Methyl Alkanes Research Articles

Increased breath markers of oxidative stress in normal pregnancy and in preeclampsia

Objectives The purpose of this study was to compare the intensity of oxidative stress in normal pregnancy, preeclampsia, and nonpregnant women using a breath test. Study design We studied primiparous women in third trimester pregnancy (38 uncomplicated, 26 with preeclampsia) and 60 nonpregnant control subjects. Volatile organic compounds (VOCs) in alveolar breath were analyzed by gas chromatography/mass spectroscopy to construct the breath methylated alkane contour (BMAC), a 3-dimensional display of abundance of C4-C20 alkanes and monomethylated alkanes. Results The mean volume under curve (VUC) of the BMAC was significantly higher in preeclampsia patients than in normal pregnant women ( P<.003) and nonpregnant control subjects ( P<.005). A predictive model employing 5 VOCs distinguished preeclampsia from uncomplicated pregnancy (sensitivity = 92.3%, specificity = 89.7%; cross-validated sensitivity = 88.5%, specificity = 79.3%). Conclusion A breath test significantly demonstrated greater oxidative stress in women with preeclampsia than in uncomplicated pregnancy and nonpregnant control subjects. The breath test accurately identified women with established preeclampsia, but further studies are required to determine if this test can predict the onset of disease.

A Theoretical Investigation of the Methylation of Alkenes with Methanol over Acidic Zeolites

A quantum chemical investigation of the methylation of alkenes by methanol over acidic zeolite catalysts has been performed. A cluster model consisting of four tetrahedrally coordinated atoms (T-atoms) has been used to represent the zeolite. The activation barrier for methylation is dependent on the size and substitution pattern of the alkene reactant. The results can be rationalized by considering the relative energies of the carbocations that describe the reaction path. Implications for methanol-to-hydrocarbon chemistry are discussed. The effect of zeolite acidity on the activation barrier has also been probed. The barrier decreases when the catalyst acidity is increased.

A Theoretical Investigation of the Methylation of Methylbenzenes and Alkenes by Halomethanes over Acidic Zeolites

The reactivity of chloromethane, bromomethane, and iodomethane over acidic zeolite catalysts has been probed, using the methylation of ethene, propene, and toluene as model reactions. Adsorbed reactants, transition states, and adsorbed products have been investigated. A cluster model has been used to represent the zeolite. Both the associative (direct) mechanism and the dissociative mechanism (via the formation of surface methoxide groups) have been studied. Quantum chemistry predicts that the associative pathway is favored over the dissociative mechanism. A weak trend in reactivity is found among the halomethanes, indicating the following order of reactivity: MeI > MeBr > MeCl. The results are compared with recent theoretical studies on methanol reactivity.

Breath markers of oxidative stress in patients with unstable angina.

Cardiac chest pain is accompanied by oxidative stress, which generates alkanes and other volatile organic compounds (VOCs). These VOCs are excreted in the breath and could potentially provide a rational diagnostic marker of disease. The breath methylated alkane contour (BMAC), a 3-dimensional surface plot of C4-C20 alkanes and monomethylated alkanes, provides a comprehensive set of markers of oxidative stress. In this pilot study, we compared BMACs in patients with unstable angina pectoris and in healthy volunteers. Breath VOCs were analyzed in 30 patients with unstable angina confirmed by coronary angiography and in 38 age-matched healthy volunteers with no known history of heart disease (mean age +/- SD, 62.7 +/- 12.3 years and 62.5 +/- 10.0, not significant). BMACs in both groups were compared to identify the combination of VOCs that provided the best discrimination between the 2 groups. Forward stepwise entry discriminant analysis selected 8 VOCs to construct a predictive model that correctly classified unstable angina patients with sensitivity of 90% (27 of 30) and specificity of 73.7% (28 of 38). On cross-validation, sensitivity was 83.3% (25 of 30) and specificity was 71.1% (27 of 38). We conclude that the breath test distinguished between patients with unstable angina and healthy control subjects.

Cuticular hydrocarbons of Drosophila birchii and D. serrata: identification and role in mate choice in D. serrata.

