Unsubstituted Aromatic Hydrocarbons Research Articles

Potential for the anaerobic oxidation of benzene and naphthalene in thermophilic microorganisms from the Guaymas Basin.

Unsubstituted aromatic hydrocarbons (UAHs) are recalcitrant molecules abundant in crude oil, which is accumulated in subsurface reservoirs and occasionally enters the marine environment through natural seepage or human-caused spillage. The challenging anaerobic degradation of UAHs by microorganisms, in particular under thermophilic conditions, is poorly understood. Here, we established benzene- and naphthalene-degrading cultures under sulfate-reducing conditions at 50°C and 70°C from Guaymas Basin sediments. We investigated the microorganisms in the enrichment cultures and their potential for UAH oxidation through short-read metagenome sequencing and analysis. Dependent on the combination of UAH and temperature, different microorganisms became enriched. A Thermoplasmatota archaeon was abundant in the benzene-degrading culture at 50°C, but catabolic pathways remained elusive, because the archaeon lacked most known genes for benzene degradation. Two novel species of Desulfatiglandales bacteria were strongly enriched in the benzene-degrading culture at 70°C and in the naphthalene-degrading culture at 50°C. Both bacteria encode almost complete pathways for UAH degradation and for downstream degradation. They likely activate benzene via methylation, and naphthalene via direct carboxylation, respectively. The two species constitute the first thermophilic UAH degraders of the Desulfatiglandales. In the naphthalene-degrading culture incubated at 70°C, a Dehalococcoidia bacterium became enriched, which encoded a partial pathway for UAH degradation. Comparison of enriched bacteria with related genomes from environmental samples indicated that pathways for benzene degradation are widely distributed, while thermophily and capacity for naphthalene activation are rare. Our study highlights the capacities of uncultured thermophilic microbes for UAH degradation in petroleum reservoirs and in contaminated environments.

The occurrence and distribution of phenylated aromatic hydrocarbons and terphenyls during heating experiments of marine and lacustrine source rocks in a semi-open system

This study aimed to explore the presence and distribution of phenyl substituted polycyclic aromatic hydrocarbons (Ph-PAHs) and polyphenyls in one Middle Devonian marine carbonate sample from the Western Canada Sedimentary Basin and one Cretaceous lacustrine shale sample from the Songliao Basin by artificially heating them in a semi-open system. GC-MS analysis of residual extracts and expelled oils showed no trace of phenylnaphthalenes, terphenyls, phenylphenanthrenes and binaphthyls in original extract and low temperature pyrolysates, but these compounds emerged at high temperatures, indicating a pyrogenic origin. Compared to the lacustrine shale, the marine carbonate consistently exhibited higher concentrations of these compounds that are likely controlled by the depositional environment. The main reaction mechanisms involve consecutive reactions of free radicals, with phenylation of unsubstituted aromatic hydrocarbons being the predominant reaction under high thermal stress. While oxidized organic matter may facilitate the formation of Ph-PAHs and polyphenyls, they can be formed from any type of organic matter. The presence of metal oxidants was found to be unnecessary, as high temperatures alone were sufficient to promote the formation of these compounds. Thus, their occurrence indicates abnormal heating regardless of the maturity of the host rocks. Phenylnaphthalenes were produced earlier than other Ph-PAHs and polyphenyls. While stable Ph-PAH and terphenyl isomers increase with simulation temperature, thermal maturity parameters derived from phenyl-PAHs and polyphenyl are insensitive to maturity except for p-TrP/m-TrP, which increases almost linearly with temperature in Brightholme shale. Ph-PAHs and polyphenyls were found to be less thermodynamically stable than conventional PAHs. Under extremely high thermal stress, they lose hydrogen atoms and transform into more stable condensed PAHs. The study provides valuable insights into the occurrence of high abundance phenyl-aromatic and polyphenyl compounds in sedimentary organic matter, enhancing our understanding of these complex organic molecules.

