The accumulation of xenobiotics, such as halogenated phenols, in soils, sediments, and living organisms is primarily a partitioning process between an aquatic and organic phase. It is traditional to use a bulk octanol-water system to model the partitioning of a compound between complex biological lipid membranes and aqueous media. The octanol-water partition coefficient Kow successfully approximates the lipid membrane-water partition coefficient Ki;pw of neutral compounds. However, the ionized species of substituted phenols were shown (Smejtek, et al. 1993; Escher, et al. 1996) to have a higher affinity for lipid membranes than predicted from octanol-water results. Data on the octanol-water partition coefficients of molecular ions is limited. In order to compare the partitioning of neutral and charged species of halogenated phenols between the lipid membrane-water and octanol-water systems, we measured the pH dependent distribution of ionized and neutral 2,3,4,6-tetrachlorophenol (TeCP), pentafluorphenol (PFP), pentachlorophenol (PCP), and pentabromophenol (PBP). For the neutral (HA) species of each phenol, log Kow was found to be 4.28 (TeCP), 2.79 (PFP), 4.77 (PCP), and approximately 4.67 (PBP). For the ionized (A-) species of each phenol, log Kow was found to be 0.48 (TeCP), -0.85 (PFP), 1.16 (PCP), and 1.77 (PBP). These results are compared with sorption data on halophenols in a lipid membrane-water system (Smejtek, et al. 1996). This study shows that ionized halophenols have an affinity for lipid membranes about two to three orders of magnitude greater than for octanol. The usefulness of the octanol-water model as a predictor of lipid membrane-water partitioning for ionizable compounds, such as halogenated phenols, is questioned in view of the present results. Two thermodynamic Kow prediction models based on molecular properties are discussed in the context of the octanol-water partitioning of ionized and neutral compounds. The partition coefficients predicted by the molar volume based model (Gobas, et al. 1988) correlates with the experimental Kow results of this study better than Kow results predicted from the molecular surface area based model (Smejtek, et al. 1996). Results of this study support the hypothesis that partitioning of xenobiotics in the octanol-water system is fundamentally different than partitioning in lipid membrane-water systems.
| Identifer | oai:union.ndltd.org:pdx.edu/oai:pdxscholar.library.pdx.edu:open_access_etds-6279 |
| Date | 06 May 1996 |
| Creators | Word, Robert |
| Publisher | PDXScholar |
| Source Sets | Portland State University |
| Detected Language | English |
| Type | text |
| Format | application/pdf |
| Source | Dissertations and Theses |
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