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PubMed2006

Halogen bonding: the sigma-hole. Proceedings of "Modeling interactions in biomolecules II", Prague, September 5th-9th, 2005.

Halogen bonding refers to the non-covalent interactions of halogen atoms X in some molecules, RX, with negative sites on others. It can be explained by the presence of a region of positive electrostatic potential, the sigma-hole, on the outermost portion of the halogen's surface, centered on the R-X axis. We have carried out a natural bond order B3LYP analysis of the molecules CF(3)X, with X = F, Cl, Br and I. It shows that the Cl, Br and I atoms in these molecules closely approximate the [Formula: see text] configuration, where the z-axis is along the R-X bond. The three unshared pairs of electrons produce a belt of negative electrostatic potential around the central part of X, leaving the outermost region positive, the sigma-hole. This is not found in the case of fluorine, for which the combination of its high electronegativity plus significant sp-hybridization causes an influx of electronic charge that neutralizes the sigma-hole. These factors become progressively less important in proceeding to Cl, Br and I, and their effects are also counteracted by the presence of electron-withdrawing substituents in the remainder of the molecule. Thus a sigma-hole is observed for the Cl in CF(3)Cl, but not in CH(3)Cl.

Halogens↗

PubMed1990

Reactivity of ebselen and related selenoorganic compounds with 1,2-dichloroethane radical cations and halogenated peroxyl radicals.

The reactivity of ebselen, 2-phenyl-1,2-benzisoselenazol-3(2H)one, and structurally related analogues was studied by pulse radiolysis. The rate constant for the reaction of ebselen with trichloromethylperoxyl radicals was determined to be 2.9 X 10(8) M-1 s-1, while its sulfur analogue, 2-phenyl-1,2-benzisothiazol-3(2H)one, was oxidized at much lower rates, k less than or equal to 10(7) M-1 s-1. Among several derivatives studied, the only other compound that exhibited a high rate constant was 2-(methylseleno)-benzoic acid-N-phenylamide. Oxidation of ebselen by other halogenated peroxyl radicals was also carried out and revealed a direct relationship between rate constant and the degree of halogenation of the oxidant. The transient radicals generated during oxidation of ebselen and the analogues were characterized by optical absorption and conductivity measurements and were attributed to one-electron-oxidized radical cations. The oxidation potentials were determined by cyclic voltammetry. Comparative evaluation of the in vitro behavior during microsomal lipid peroxidation revealed ebselen to be the most potent antioxidant of the compounds investigated, 2-(Methylseleno)-benzoic acid-N-phenylamide, despite its high rate constant for oxidation by halogenated peroxyl radicals, was found to be a poor antioxidant. The rate constant of oxidation of ebselen by trichloromethylperoxyl radicals is comparable to that of alpha-tocopherol under similar conditions, underscoring the potential pharmacological interest of ebselen as an antioxidant.

Antioxidants↗

PubMed1995

Toxicology of halogenated aliphatic hydrocarbons: structural and molecular determinants for the disturbance of chromosome segregation and the induction of lipid peroxidation.

The induction of mitotic chromosome malsegregation, mitotic arrest and lethality by a set of 55 halogenated hydrocarbons was investigated. To this aim, genetic assays in the mould Aspergillus nidulans, able to provide precise quantitative information on the end-points studied, were used throughout the work. The experimental data obtained were used to develop QSAR models for the induction of aneuploidy, which pointed to a major role of electrophilicity as molecular determinant for the aneugenic potential of the halogenated hydrocarbons investigated. Within the hypothesis of a link between the electrophilicity of haloalkanes and their propensity to undergo a reductive biotransformation, with production of free radical species, a subset of 27 compounds was also tested for the ability to induce lipid peroxidation in rat liver microsomes in vitro. The results obtained indicate a partial coincidence between the abilities to initiate lipid peroxidation and to disturb chromosome segregation at mitosis. The data base obtained was also used to investigate the relationship between chemical structure and peroxidative potential. The analysis indicated that electronic and structural parameters related to the ease of homolitic cleavage of the carbon-halogen bond play a pivotal role as determinants for the peroxidative character of haloalkanes.