Influence of thermal maturity on carbon isotopic composition of individual aromatic hydrocarbons during anhydrous closed-system pyrolysis

Anhydrous closed-system pyrolysis experiments were conducted on the pure alkyl naphthalenes, n-alkanes and three crude oils at the highly to over mature stage in order to investigate compositional and isotopic changes of individual aromatic hydrocarbons during laboratory maturation. On the whole, aromatic hydrocarbons have better thermal stability than n-alkanes. While the carbon isotopic composition of substituted aromatic hydrocarbons exhibits a positive correlation with increasing thermal maturity, there is no significant fractionation in the carbon isotopic composition of unsubstituted aromatic hydrocarbons. This finding suggests that the carbon isotopic distribution of unsubstituted aromatic hydrocarbons can be used as an effective tool for oil-oil and oil-source correlation, especially for highly-over mature fields.

Effects of methyl group on aromatic hydrocarbons on the nanostructures and oxidative reactivity of combustion-generated soot

The substituted and unsubstituted aromatic hydrocarbons, present in transportation fuels such as gasoline and diesel, are thought to be responsible for most of the soot particles produced during their combustion. However, the effects of the substituted alkyl groups on the aromatic hydrocarbons on their sooting tendencies, and on the physical and chemical properties of soot produced from them are not well understood. In this work, the effect of the presence of methyl groups on aromatic hydrocarbons on their sooting propensity, and on the oxidative reactivity, morphology, and chemical composition of soot generated from them in diffusion flames is studied using benzene, toluene, and m-xylene as fuels. Several experimental techniques including high resolution transmission electron microscopy and X-ray diffraction are used to identify the morphological changes in soot, whereas the elemental and thermo-gravimetric analyses, electron energy loss spectroscopy, and Fourier transform infrared spectroscopy are used to study the changes in its chemical properties and reactivity. The activation energies for soot oxidation are calculated at different conversion levels, and a trend in the reactivity of soots from benzene, toluene and m-xylene is reported. It is observed that the sizes of primary particles and graphene-like sheets, and the concentrations of aliphatics and oxygenated groups in soot particles decreased with the addition of methyl group(s) on the aromatic ring. The physicochemical changes in soot are found to support the oxidative reactivity trends.

Electric field dependence of ion mobilities of aromatic compounds with different ionic mass and different functional groups

Ion mobility spectrometry with high electric fields complements the information from the spectra obtained using conventional time-of-flight ion mobility spectrometry by providing the field dependence of ion mobility. The formation and dissociation of ion clusters depending on the differences between high and low field strength is one process which has a considerable influence on the peak position in the resulting spectra. This process can be affected by the properties of substances. Therefore, one objective of our study was to determine possible influences of the nature and position of functional groups on the field dependence of ion mobilities. For this purpose, we investigated sets of compounds with different functional groups including chlorobenzenes, toluenes, anilines and phenols. Furthermore, it is known that ionic mass is an important parameter influencing the field dependence of ion mobility. Therefore, we also investigated unsubstituted aromatic hydrocarbons with different ionic masses. Although an influence of functional groups on the field dependence of ion mobilities was found, the ionic mass has a more significant influence. However, some isomers can be separated by applying high field amplitudes. Furthermore, the influence of ionic mass on the peak position is also more significant using high field amplitudes.

KINETICS OF FORMATION OF (±)-ANTI-7, 8-DIHYDROXY-9α,10α -EPOXY-7, 8, 9, 10-TETRAHYDROBENZO[A]PYRENE ADDUCTS WITH MOUSE AND HUMAN HEMOGLOBIN

