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PubMed2003

What is the nature of the first-formed intermediates in the electrophilic halogenation of alkenes, alkynes, and allenes?

The pi complexes first formed as essential intermediates from alkenes, alkynes, and allenes with bromine have been investigated in different solvents by UV-spectroscopy in combination with stopped-flow techniques allowing the determination of the equilibrium constants, K(f). Using alkenes with sterically protected double bonds, such as di-tert-butylstilbene and tetraneopentylethylene, the reaction stops at the stage of the 1:1 and 1:2 pi complex of the alkene with bromine as persistent species in 1,2-dichlorethane as solvent. Calculations by state-of-art ab initio and DFT methods reproduces the experimentally determined thermodynamic values quite well, and reveal the preferred structures and nature of both complexes for ethene, ethyne, and allene. Consideration of the entropy term reveals that complexes are stabilized in solution owing to reduction of the entropy loss by restricted translations and rotation. According to calculations these species are Mulliken-outer-type complexes with no or little charge transfer from bromine to the double or triple bond, respectively. The 1:2 complex has a close structural relationship to the bromonium- or bromirenium ion, which is the subsequent intermediate on the reaction coordinate. Steric influences show a strong effect on the K(f) value, which can be explained by the polarizibility of the parent system. Addition-elimination often occurs. In bromination of adamantylidenadamantane and its derivatives the reaction stops at the stage of the bromonium ion. The effect of various polar groups situated in equatorial homoallyl positions on the stability of corresponding pi complex and bromonium ion has been studied in this series.

Alkadienes↗
PubMed1988

Mutagenicity of halogenated propanes and their methylated derivatives.

1,2,3-Tribromopropane, 1,2,3-trichloropropane, and 1,2-dibromo-3-chloropropane are mutagenic in strains TA1535 and TA100 of Salmonella typhimurium, but only in the presence of rat liver S9 mix. This requirement for metabolic activation was unexpected for an alkyl halide and thus suggested the metabolic formation of the 2-keto derivatives (di-haloacetone). The 2-methyl derivatives of the halopropane compounds did not induce a doubling of revertants compared to controls. It was demonstrated that none of these compounds is converted into a secondary material that could be determined as structurally different by gas chromatography. These observations suggest that lack of mutagenicity of the methylated compounds is a manifestation of a steric effect.

Biotransformation↗
PubMed2002

Facile distinction of neutral and acidic tetraether lipids in archaea membrane by halogen atom adduct ions in electrospray ionization mass spectrometry.

Calditocaldarchaeol (neutral tetraether lipid) from Sulfolobus acidocaldarius (acidothermophilic archaea) and intact total lipid from the thermoacidophilic archaea Sulfolobus sp. was examined by electrospray ionization time-of-flight mass spectrometry in the negative-ion mode using high resolution. When the sample was injected as a solution in a 3:1 mixture of methanol (MeOH) and chloroform (CHCl(3)) using an infusion system, the total ether lipid afforded molecular-related ions as [M - H](-) for acidic polar lipids containing a phosphoric or sulfuric group, and as [M + Cl](-) ion for neutral glycolipids. The attachment of chloride was confirmed by the observation of [M + Br](-) ion, instead of [M + Cl](-) ion, when a 3:1 mixture of MeOH and CHBr(3) was used in place of MeOH-CHCl(3) as the solvent. The composition of tetraether neutral glycolipids that are different from each other only in the number of five-membered rings in the isoprenoid chain was determined on the basis of the isotope-resolved mass spectrum of [M + Cl](-) ions. As for acidic tetraether lipids, molecular-related ions [M - H](-)) were not observed when the 3:1 MeOH-CHBr(3) mixture was used as the solvent. These results together afforded a facile method of distinguishing neutral from acidic tetraether lipids in intact total lipids of acidothermophilic archaea. This method was applied to determine the difference of the number of five-membered rings in isoprenyl chains of neutral tetraether glycolipids yielded by the Sulfolobus sp. grown at different temperatures. Discrimination of neutral tetraether glycolipids from acidic tetraether lipids in the total lipids obtained from Thermoplasma sp. was also achieved by this method.

Animals↗
PubMed1991

Evidence of steric factors in the fungitoxic mechanisms of 8-quinolinol and its 5- and 7-halogenated analogues.

Antifungal studies were made of mixtures of minimal inhibitory concentrations (MICs) of 8-quinolinol and its 5- and 7-halo analogues against six fungi: Aspergillus niger, A. oryzae, Trichoderma viride, Myrothecium verrucaria, Mucor cirinelloides, and Trichophyton mentagrophytes. Mixtures of 8-quinolinol with 5- or 7-fluoro-8-quinolinol and of 5- and 7-fluoro-8-quinolinol showed additive activity, and their respective toxicities were reversed by L-cysteine. These results suggested a common mechanism of activity for the three toxicants. Potentiation of the fungitoxicity of mixtures of 8-quinolinol and its 5- and 7-chloro, bromo, and iodo analogues, as well as mixtures of 5- and 7-chloro, 5- and 7-bromo, and 5- and 7-iodo-8-quinolinols, along with the absence of protection of the fungi by L-cysteine from the toxicities of these compounds was observed. This suggested that the modes of action of these compounds were different from each other and from 8-quinolinol and the 5- and 7-fluoro analogues. The geometry of 8-quinolinol as influenced by substituents in the 5- and 7- positions of the molecule determines its site(s) of fungitoxicity.