Aneuploidy↗

PubMed1992

Halogenated alcohols as solvents for proteins: FTIR spectroscopic studies.

Fourier transform infrared (FTIR) spectroscopy has been applied to investigate the secondary structure of proteins and polypeptides in halogenated alcohols. Each alcohol studied was able, as a pure liquid, to induce conversion of the beta-sheet protein concanavalin A into a predominantly alpha-helical configuration. In 2H2O/alcohol mixtures, helicogenisis was also apparent, decreasing in the order dichloroethanol greater than bromoethanol greater than trifluoroethanol greater than chloroethanol greater than fluoroethanol. At concentrations below those found to be helicogenic, disruption of the protein secondary structure by the alcohols resulted in pronounced aggregation. At concentrations insufficient to cause noticeable disruptions of the secondary structure at room temperature, the thermal stability of the protein was greatly reduced. We suggest the helicogenic effect exhibited by halogenated alcohols to be related to a combination of a relatively low dielectric constant and a high dipole moment, the latter causing disruption of the internal hydrogen bond networks and the former causing refolding to a helical configuration. The results presented here highlight the risk of using halogenated alcohols, both as solvents for proteins and as a test of the intrinsic capacity of proteins and peptides to adopt helical secondary structures.

Alcohols↗

PubMed1989

Uptake of 63Ni2+ in the central and peripheral nervous system of mice after oral administration: effects of treatments with halogenated 8-hydroxyquinolines.

Oral administration of 63Ni2+ to mice resulted in higher levels of the metal in the sciatic nerves, the trigeminal ganglia, spinal nerve roots and the spinal cord than in the cerebellum and the frontal cortex. In the sciatic nerves, the trigeminal ganglia and the spinal nerve roots a high vascular permeability may explain the preferential uptake of the 63Ni2+. The higher accumulation of the 63Ni2+ in the spinal cord than in the examined structures of the brain may be related to a diffusion from the cerebrospinal fluid. Administrations of halogenated 8-hydroxyquinolines [5,7-diiodo-8-hydroxyquinoline (diI-HQ), 5,7-dibromo-8-hydroxyquinoline (diBr-HQ), 5,7-dichloro-8-hydroxyquinoline (diCl-HQ) or 5-chloro-7-iodo-8-hydroxyquinoline (Cl-I-HQ)] together with the 63Ni2+ resulted in increased levels of the metal both in neural and extraneural tissues in comparison with mice given the 63Ni2+ alone. In most instances the tissue-levels of 63Ni2+ were 2-4 times higher in the animals treated with the halogenated 8-hydroxyquinolines than in the controls. Determinations of chloroform/water partition coefficients showed that lipophilic chelates are formed between each of these compounds and the nickel. Our results are probably explained by a facilitated passage of the complexed metal over the walls of the stomach and the intestine. The halogenated 8-hydroxyquinolines are linked with the SMON-syndrome, a disease which occurred primarily in Japan. The possibility that this syndrome may be causally connected with a concomitant intake of these drugs and metals is discussed.

Administration, Oral↗

PubMed1994

Detection of p53 and histopathological classification of skin tumours induced by halogen lamps in hairless mice.

Skin lesions induced by exposure of three strains of female hairless mice to the light emitted by uncovered halogen quartz lamps were subjected to histopathological analysis. We examined 170 representative specimens out of a total of 597 skin lesions, i.e. 38 out of 74 SKH-1 mice, 110 out of 472 MF-1 mice, and 42 out of 51 C3H mice. The results provided evidence of various types of alterations, including preneoplastic changes, such as epidermal hyperplasia, and benign tumours, such as papillomas, as well as tumours with an increasing degree of malignancy, i.e., keratoacanthoma-like tumours, appendage/basal tumours, actinic keratoses/carcinomas in situ, and squamocellular carcinomas. SKH-1 was the most sensitive strain to the far-ultraviolet wavelengths delivered by halogen lamps, as shown not only by the shortest latency time and the highest multiplicity of skin lesions but also by the highest frequency of malignant tumours. Some areas of atypical melanocyte proliferation were only detected in C3H pigmented mice. Eighty-two of the lesions excised from MF-1 mice were additionally examined for p53 protein by immunohistochemical methods. Formalin-fixed, paraffin-embedded sections and frozen sections were analyzed in parallel by using polyclonal CM-1 antibody and monoclonal PAb240 antibody, respectively. A positive response for p53 was only observed in squamocellular carcinomas, and was related to the size of cancers; in fact, six out of 10 cancers of 10-30 mm in diameter were positive, whereas all 16 cancers of 2-9 mm in diameter were negative. All six positive squamocellular carcinomas were detected by using the CM-1 antibody, which recognizes both wild-type and mutant forms of p53 protein, and five of them were also positive with the PAb 240 antibody, which only recognizes the mutant form. Thus, p53 mutation appears to be a late event in the development of halogen-induced skin tumours in hairless mice, requiring a severe degree of malignancy and an advanced stage of the neoplastic mass growth.