Previous studies on the metabolic activation of polycyclic aromatic hydrocarbons have shown conclusively that activation to reactive electrophilic intermediates occurs via metabolism of the unsubstituted aromatic hydrocarbon to electrophilic diol-epoxide intermediates. These reactive electrophilic derivatives undergo reaction with various biological nucleophilic centers to form covalent modifications, which can be assessed by a variety of analytical techniques. While the formation of these adducts with various cellular nucleophiles has been clearly established, the relationships between relative rates of formation of various adducts in biological systems has not been significantly investigated. In the present study, we investigated the pharmacokinetics of reaction of the ultimate carcinogenic metabolite of benzo(a)pyrene, (±)-anti-7,8-dihydroxy-9α,10α -epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE), with both male C57BL/6 mouse red cell hemoglobin and human red cells, in vitro. Comparative pharmacokinetic studies using both mouse and human hemoglobin were carried out by incubation of the packed red cells together with solutions of (±)-anti-7,8-dihydroxy-9α,10α -epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene. Similar rate constants for the binding of the epoxide to hemoglobin in both mouse and human were found, suggesting a similarity of amino acids that were adducted in both species. Overall binding of (±)-anti-7,8-dihydroxy-9α,10α -epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene to mouse hemoglobin was found to be slightly higher than that found using human hemoglobin, suggesting the potential of a facilitation of the binding of the epoxide with mouse protein or an additional reactive amino acid to which the epoxide was bound. In parallel studies, mice were injected intraperitoneally with benzo(a)pyrene to compare the kinetics of binding of this carcinogen to hemoglobin, in vivo. Blood samples were obtained at specific intervals, and the kinetics of binding to hemoglobin characterized. These results suggested that the binding to hemoglobin in the in vivo studies was slightly lower than that compared to the in vitro assessments. However the differences in adduct levels strongly suggests that other metabolic pathways are involved in the metabolic activation of benzo(a)pyrene as well as other related carcinogenic compounds.

BENZO(A)PYRENE METABOLISM: ROLE OF BIOALKYLATION

Previous studies of the metabolic activation of chemical carcinogens including benzo(a)pyrene have demonstrated that selected aromatic hydrocarbons can undergo a series of biological substitution reactions in tissue cytosol preparations when in the presence of the methyl donor S-adenosyl-L-methionine. Although identification of the methylated metabolites of carcinogens had been tentatively established using liquid chromatographic techniques, confirmation of the identity of these methylated derivatives has remained elusive. In the present study, the carcinogen benzo(a)pyrene was found to undergo methylation reactions in the presence of rat liver cytosol preparations to yield products that were identified as methylated, formyl and hydroxyalkyl derivatives of the parent carcinogen. Identification and confirmation of the identity of the 6-methylbenzo(a)pyrene metabolite was confirmed by both UV detection, gas chromatographic and mass spectral identification, and NMR analysis of metabolic fractions obtained from incubations of benzo(a)pyrene. The results conclusively demonstrate that the carcinogen benzo(a)pyrene undergoes biological substitution reactions to yield 6-methylbenzo(a)pyrene in the presence of cytosolic enzymes. The presence of 6-methylbenzo(a)pyrene as a metabolite of benzo(a)pyrene illustrates and confirms the presence of centers of reactivity in unsubstituted aromatic hydrocarbons. These centers of reactivity serve as sites for biochemical substitution reactions, yielding methyl substituted hydrocarbons. Furthermore, these methylated metabolites of polycyclic aromatic hydrocarbons are further metabolically activated to hydroxyalkyl and formyl derivatives which can be subsequently activated to electrophilic esters leading to DNA and protein reactive species.