The cuticular hydrocarbon compositions of two sympatric species of Australian Drosophila in the montium subgroup of the melanogaster group that use cuticular hydrocarbons in mate recognition have been characterized. Drosophila birchii has 34 components in greater than trace amounts, with a carbon number range of C20 to C33. Drosophila serrata has 21 components above trace level and a carbon number range of C24 to C31. These two species share eight hydrocarbon components, with all but two of them being monoenes. For both species, the (Z)-9-monoenes are the predominant positional isomer. The hydrocarbons of D. birchii are n-alkanes, n-alkenes (Z)-5-, (Z)-7-, (Z)-9-, and (Z)-11-), low to trace levels of homologous (Z,Z)-7,11- and (Z,Z)-9,13-dienes; and trace amounts of (Z,Z)-5,9-C25:2, a major component of D. serrata. Only one methyl branched hydrocarbon was detected (2-methyl C28), and it occurred at very low levels. The hydrocarbons of D. serrata are dominated by a homologous series of (Z,Z)-5,9-dienes, and notably, are characterized by the apparent absence of n-alkanes. Homologous series of (Z)-5-, (Z)-7-, and (Z)-9-alkenes are also present in D. serrata as well as 2-methyl alkanes. Drosophila serrata females display strong directional mate choice based on male cuticular hydrocarbons and prefer D. serrata males with higher relative abundances of the 2-methyl alkanes, but lower relative abundances of (Z,Z)-5.9-C24:2 and (Z)-9-C25:1.

Reevaluation of the chemical secretion of the sternal glands of Polistes social wasps (Hymenoptera Vespidae)

In a previous paper we reported the presence of long chain fatty acids (lauric, myristic, palmitoleic, palmitic, linoleic, oleic, stearic) in the sternal glands of social wasps of the genus Polistes. These glands had been reported by several authors to be the source of ant repellents that these wasps apply on their nests. Among the identified acids, we found that the unsaturated ones had an ant repellent effect. More recently, sampling (through SPME) from the bristly area corresponding to the sternal glands (van der Vecht organ) of live P. dominulus revealed that this contains the same hydrocarbons as the cuticle and only minor quantities of fatty acids. This is in contrast to the amount of fatty acids we previously found in excised glands sampled either directly by headspace SPME or by solid sampling following derivatisation with diazomethane. By repeating the analysis on the sternal glands and on some tissues with no glandular function, we found that the same fatty acids were ubiquitously present. Furthermore, new bioassays have shown that long chain methyl esters, alkanes and alkenes have a similar repellent effect to fatty acids on the ant Monomorium pharaonis, provided they are liquid at room temperature. These results questioned our previous report that fatty acids are characteristic components of sternal gland secretion and suggest that other long chain compounds may also play a significant role in the defence of the nest.

Increased oxidative stress in younger as well as in older humans

Background: The free radical theory of aging is based upon the adverse effects of oxidative stress (OS), and indices of OS generally increase with advancing age. However, since OS may also be a normal physiological response in youth, when reactive oxygen species (ROS) act as signal transducers during normal growth and development, we compared markers of OS in normal humans over a wide spectrum of different ages. Methods: Fasting breath samples were collected from 102 healthy volunteers (age 9 to 89 years) and volatile organic compounds (VOCs) were assayed by gas chromatography and mass spectroscopy. The intensity of OS in each volunteer was estimated by the breath methylated alkane contour (BMAC), a three-dimensional display of the abundance of C4–C20 alkanes and monomethylated alkanes. The collective abundance of these VOCs in a breath sample was reduced to a single value, the volume under curve (VUC), and correlated with chronological age. Results: Compared to subjects aged 20–40 years, the mean BMAC VUC was significantly increased in subjects aged <20 ( p<0.0001) and >40 years ( p<0.001). A cubic function correlated BMAC VUC ( x) with chronological age ( y): y=33.7−3.29 x+0.072 x 2−0.0004 x 3 ( r=0.48). Conclusions: Breath markers of OS were significantly increased both in younger and in older subjects, compared to those aged 20–40 years. Increased OS in older subjects was consistent with previous reports, but increased OS in younger subjects aged <20 years is a new observation; this may be a normal physiological response in youth.

