developed for estimation of the partition coefficients with varying success and applicability. These methods are cable of predicting environmental behaviors not only for common environmental pollutants but also for those chemicals that have not yet been synthesized or those that cannot be examined experimentally due to their extremely hazardous nature. According to the descriptors used, these methods can be classified into two groups: empirical and theoretical methods [12]. Hansen et al. [1] used the degree of chlorination in the orthoposition of the PCB molecule as predictor variable together with either the calculated molecular total surface area (TSA) or the gas chromatographic relative retention times (RRT) to predict the log 𝐾OW and soil sorption coefficients (log 𝐾OC ) for all 209 PCB congeners which were based on 53 and 48 experimental data points, respectively. Also, reportin (...truncated)