BIOALKYLATION OF BENZ(A)ANTHRACENE: IMPLICATIONS FOR CARCINOGENESIS

Previous studies on the metabolic activation of unsubstituted polycyclic aromatic hydrocarbons have largely focused on the formation of oxidation products as a result of reaction of the parent hydrocarbon with microsomal enzymes. These products of reaction have included both epoxide as well as a diol epoxide metabolites. In addition, formation of quinones as well as hydroxyl substituted derivatives of various polycyclic aromatic hydrocarbons have been investigated as metabolic endpoints. Several studies have taken place illustrating the formation of these metabolites, as well as the formation of biologically relevant adducts in a variety of animal models as well as cell culture preparations. Although these metabolic products have been widely characterized and studied in a variety of systems, there have been limited studies characterizing the formation of methyl substituted derivatives of unsubstituted aromatic hydrocarbons, and the subsequent formation of hydroxyalkyl derivatives of various compounds. The present study investigates the formation of both alkyl and hydroxyalkyl substituted derivatives of the unsubstituted polycyclic aromatic hydrocarbon benz(a)anthracene in preparations of rat liver cytosol fortified with S-adenosyl-L-methionine. The results demonstrate that methylated derivatives are formed as metabolic products, and that the identity of these products are both the 7-methybenz(a)anthracene and 7,12-dimethylbenz(a)anthracene. These results confirm that aromatic hydrocarbons undergo biological substitution reactions at specific centers of reactivity of the unsubstituted molecule yielding bioalkylated derivatives.

Methylation is the initial reaction in anaerobic naphthalene degradation by a sulfate‐reducing enrichment culture

The sulfate-reducing culture N47 can utilize naphthalene or 2-methylnaphthalene as the sole carbon source and electron donor. Here we show that the initial reaction in the naphthalene degradation pathway is a methylation to 2-methylnaphthalene which then undergoes the subsequent oxidation to the central metabolite 2-naphthoic acid, ring reduction and cleavage. Specific metabolites occurring exclusively during anaerobic degradation of 2-methylnaphthalene were detected during growth on naphthalene, i.e. naphthyl-2-methyl-succinate and naphthyl-2-methylene-succinate. Additionally, all three enzymes involved in anaerobic degradation of 2-methylnaphthalene to 2-naphthoic acid that could be measured in vitro so far, i.e. naphthyl-2-methyl-succinate synthase, succinyl-CoA:naphthyl-2-methyl-succinate CoA-transferase and naphthyl-2-methyl-succinyl-CoA dehydrogenase were also detected in naphthalene-grown cells with similar activities. Induction experiments were performed to study the growth behaviour of the cell when transferred from naphthalene to 2-methylnaphthalene or vice versa. When the cells were transferred from naphthalene to 2-methylnaphthalene they grew immediately, indicating that no new enzymes had to be induced. On the contrary, the transfer of cells from 2-methylnaphthalene to naphthalene caused a lag-phase of almost 100 days demonstrating that an additional catabolic enzyme has to be activated in this case. We propose the methylation as a novel general mechanism of activation reactions in anaerobic degradation of unsubstituted aromatic hydrocarbons.

Bioremediation of PAH-contaminated soil by composting: a case study.

Composting technique was used for bioremediation of industrial soil originating from a former tar-contaminated site. The composting process was regulated by aeration to keep optimal temperature gradient and concentrations of O2 and CO2 inside the composting pile. The efficiency of bioremediation was evaluated by performing analysis of 11 individual three- to six-ring unsubstituted aromatic hydrocarbons (PAH) and estimating of changes in ecotoxicity of the contaminated soil. After 42 d of composting, PAH with 3-4 rings were removed from 42 to 68%, other higher-molar mass PAH from 35 to 57%. Additional 100 d of compost maturation in open-air field did not result in a further decrease of PAH. Ecotoxicity tests performed with bioluminescent bacteria Vibrio fischerii showed a decrease in toxicity both after composting and maturation phases. However, toxicity tests on mustard-seed germination did not reveal any significant changes during composting and maturation phases.

Mycoremediation of PAH-contaminated soil.