Effect of oxygen on breath markers of oxidative stress.

Supplemental oxygen is often administered to induce hyperoxia in nonhypoxic patients for indications such as chest pain, despite lack of evidence of clinical benefit. Induced hyperoxia is potentially toxic, since it may increase oxidative stress and peroxidative damage to deoxyribonucleic acid, lipids and proteins. The aim of this study was to establish whether supplemental oxygen induces oxidative stress in nonhypoxic subjects. Breath markers of oxidative stress were measured in 31 healthy subjects before and after breathing 28% oxygen at 2.0 L x min(-1) via nasal prongs for 30 min while resting. The criterion standard of oxidative stress was the breath methylated alkane contour (BMAC), a three-dimensional plot of the alveolar gradients of C4-C20 alkanes and monomethylated alkanes produced by lipid peroxidation. Volatile organic compounds (VOCs) in breath were assayed by gas chromatography and mass spectroscopy, and the BMACs before and after oxygenation were compared. Following oxygenation, there was a significant increase in mean volume under the curve of the BMAC and in alveolar gradients of three VOCs: 3-methyltridecane, 3-methylundecane and 5-methylnonane. Breath markers of oxidative stress were significantly increased in normal volunteers breathing supplemental oxygen for 30 min.

Investigation of entropy effects during sorption of mixtures of alkanes in MFI zeolite

We have carried out a comprehensive study of sorption of mixtures of alkanes, in the 1–7 C atom range, in MFI zeolite using configurational-bias Monte Carlo (CBMC) simulations. The isotherm characteristics of various binary, ternary and quaternary mixtures have been investigated. Our studies show that two types of entropy effects have a significant influence on mixture sorption: 1. Size entropy effects arise due to differences in the saturation loading of the pure components. Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the “gaps” within the zeolite matrix at high molecular loadings. 2. Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and mono-methyl alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules “pack” more efficiently within the MFI matrix. For a mixture of mono-methyl and di-methyl alkanes with the same number of C atoms, configurational entropy effects favour the single branched isomer. Configurational entropy effect comes into play when the loading exceeds four molecules per unit cell, when all the intersection sites are occupied, and results in the following hierarchy of sorption strengths: linear alkanes ⪢ mono- methyl alkanes ⪢ di- methyl alkanes . In all cases, the mixture isotherms can be predicted with good accuracy using the ideal adsorbed solution theory (IAST). CBMC simulations of sorption of an 8-component mixture containing n-pentane ( n-C 5), 2-methylbutane (2MB), n-hexane ( n-C 6), 2-methylpentane (2MP), 2,2-dimethylbutane (22DMB), n-heptane ( n-C 7), 2-methylhexane (2MH) and 2,2-dimethylpentane (22DMP) show that both size and configurational entropy effects contribute, leading to a sorption hierarchy depending on the degree of branching, linear alkanes ⪢ mono- methyl alkanes ⪢ di- methyl alkanes . This result has considerable potential for commercial application in the petroleum industry in catalytic isomerization process where it is necessary to isolate the di-branched alkanes which are preferred ingredients in gasoline.

Exopolysaccharides of Xanthomonas pathovar strains that infect rice and wheat crops.