Animals↗

PubMed2005

Halogenated and isosteric cytisine derivatives with increased affinity and functional activity at nicotinic acetylcholine receptors.

A series of pyridone ring-modified derivatives of (7R,9S)-(-)-cytisine were evaluated for affinity and functional activity at neuromuscular alpha1beta1gammadelta, ganglionic alpha3beta4, and central neuronal alpha4beta2 subtypes of nicotinic receptors. Halogenation at the 3-position improved affinity and functional activity, while substitution at the 5-position led to modest decreases in both, and disubstitution led to near abolition of functional activities and could be correlated with the electron-withdrawing ability of the halogen. Subtype selectivities of the halogenated derivatives were altered relative to cytisine in a substitution-dependent manner. Caulophylline methiodide was less potent than cytisine, but retained significant activity. Thiocytisine was relatively weak in potency and efficacy, but was significantly selective for the alpha4beta2 subtype.

Alkaloids↗

PubMed2005

Time dependence of composite shrinkage using halogen and LED light curing.

OBJECTIVES: The polymerization shrinkage of light cured dental composites presents the major drawback for these aesthetically adaptable restorative materials. LED based light curing technology has recently become commercially available. Therefore, the aim of the present study was to investigate if there was a statistically significant difference in linear and volumetric composite shrinkage strain if a LED LCU is used for the light curing process rather than a conventional halogen LCU. METHODS: The volumetric shrinkage strain was determined using the Archimedes buoyancy principle after 5, 10, 20, 40 s of light curing and after 120 s following the 40 s light curing time period. The linear shrinkage strain was determined with a dynamic mechanical analyzer for the composites Z100, Spectrum, Solitaire2 and Definite polymerized with the LCUs Trilight (halogen), Freelight I (LED) and LED63 (LED LCU prototype). The changes in irradiance and spectra of the LCUs were measured after 0, 312 and 360 min of duty time. RESULTS: In general there was no considerable difference in shrinkage of the composites Z100, Spectrum or Solitaire2 when the LED63 was used instead of the Trilight. There was, however, a statistically significant difference in shrinkage strain when the composite Definite was polymerized with the LED63 instead of the Trilight. The spectrum of the Trilight changed during the experiment considerably whereas the LED63 showed an almost constant light output. The Freelight I dropped considerably in irradiance and had to be withdrawn from the study because of technical problems. SIGNIFICANCE: The composites containing only the photoinitiator camphorquinone showed similar shrinkage strain behaviour when a LED or halogen LCU is used for the polymerization. The irradiance of some LED LCUs can also decrease over time and should therefore be checked on a regular basis.

Analysis of Variance↗

PubMed2006

Halogenated organic compounds in archived whale oil: a pre-industrial record.

To provide additional evidence that several halogenated organic compounds (HOCs) found in environmental samples are natural and not industrially produced, we analyzed an archived whale oil sample collected in 1921 from the last voyage of the whaling ship Charles W. Morgan. This sample, which pre-dates large-scale industrial manufacture of HOCs, contained two methoxylated polybrominated diphenyl ethers (MeO-PBDEs), five halogenated methyl bipyrroles (MBPs), one halogenated dimethyl bipyrrole (DMBP), and tentatively one dimethoxylated polybrominated biphenyl (diMeO-PBB). This result indicates, at least in part, a natural source of the latter compounds.

