Metabolic degradation, inducing potency, and metabolites of fluorinated and chlorinated-fluorinated dibenzodioxins and dibenzofurans

The OH-initiated atmospheric chemical reactions of polyfluorinated dibenzofurans and polychlorinated dibenzofurans: A comparative theoretical study

The atmospheric chemical reactions of some polyfluorinated dibenzofurans (PFDFs) and polychlorinated dibenzofurans (PCDFs), initiated by OH radical, were investigated by performing theoretical calculations using density functional theory (DFT) and B3LYP/6-311++G(2df,p) method. The obtained results indicate that OH addition reactions of PFDFs and PCDFs occurring at C_1∼4 and C_A sites are thermodynamic spontaneous changes and the branching ratio of the PF(C)DF-OH adducts is decided primarily by kinetic factor. The OH addition reactions of PFDFs taking place at fluorinated C_1∼4 positions are kinetically comparable with those occurring at nonfluorinated C_1∼4 positions, while OH addition reactions of PCDFs occurring at chlorinated C_1∼4 sites are negligible. The total rate constants of the addition reactions of PFDFs or PCDFs become smaller with consecutive fluorination or chlorination, and substituting at C₁ position has more adverse effects than substitution at other sites. The succedent O₂ addition reactions of PF(C)DF-OH adducts are thermodynamic nonspontaneous processes under the atmospheric conditions, and have high Gibbs free energies of activation (Δ_rG^≠). The substituted dibenzofuranols are the primary oxidation products for PCDFs under the atmospheric conditions. However, other oxidative products may also be available for PFDFs besides substituted dibenzofuranols.

Photodegradation of Polyfluorinated Dibenzo-p-Dioxins in Organic Solvents: Experimental and Theoretical Studies

Eighteen polyfluorinated dibenzo-p-dioxins (PFDDs) were synthesized by pyrolysis of fluorophenols. Using a 500 W Xe lamp as the light source, the PFDDs photodegradation kinetics in n-hexane were investigated. The photolysis reactions obeyed the pseudo-first-order rate equation, and higher fluorinated PFDDs tended to photolyze more slowly. Theoretically calculated parameters reflecting the molecular structural properties were used to develop a new model of PFDDs photolysis rates. The results indicated that the substitution pattern for fluorine atoms and the C-O bond length were major factors in the photolysis of PFDDs. We selected octafluorinated dibenzo-p-dioxin (OFDD) as a representative PFDDs to explore the influence of solvent on the photolysis rate of PFDDs, and the results indicated that neither the polarity nor donor hydrogen of organic solvents are independent influencing factors. Mechanistic pathways for the photolysis of OFDD in n-hexane were first studied. The results indicated that photodegradation of OFDD produces octafluorinated dihydroxybiphenyls, octafluorinated phenoxyphenols, and fluorinated phenols. The major pathway for photodegradation of OFDD was C-O bond cleavage. Defluorination reactions did not occur during the photolysis process.

Theoretical study on the OH-initiated oxidation mechanism of polyfluorinated dibenzo-p-dioxins under the atmospheric conditions

Density functional theory (DFT) calculations at the B3LYP/6-311++G(2df,p) level of theory have been carried out to investigate the atmospheric oxidation mechanisms of some polyfluorinated dibenzo-p-dioxins (PFDDs), initiated by OH radical. The computed results show that all OH addition reactions of PFDDs are thermodynamically spontaneous processes and the branch ratio of the PFDD-OH adducts is determined by the magnitude of the Gibbs free energies of activation (Δ(r)G(≠)) and hence rate constants (k) for addition reactions. The OH reactions with all studied PFDDs are dominated by Cγ-addition and the total rate constants for OH addition decrease with increasing the number of fluorine atom substituting at α positions. Under the atmospheric conditions, the subsequent O2 addition reactions of PFDD-OH adducts occur hardly thermodynamically and are slow kinetically. For PFDD-α(β)-OH adducts without F atom at same positions the main reaction pathway is H abstraction by O2, while PFDD-γ-OH adducts will undergo fused-ring C-O bond cleavage, affording the substituted phenoxy radicals.