In order to understand the mode of action of the taxonomically related pathogens Xanthomonas campestris pv. translucens, Xanthomonas oryzae pv. oryzae, and Xanthomonas oryzae pv. oryzicola, which attack wheat and rice crops, we examined the compositional differences of their exopolysaccharides (EPSs). Maximum production of polysaccharide in shake cultures of these pathogens was observed between 24 and 72 h. X. campestris pv. translucens, the leaf streak pathogen of wheat, produced a higher amount of polysaccharide (46.97 microg/ml) at 72 h compared to X. oryzae pv. oryzae (42.02 microg/ml), the bacterial blight pathogen of rice, and X. oryzae pv. oryzicola (41.91 microg/ml), the bacterial leaf streak pathogen of rice. Infrared (FTIR) spectra suggested that the polysaccharides of all three Xanthomonas pathovar strains have an -OH group with intermolecular hydrogen bonding, a C-H group of methyl alkanes, an aldehyde (RCHO) group, a C=C or C=O group, and a C-O group. FTIR spectra also revealed the presence of an acid anhydride group in X. oryzae pv. oryzae, a secondary aromatic or aliphatic amine group in X. campestris pv. translucens, and a primary aromatic or aliphatic amine group in X. oryzae pv. oryzae and X. oryzae pv. oryzicola. Nuclear magnetic resonance (NMR) spectra revealed the presence of unsubstituted sugars, an acetyl amine of hexose or pentose, and a beta-anomeric carbon of hexose or pentose in the polysaccharides of all bacteria. NMR spectra also identified the alpha-anomeric carbon of hexose or pentose in all strains, and a branching at the fourth carbon of the sugar only in X. campestris pv. translucens; the presence of an uronic acid molecule (acid anhydride group) in X. oryzae pv. oryzae; and a deoxy sugar, rhamnose, in X. oryzae pv. oryzicola.

Lack of intraspecific aggression in the ant Tetramorium bicarinatum: a chemical hypothesis.

Tetramorium bicarinatum (Myrmicinae) is an ant species frequently found in tropical and subtropical areas, particularly in Africa, Southeast Asia (Japan), and South America (Brazil). The species is polygynous, reproduces by budding, and has sterile workers. Since the nests are widely distributed in a given area, the problem arises of territorial defense against conspecifics. Because not all ants defend territories, we assessed the defensive behavior of T. bicarinatum workers through intraspecific and interspecific aggressiveness tests. A detailed behavioral study of the interactions between workers from several different colonies of T. bicarinatum (originating from Japan and Brazil) showed that workers do not discriminate against conspecific nonnestmate individuals, but they are highly aggressive towards allospecifics (Myrmica rubra, Myrmicinae). The results suggest that each colony from this ant species possesses a similar colonial odor. Chemical analyses of the cuticular hydrocarbons of these species were made with gas chromatography coupled to mass spectrometry. Results showed that the different colonies of T. bicarinatum possess a common chemical profile mainly composed of straight-chain alkanes and alkenes, while M. rubra possess more methyl-branched alkanes. We suggest that methyl alkane cues play a determining role in colonial recognition and that these results could explain the underlaying basis of the lack of intraspecific aggressiveness in T. bicarinatum.

Effect of age on the breath methylated alkane contour, a display of apparent new markers of oxidative stress

Reactive oxygen species (ROS) are toxic byproducts of mitochondrial energy production that inflict oxidative stress, a constant barrage of damage to DNA, proteins, lipids, and other biologically important molecules. Oxidative stress has been implicated as a pathologic mechanism in aging and in several diseases. We developed a display of apparent new markers of oxidative stress in human beings, the breath methylated alkane contour (BMAC). The BMAC is a three-dimensional display of C4 to C20 alkanes and monomethylated alkanes in breath, with x-axis = carbon chain length, z-axis = methylation site, and y-axis = alveolar gradient (relative abundance in breath minus relative abundance in room air). In 102 normal human subjects of 9 to 89 years of age, alveolar gradients of components of the BMAC increased significantly with age. The mean alveolar gradient of all components of the BMAC varied from negative in the youngest quartile (ages 9 to 31 years) to positive in the oldest quartile (ages 74 to 89 years)(P < 2.10–9). These findings were consistent with an increase in oxidative stress with advancing age, although an age-related decline in clearance by cytochrome p450 may have contributed. The BMAC provides a display of apparent new markers of oxidative stress with potential applications in aging research, clinical diagnosis, pharmacology, and toxicology. (J Lab Clin Med 2000;136:243-9)

Origin of free and bound mid-chain methyl alkanes in oils, bitumens and kerogens of the marine, Infracambrian Huqf Formation (Oman)

The Huqf oil from Oman and other time-equivalent oils are characterized by relatively large contents of mid-chain branched monomethyl alkanes. The Huqf oil, the saturated hydrocarbon fractions, the desulfurized polar fractions of the bitumens and the kerogen pyrolysates of three potential source rocks from the Huqf Formation (Oman) were analyzed in order to resolve the origin of these mid-chain branched monomethyl alkanes. Using off-line pyrolysis and chemical degradation in combination with compound specific carbon isotope analysis, it was assessed that the mid-chain branched monomethyl alkanes are not rearrangement products but are derived from a biological source, probably from lipids with C28+ carbon skeletons and methyl branching at the 12- or 13-position. These compounds were incorporated in the kerogen through reactions of sulfur with functional groups at different positions in the precursor lipid(s).