Animals↗

PubMed2004

Uncoupling-mediated generation of reactive oxygen by halogenated aromatic hydrocarbons in mouse liver microsomes.

Studying liver microsomes from 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-induced or vehicle-treated (noninduced) mice, we evaluated the in vitro effects of added chemicals on the production of reactive oxygen due to substrate/P450-mediated uncoupling. The catalase-inhibited NADPH-dependent H(2)O(2) production (luminol assay) was lower in induced than noninduced microsomes. The effects of adding chemicals (2.5 microM) in vitro could be divided into three categories: Group 1, highly halogenated and coplanar compounds that increased H(2)O(2) production at least 5-fold in induced, but not in noninduced, microsomes; Group 2, non-coplanar halogenated biphenyls that did not affect H(2)O(2) production; Group 3, minimally halogenated biphenyls and benzo[a]pyrene that decreased H(2)O(2) production. Molar consumption of NADPH and O(2) and molar H(2)O(2) production (o-dianisidine oxidation) revealed that Group 1 compounds mostly increased, Group 2 had no effect, and Group 3 decreased the H(2)O(2)/O(2) and H(2)O(2)/NADPH ratios. Microsomal lipid peroxidation (thiobarbituric acid-reactive substances) was proportional to H(2)O(2) production. Although TCDD induction decreased microsomal production of H(2)O(2), addition of Group 1 compounds to TCDD-induced microsomes in vitro stimulated the second-electron reduction of cytochrome P450 and subsequent release of H(2)O(2) production. This pathway is likely to contribute to the oxidative stress response and associated toxicity produced by many of these environmental chemicals.

Animals↗

PubMedDate not supplied

The lipid peroxidation model for halogenated hydrocarbon toxicity. Kinetics of peroxyl radical processes involving fatty acids and Fe(III) porphyrins.

The toxicity of halogenated alkanes originates from their metabolism by cytochrome P-450 which leads to the formation of reactive intermediates. In particular, peroxyl radicals derived from the halogenated compounds are believed to induce peroxidative chain degradation of lipids. To examine this hypothesis, radical reactions in a system involving FeIII-deuteroporphyrin as a model of cytochrome P-450, fatty acids or cholesterol, and carbon tetrachloride or the anesthetic agent halothane are studied by means of pulse radiolysis. It is shown that haloperoxyl radicals react with the fatty acids in competition with their reaction with the ferriporphyrin. Moreover, the secondary fatty acid peroxyl radicals also react efficiently with the porphyrin. A model for halogenated alkane toxicity is discussed in terms of these new findings. The importance of local oxygen concentration and structural arrangement of fatty acids around cytochrome P-450 are emphasized.

Cytochrome P-450 Enzyme System↗

PubMed1999

Halogenated derivatives of boldine with high selectivity for alpha1A-adrenoceptors in rat cerebral cortex.

The selectivity of 3-nitrosoboldine and different halogenated derivatives of boldine (3-bromoboldine, 3,8-dibromoboldine and 3-chloroboldine) for alpha1-adrenoceptor subtypes was studied by examining [3H]-prazosin competition binding in rat cerebral cortex. In the competition experiments [3H]-prazosin binding was inhibited completely by all the compounds tested. The inhibition curves displayed shallow slopes which could be subdivided into high and low affinity components. The relative order of affinity and selectivity for alpha1A-adrenoceptors was 3-bromoboldine = 3,8-dibromoboldine = 3-chloroboldine > boldine > 3-nitrosoboldine. The competition curves for 3-bromoboldine remained shallow and biphasic following chloroethylclonidine treatment. Whereas the relative contribution of the high affinity sites increased, the 3-bromoboldine affinities at its high and low affinity sites remained similar to those obtained in untreated membranes. 3-Bromoboldine, 3,8-dibromoboldine, 3-chloroboldine and 3-nitrosoboldine did not significantly displace [3H]-(+)-cis-diltiazem binding to rat cerebral cortex membranes. This activity was lower than that shown by boldine. Compared to boldine, halogen (bromine or chlorine) substitution at position 3 increases the alpha1A-adrenoceptor subtype selectivity and decreases the affinity for the benzothiazepine binding site at the calcium channel. Further halogen substitution at position 8 did not significantly improve this activity with respect to 3-bromoboldine. In contrast, the NO substitution at position 3 of boldine (3-nitrosoboldine) gives a loss of affinity and selectivity for alpha1-adrenoceptor subtypes.