Antifungal Agents↗
PubMed2001

Application of the ethoxyresorufin-O-deethylase (EROD) assay to mixtures of halogenated aromatic compounds.

The ethoxyresorufin-O-deethylase (EROD) assay monitors the induction of the xenobiotic-metabolizing enzyme cytochrome P-450 (CYP) 1A1 and is a widely used biomarker for exposure of wildlife to substances that bind the aryl hydrocarbon (Ah) receptor. In this work the induction of EROD activity by single compounds and binary mixtures in primary rat hepatocytes was compared with the predictions of a kinetic model involving mixtures of inducers. The inducing agents were also examined for their ability to activate the Ah receptor to its DNA-binding form and for their ability to act as competitive inhibitors for CYP 1A1. Xenobiotics that failed to activate the Ah receptor did not induce EROD activity. Competitive inhibition for CYP 1A1 between the xenobiotic and 7-ethoxyresorufin caused EROD activity to fall at high xenobiotic concentrations. Competition for a limited number of Ah receptor sites depressed the EROD activity of a strong inducer such as 2,3,7,8-tetrachlorodibenzo-p-dioxin at high concentrations of a weak inducer. Application of the kinetic model to the example of a mixture of low concentrations of dibenzo-p-dioxins and much higher concentrations of polychlorinated biphenyls indicated that EROD assays often seriously underestimate the true potency of an environmental sample. Hence the EROD assay cannot be used for determining dioxin equivalent concentrations using the toxic equivalence factor approach.

Animals↗
PubMed1996

Lipid peroxidation and formation of 8-hydroxydeoxyguanosine from acute doses of halogenated acetic acids.

Chlorinated, brominated, and mixed bromochloro acetates are major by-products of water disinfection by chlorine or ozone. The chlorinated acetates, trichloroacetate (TCA) and dichioroacetate (DCA), are carcinogenic in rodents. Brominated analogs of TCA and DCA have received little study. TCA and DCA induce lipid peroxidation in the livers of rodents when administered acutely. Oxidative stress can also result in oxidative damage to DNA, most commonly measured as increases in 8-hydroxydeoxyguanosine (8-OHdG) adducts. In this study, the ability of acute doses of TCA, DCA, dibromoacetate (DBA), bromodichloroacetate (BDCA), and bromochloroacetate (BCA) to induce lipid peroxidation and 8-OHdG formation was examined. Male B6C3F1 mice developed significant increases in 8-OHdG/dG ratios in nuclear DNA isolated from livers when treated with haloacetates. The extent of 8-OHdG formation appeared to be related to the ability to induce thiobarbituric acid-reactive substances (TBARS). The order of potency was DBA = BCA > BDCA > DCA > TCA. The induction of 8-OHdG was found to be generally more sensitive to treatment with haloacetates than the TBARS response. Significantly elevated levels of 8-OHdG were observed at doses of DBA, BCA, and BDCA as low as 30 mg/kg. We suggest that formation of 8-OHdG by brominated haloacetates may contribute to their toxicological effects.

8-Hydroxy-2'-Deoxyguanosine↗
PubMed1997

Physiologically based modeling of nonsteady state dermal absorption of halogenated methanes from an aqueous solution.

Dermal absorption of organic chemicals from aqueous solutions are a concern in both the workplace and the home. Organic chemicals are generally not very soluble in water and the exposure may never reach steady state because the concentration of chemical decreases during the exposure. In vivo animal studies which mimic human exposures, but are carefully controlled, are one way to measure absorption. Whole animal studies are superior to excised skin measurements, because the physiological responses including blood flow, metabolism, and biological defenses are present. In this study, we develop a physiologically based model for nonsteady state exposures to organic chemicals in aqueous solutions. A key feature of this model is a compartment which describes loss of chemical in the exposure solution due to absorption into the skin. We exposed rats to a range of aqueous concentrations of dibromomethane (2.4 to 9.4 mg/ml) and bromochloromethane (3.6 to 12.8 mg/ml) and measured blood concentrations during 24-hr exposures. The blood concentrations peaked at about 1-2 hr and diminished to nearly nothing at 24 hr. Physiologically based models were used to estimate permeability coefficients for each of the exposures, although none of the exposures reached steady state due to the decreasing concentration of chemical on the surface of the skin. A constant permeability coefficient adequately described the blood concentrations during the prolonged exposure. Physiologically based models can be used to estimate permeability coefficients when the concentration of chemical on the skin is not constant. These permeability parameters can subsequently be used for assessing the risks in human exposure situations.

Administration, Cutaneous↗