Semiochemicals of rose aphid, black citrus aphid (Hemiptera: Aphididae) and greenhouse thrips (Thysanoptera: Thripidae)

Macrosiphum rosae L. or rose aphid. Toxoptera citricida Kirkaldy or black citrus aphid (Hemiptera: Aphididae) and Heliothrips haemorrhoidalis Bouche or greenhouse thrips (Thysanoptera: Thripidae) are serious cosmopolitan phytophagous pests that can cause severe damage to many cultivated crops.The aim of this study was to investigate the chemical signals utilized by these pests in order to carry out their everyday functions. This was achieved by determining the nature of volatile compounds in the secretions of the rose aphid, the black citrus aphid and the greenhouse thrips.The results obtained showed similarities not only between the two aphid species but also between aphids and thrips with acids and their methyl esters, aldehydes and alkanes being common components of the secretions.E-β-farnesene, a known alarm substance in many aphid species, was confirmed to be a constituent of the secretions of the rose aphid and was isolated for the first time from the black citrus aphid. Results indicated that the s...

Terminal Proterozoic mid-shelf benthic microbial mats in the Centralian Superbasin and their environmental significance

A combined sedimentological and biogeochemical study has been conducted on several Terminal Proterozoic mid-shelf microbial mat facies from the Centralian Super-basin. Isotopic and organic geochemical analysis of the bitumen and kerogen indicated that two sources of organic matter from ‘planktonic’ and ‘benthic microbial-mat’ populations contributed to the sediment. The ‘planktonic’ source provided a suite of n-alkanes with <C 20 predominance and the odd n-alkanes >C 20, whereas, the ‘benthic’ source contributed an overlay of n-alkanes >C 20 with a strong even preference, together with mid-chain methyl alkanes. Kerogen and biomarkers derived from the microbial mat were found to be depleted in 13C relative to planktonic material. Pyrite in the micorbial mats was also found to be depleted in 34S compared to surrounding facies. The combination of these observations suggested that the mats may have been at least partly composed of sulfide oxidising bacteria. These organisms have specific environmental tolerances that set limits on palaeo-environment. Their requirement for oxygen indicates that the water column above the mid-shelf could not have been anoxic. Accordingly, from the results and age determinations reported here, it would appear that mid-shelf environments of the Centralian Superbasin of Australia were seeing significant levels of oxygen through the Ediacarian.

Sorption-Induced Diffusion-Selective Separation of Hydrocarbon Isomers Using Silicalite

In this paper we demonstrate a new principle for separation of linear and branched (2-methyl)alkanes, in the five to seven carbon atom range, by means of permeation through a silicalite membrane. The permeation selectivity relies on subtle interplay between sorption and diffusion. The required sorption isotherms for the pure components and mixtures are generated using configurational-bias Monte Carlo (CBMC) simulations. The CBMC simulations of the mixture isotherm show a curious maximum in the loading of 2-methyl alkane; this loading decreases to almost zero with increased pressures. The high sorption selectivity for the linear alkane is due to entropic effects; the linear alkane has a higher “packing” efficiency than the branched alkane within the zeolite structure. Calculations for a 50−50 mixture of n-hexane (n-C6) and 2-methylpentane (2MP), for example, show that the higher sorption selectivity for the linear alkane has the effect of enhancing the flux of n-C6 through the silicalite membrane by up to a factor of 60 above that of 2MP. Experimental evidence to support our new separation principle is provided by permeation data of Funke et al.2

An evaluation of atomic and molecular mixture rules and group additivity concepts for the estimation of radiation absorption by long-chained, saturated hydrocarbons at vacuum UV and soft X-ray energies