Mechanism and rate constants for complete series reactions of 19 fluorophenols with atomic H

Fluorine-containing halogenated fluorophenol may have effect as intermediate species involved in the formation of polyfluorinated dibenzo-p-dioxin/dibenzofurans (PFDDs/Fs). The mechanism for the atomic H initiated reactions with complete series of nineteen fluorophenol congeners was studies using the density functional theory. At the MPWB1K/6-31+G(d,p) level, the geometries and frequencies of reactants, transition states, and products were obtained, and the accurate energetic values were acquired at the MPWB1K/6-311+G(3df,2p) level. The rate constants were evaluated by the canonical variational transition-state theory with the small curvature tunneling contribution over a wide temperature range of 600-1000 K. The study shows that the intramolecular hydrogen-bond in the ortho-substituted FPs as well as the inductive effect of the electron-withdrawing fluorine and steric repulsion of multiple substitutions may ultimately be responsible for the relative strength of the O-H bonds in FPs. The results can be used for further studies on PFDD/Fs formation mechanism.

Theoretical study on hydrophilicity and thermodynamic properties of polyfluorinated dibenzofurans

Logarithm values of octanol-water partition coefficients (logK(ow)) of polyfluorinated dibenzofurans (PFDFs) were calculated based on group contributions, and their thermodynamic properties in the ideal gas state at 298.15 K and 1.013 × 10(5) Pa were calculated using a combination of quantum mechanical computations performed with the Gaussian 03 program at the B3LYP/6-311G(*) level. The isodesmic reactions were designed to calculate standard enthalpy of formation (ΔH(f)(θ)) and standard free energy of formation (ΔG(f)(θ)) of PFDF congeners. By establishing relationships between these properties (including logK(ow), standard state entropy S(θ), ΔH(f)(θ),ΔG(f)(θ)) and the number and position of fluorine atom substituents (N(PFS)), it was found that the fluorine substitution pattern strongly influences all of these properties of the compounds. The relation curve of logK(ow) values varying with the fluorine substitution pattern presented a three-level sawtooth shape. Intramolecular repulsive forces exist between an oxygen-adjacent fluorine atom on phenyl ring and the oxygen of dibenzofuran, between ortho-substituted fluorine atoms on a phenyl ring, and between two adjacent fluorine atoms on different phenyl rings. Their repulsion energies were ascertained by comparing ΔG(f)(θ) values to be approximately 14 kJ mol(-1)-17 kJ mol(-1), 18 kJ mol(-1)-22 kJ mol(-1) and 7 kJ mol(-1)-9 kJ mol(-1), respectively. According to the relative magnitude of their ΔG(f)(θ), the relative stability order of PFDF congeners was theoretically proposed.

Studies of thermodynamic properties and relative stability of a series of polyfluorinated dibenzo-p-dioxins by density functional theory

The thermodynamic properties of 75 polyfluorinated dibenzo-p-dioxins (PFDDs) in the ideal gas state at 298.15K and 1.013x10(5) Pa have been calculated at the B3LYP/6-311G* level using Gaussian 03 program. The isodesmic reactions were designed to calculate standard enthalpy of formation (DeltaH(f)(degrees)) and standard free energy of formation (DeltaG(f)(degrees)) of PFDDs congeners. The relations of these thermodynamic parameters with the number and position of fluorine atom substitution (N(PFS)) were discussed, and it was found that there exist high correlations between thermodynamic parameters (entropy (S(degrees)), DeltaH(f)(degrees) and DeltaG(f)(degrees)) and N(PFS). According to the relative magnitude of their DeltaG(f)(degrees), the relative stability order of PFDD congeners was theoretically proposed.