Animals↗

PubMed2001

Formation of volatile halogenated by-products during chlorination of isoproturon aqueous solutions.

The present paper deals with the formation of volatile halogenated by-products (POX) during the chlorination (160 mg/l) of aqueous solutions of the herbicide isoproturon (40 mg/l). Chlorination reactions have been carried out over 48 h, at ambient temperature, at two pHs (6 and 9) and in the presence or not of bromide ions (80 mg/l). The main results obtained have been as follows: (1) in the presence of bromide, isoproturon degradation is rather fast and it results affected by pH, complete isoproturon degradation is achieved within 1 and 15 min at pH 6 and 9, respectively; (2) in the absence of bromide herbicide degradation is slow (complete degradation is achieved within 180 min) and it is not affected by pH; (3) at pH 6, regardless of the presence of bromide, the maximum amount of POX formed is low (approximately 15 micromol X-/l) and remains constant during the reaction; (4) at pH 9 the amount of POX formed is far greater and continuously increases during the reaction, reaching a value of about 110 micromol X-/l after 48 h; (5) two different groups of by-products have been identified by solid phase micro extraction (SPME)-gas chromatography (GC)-mass spectrometry (MS) for the reactions carried out with or without bromide; among them, aliphatic as well as aromatic by-products containing chlorine, bromine or both halogens are present even though the most abundant are halogenated-methane derivatives (haloforms); pH value affects the amount of these by-products but does not modify their chemical nature.

Chlorine↗

PubMed2001

Syntheses and evaluation of halogenated cytisine derivatives and of bioisosteric thiocytisine as potent and selective nAChR ligands.

We have developed one-step syntheses of halogenated derivatives of (-)-cytisine featuring a halogen substituent at positions 3, 5 or 3 and 5 of the 2-pyridone fragment, and prepared the novel bioisosteric thiocytisine by oxygen-sulphur exchange. The affinities of these pyridone-modified analogs of (-)-cytisine for (alpha 4)(2)(beta 2)(3) and alpha 7* nAChRs in rat forebrain membranes were determined by competition with (+/-)-[(3)H]epibatidine and [(3)H]MLA, respectively. The 3-halocytisines 7 possess subnanomolar affinities for (alpha 4)(2)(beta 2)(3) nAChRs, higher than those found for (-)-cytisine as well as for the 5-halocytisines 8 and 3,5-dihalocytisines 6. In contrast to the parent alkaloid the 3-halogenated species display much a higher affinity for the alpha 7* nAChR subtype. The most potent molecule was 3-bromocytisine (7b) with preferential selectivity (200-fold) for the (alpha 4)(2)(beta 2)(3) subtype [K(i)=10 pM (alpha 4 beta 2) and 2.0 nM (alpha 7*)]. Replacement of the lactam with a thiolactam pharmacophore to thiocytisine (12) resulted in a subnanomolar affinity for the (alpha 4)(2)(beta 2)(3) nAChR subtype (K(i)=0.832 nM), but in a drastic decrease of affinity for the alpha 7* subtype; thiocytisine (12) has a K(i) value of 4000 nM (alpha 7*), giving a selectivity of 4800-fold for the neuronal (alpha 4)(2)(beta 2)(3)-nAChR and thus displaying the best affinity-selectivity profile in the series under consideration.

Aconitine↗

PubMed2000

Ab initio study of reactions between halogen atoms and various fuel molecules by Gaussian-2 theory.