The feasibility of using atomic and molecular mixture rules as well as group additivity concepts for predicting valence shell photoabsorption oscillator strengths (cross sections) for long-chained alkane molecules has been investigated over a wide energy range from 18 to 220 eV. The predictions are discussed with reference to recently reported experimental measurements (Chem. Phys. 173 (1993) 209) for normal alkanes, C n H 2 n+2 ( n=1–8). Atomic mixture rules based on either theoretical or experimental atomic oscillator strength sums are found to be unsatisfactory, giving very large errors at most photon energies. A wide range of molecular mixture rules based on linear combinations of measured oscillator strength values for small ‘component’ alkane molecules and molecular hydrogen have also been evaluated. Although good agreement with experiment is obtained with some linear combinations, many others result in substantial errors. Molecular mixture rules constructed using oscillator strength for larger component alkanes generally give better estimates of the experimentally measured data; however, since no other a priori physical or chemical reasons can be advanced for any particular choice of molecular mixture rule, this procedure is unsatisfactory for general application. In contrast, a group additivity procedure based on oscillator strength estimates for the methylene (CH 2) and methyl (CH 3) alkane group fragments, derived entirely from the photoabsorption measurements for lower alkanes, provides excellent agreement with the measured oscillator strengths for C 8H 18 over the entire energy range studied (18–220 eV). The absolute photoabsorption group oscillator strengths derived for the CH 2 and CH 3 fragments should be applicable to assessing the contributions from saturated hydrocarbon groupings to vacuum UV and soft X-ray absorption in larger chemical and biochemical systems.

Aliphatic hydrocarbons of the Murchison meteorite

The indigenous organic compounds of carbonaceous chondrites have been difficult to characterize because of problems arising from terrestrial contamination. The fall of the Murchison meteorite (CM2) provided pristine samples which allowed the resolution of some prior ambiguities as, for example, in the case of the amino acids. However, the nature of the aliphatic hydrocarbons has remained unclear. Shortly after the Murchison fall, one laboratory found them to be mainly cycloalkanes; another found, in order of abundance, branched alkanes, olefins, and cycloalkanes; while a third reported predominantly n-alkanes followed by methyl alkanes and olefins. We have reinvestigated this question using benzene-methanol as the extraction solvent, silica-gel chromatography for fractionation of the extract, and GC-MS, and IR and NMR spectroscopic techniques for the analyses. When interior samples were obtained and the analyses carried out under conditions that minimized environmental contaminants, we have found the principal aliphatic components of the Murchison meteorite to be a structurally diverse suite of C 15 to C 30 branched alkyl-substituted mono-, di-, and tricyclic alkanes. Comparative analyses were carried out on the Murray (CM2), Allende (CV3), and New Concord (L6) chondrites that illustrate the nature of the contamination problem encountered with carbonaceous chondrites.

The use of GC-MS-MS for the characterization of naturally occurring methyl alkanes

Biomedical & Environmental Mass SpectrometryVolume 18, Issue 10 p. 939-941 Short Communication The use of GC-MS-MS for the characterization of naturally occurring methyl alkanes James F. Carter, James F. Carter Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TSSearch for more papers by this authorJosef Rechka, Josef Rechka Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TSSearch for more papers by this authorNeil Robinson, Neil Robinson Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TSSearch for more papers by this author James F. Carter, James F. Carter Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TSSearch for more papers by this authorJosef Rechka, Josef Rechka Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TSSearch for more papers by this authorNeil Robinson, Neil Robinson Organic Geochemistry Unit, School of Chemistry, University of Bristol, Cantocks Close, Bristol BS8 1TSSearch for more papers by this author First published: October 1989 https://doi.org/10.1002/bms.1200181016Citations: 5AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article.Citing Literature Volume18, Issue10October 1989Pages 939-941 RelatedInformation

Phase behavior in the critical region of six binary mixtures of 2-methyl alkanes

Diagramme PTx de six systemes binaires formes par le methyl-2-pentane, methyl-2-hexane, methyl-2-heptane et le methyl-2-octane