Kinetics and organotropy of some polyfluorinated dibenzo-p-dioxins and dibenzofurans (PFDD/PFDF) in rats

While polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) and the corresponding polybrominated congeners must be considered as animal teratogens and carcinogens, little information is available on corresponding polyfluorinated compounds (PFDD/PFDF). Kinetic studies on a few fluorinated dibenzo-p-dioxins and dibenzofurans revealed a rapid elimination, suggesting a much lower toxicity than the corresponding polychlorinated and polybrominated congeners. In order to obtain further clues on the possible toxicity, the kinetics and organ distribution (in liver, thymus and adipose tissue) of a PFDD/PFDF-mixture were studied in Wistar rats after intravenous application. The congeners investigated included four of the 2,3,7,8-substituted, and four of the not-2,3,7,8-substituted dibenzo-p-dioxins, as well as two dibenzofurans. The main result of our studies is the finding that the concentration in the thymus of several of the 2,3,7,8-substituted PFDD/PFDF greatly exceeded that in hepatic tissue. An organotropy quite different from that of the other polyhalogenated congeners must be expected, immunosuppressive effects presumably being the predominant ones. Overall, the elimination half-life of all the PFDD/PFDF studied is considerably shorter than that of the corresponding polychlorinated or polybrominated congeners, in the rat, suggesting a much lower toxicity in this species. No information is available for other species, e.g. nonhuman primates or humans.

Combinatorial bio/chemical analysis of dioxin and dioxin-like compounds in waste recycling, feed/food, humans/wildlife and the environment

The present review describes international activities using bioassays/biomarkers in combination with chemical analysis to measure the effects of dioxin and dioxin-like compounds (DLCs) in the environment. The above authors reviewed already the state-of-art bioanalytical detection methods (BDMs) for dioxins and DLCs [Environ Int (2001)]. The aim of this study will be to review applications of these bioassays/biomarkers to evaluate potential dioxins and DLCs. The present literature study lists relative potencies (REPs) of polyhalogenated dibenzo-p-dioxins and -furans (PXDD/Fs; X = Cl, Br, F), their thio analogues polychlorinated dibenzothiophenes (PCDTs) and thianthrens (PCTAs), polyhalogenated biphenyls (PXBs), polychlorinated naphthalenes (PCNs) and other Ah receptor agonists measured by several biodetectors (Tier 3 screening). The authors will discuss some examples of the applications of some of these biodetectors in biomonitoring programmes and recently occurred dioxin crisis in feed/food. The diagnosis of the biopotency of these pollutants in technical processes like thermally treated waste, waste water treatment, landfill leachate treatment, commercial PCB-mixtures, the release into the environment (soil, air and water) and the final intake into wildlife and humans will be reviewed.

Elimination kinetics and toxicity of 2,3,7,8-tetrachlorothianthren, a thio analogue of 2,3,7,8-TCDD

In comparison to the polychlorinated dibenzo-p-dioxins (PCDD) informations on their thio analogues the polychlorinated thianthrens (PCTA) are very limited. In this study we investigated the kinetics and toxicity of 2,3,7,8-tetrachlorothianthren (TCTA), the analogue of the most toxic PCDD congener 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). It was found that TCTA is rapidly eliminated in mouse liver homogenate fortified with an NADPH-regenerating system suggesting rapid metabolic degradation by liver monooxygenases. Furthermore, TCTA was rapidly eliminated from mouse liver and whole body. In accordance with this rapid elimination, a weekly dosage of 1mg TCTA per kg body weight (i.p.) over six weeks did not result in weight loss or other signs of overt toxicity in male mice. In rat hepatocytes in primary culture, TCTA was active as inducer of dioxin receptor-regulated cytochrome P4501A1 activity measured as 7-ethoxyresorufin O-deethylase (EROD). The relative inducing potency was about 0.0001 in comparison to TCDD. In spite of this molecular effects, the rapid elimination both in vitro and in vivo argues against a consideration of a TCDD equivalency factor for TCTA.