Ab initio calculations by using Gaussian-2 theory have been carried out for the reactions between halogen atoms and various fuel molecules, i.e. fluorine, chlorine, and bromine atoms vs. hydrogen, methane, ethane, ethylene, acetylene, ammonia, silane, dichlorosilane and phosphine. The activation energy for the reaction between a halogen atom and a fuel molecule seems to indicate whether the reaction between the fuel gas and the corresponding halogen gas occurs spontaneously when they are brought into contact to each other at room temperature.

Explosions↗

PubMed2003

Investigation of the reaction of peroxynitrite with propofol at acid pH: predominant production of oxidized, nitrated, and halogenated derivatives.

We reported here the reaction, in acidic conditions, of peroxynitrite (ONOO(-)) with the anaesthetic agent propofol (2,6-diisopropylphenol, PPF). The most interesting finding is that peroxynitrite is able to nitrate and to oxidize propofol leading to 4-nitropropofol, quinone, and diphenylquinone as the major products. More surprisingly, we also found that peroxynitrite is capable of halogenating propofol in the presence of halide ions, suggesting the formation of nitrosyl chloride (NOCl) or nitryl chloride (NO(2)Cl) from the reaction of peroxynitrite with chloride ions. A significant enhancement of the halogenation yield is observed with a simultaneous decrease of the yields of the other products in the presence of methanol or hydrogen peroxide. Increased halogenation of PPF probably results from the formation of peroxynitrate (O(2)NOO(-)), that further oxidizes chloride ions in hypochlorous acid (HOCl) or molecular chlorine (Cl(2)). Spontaneous decay of peroxynitrate is relatively slow in acidic medium, thus explaining the decrease of the yields of the other products. By direct EPR techniques, we also observed that this reaction occurs via a radical pathway.

Anesthetics, Intravenous↗

PubMed2005

Pyrolysis study of halogen-containing aromatics reflecting reactions with polypropylene in a posttreatment decontamination process.

Halogen-containing aromatics, mainly bromine-containing phenols, are harmful compounds contaminating pyrolysis oil from electronic boards containing halogenated flame retardants. In addition,theirformation increases the potential for evolution of polybrominated dibenzo-p-dioxins (PBDDs) and dibenzofurans (PBDFs) at relatively low temperature (up to 500 degrees C). As a model compound, 2,4-dibromophenol (DBP) was pyrolyzed at 290-450 degrees C. While its pyrolysis in a nitrogen flow reactor or in encapsulated ampules yields bromine-containing phenols, phenoxyphenols, PBDDs, and PBDFs, pyrolysis of DBP in a hydrogen-donating medium of polypropylene (PP) at 290-350 degrees C mainly results in the formation of phenol and HBr, indicating the occurrence of a facile hydrodebromination of DBP. The hydrodebromination efficiency depends on temperature, pressure, and the ratio of the initial components. This thermal behavior of DBP is compared to that of 2,4-dichlorophenol and decabromodiphenyl ether. A treatment of halogen-containing aromatics with PP offers a new perspective on the development of low-environmental-impact disposal processes for electronic scrap.

Electronics↗

PubMed2004

Influence of halogenation on the properties of uracil and its noncovalent interactions with alkali metal ions. Threshold collision-induced dissociation and theoretical studies.

The influence of halogenation on the properties of uracil and its noncovalent interactions with alkali metal ions is investigated both experimentally and theoretically. Bond dissociation energies of alkali metal ion-halouracil complexes, M+(XU), are determined using threshold collision-induced dissociation techniques in a guided ion beam mass spectrometer, where M+ = Li+, Na+, and K+ and XU = 5-fluorouracil, 5-chlorouracil, 6-chlorouracil, 5-bromouracil, and 5-iodouracil. The structures and theoretical bond dissociation energies of these complexes are determined from ab initio calculations. Theoretical calculations are also performed to examine the influence of halogenation on the acidities, proton affinities, and Watson-Crick base pairing energies. Halogenation of uracil is found to produce a decrease in the proton affinity, an increase in the alkali metal ion binding affinities, an increase in the acidity, and stabilization of the A::U base pair. In addition, alkali metal ion binding is expected to lead to an increase in the stability of nucleic acids by reducing the charge on the nucleic acid in a zwitterion effect as well as through additional noncovalent interactions between the alkali metal ion and the nucleobases.

Base Pairing↗