Out of a number of white-rot fungal cultures, strains of Irpex lacteus and Pleurotus ostreatus were selected for degradation of 7 three- and four-ring unsubstituted aromatic hydrocarbons (PAH) in two contaminated industrial soils. Respective data for removal of PAH in the two industrial soils by I. lacteus were: fluorene (41 and 67%), phenanthrene (20 and 56%), anthracene (29 and 49%), fluoranthene (29 and 57%), pyrene (24 and 42%), chrysene (16 and 32%) and benzo[a]anthracene (13 and 20%). In the same two industrial soils P. ostreatus degraded the PAH with respective removal figures of fluorene (26 and 35%), phenanthrene (0 and 20%), anthracene (19 and 53%), fluoranthene (29 and 31%), pyrene (22 and 42%), chrysene (0 and 42%) and benzo[a]anthracene (0 and 13%). The degradation of PAH was determined against concentration of PAH in non-treated contaminated soils after 14 weeks of incubation. The fungal degradation of PAH in soil was studied simultaneously with ecotoxicity evaluation of fungal treated and non-treated contaminated soils. Compared to non-treated contaminated soil, fungus-treated soil samples indicated decrease in inhibition of bioluminescence in luminescent bacteria (Vibrio fischerii) and increase in germinated mustard (Brassica alba) seeds.

Identification of Petroleum Products in Environmental Samples Using Gas Chromatography and Gas Chromatography–Mass Spectrometry

Typical analytical features of petroleum products in environmental samples were considered. Among them are typical shapes of chromatograms; peaks of n-alkanes; the ratio between n-alkanes with even and odd numbers of carbon atoms; peaks of biomarkers (isoprenanes, steranes, triterpanes, etc.); the predominance of methyl- and alkyl-substituted mono-, bi-, and polynuclear aromatic hydrocarbons over unsubstituted aromatic hydrocarbons; and other features. These features altogether enable reliable conclusions about the petroleum origin of the identified pollutants to be made.

Naphthalene and Benzene Degradation under Fe(III)-Reducing Conditions in Petroleum-Contaminated Aquifers

Naphthalene was oxidized anaerobically to CO2 in sediments collected from a petroleum-contaminated aquifer in Bemidji, Minnesota in which Fe(III) reduction was the terminal electron-accepting process. Naphthalene was not oxidized in sediments from the methanogenic zone at Bemidji or in sediments from the Fe(III)-reducing zone of other petroleum-contaminated aquifers studied. In a profile across the Fe(III)-reducing zone of the Bemidji aquifer, rates of naphthalene oxidation were fastest in sediments with the highest proportion of Fe(III), which was also the zone of the most rapid degradation of benzene, toluene, and acetate. The comparative studies attempted to elucidate factors that might account for the fact that unsubstituted aromatic hydrocarbons such as benzene and naphthalene were degraded under Fe(III)-reducing conditions at Bemidji, but not at the other aquifers examined. These studies indicated that the ability of Fe(III)-reducing microorganisms to degrade benzene and naphthalene at the Bemidji site cannot be attributed to groundwater components that make Fe(III) more available for reduction or other potential factors that were evaluated. However, unlike the other aquifers evaluated, uncontaminated sediments at the Bemidji site could be adapted for anaerobic benzene degradation merely with the addition of benzene. These findings indicate that Bemidji sediments naturally contain Fe(III) reducers capable of degradation of unsubstituted aromatic hydrocarbons.

Comparative molecular analysis of genes for polycyclic aromatic hydrocarbon degradation.

Aromatic hydrocarbons are found ubiquitously in nature. Their sources vary from biosynthesis de novo by organisms to abiotic reactions on naturally-occurring carbon deposits to chemical synthesis by man. It is generally accepted, however, that the bulk of aromatic compounds found in the environment are not of biosynthetic origin but are produced through the pyrolysis of organic materials such as the amorphous polymer lignin. Unsubstituted aromatic hydrocarbons are generally formed at high temperatures (2000°C) of pyrolysis. Intermediate temperatures of pyrolysis (400–800°C) result in the formation of alkylsubstituted aromatic hydrocarbons together with unsubstituted aromatic compounds. In contrast, petroleum is formed at low temperatures (80–150°C) and contains aromatic hydrocarbons with two or three alkyl substituents as the major components (1). The ubiquitous presence of aromatic hydrocarbons in the environment is generally thought to result from the deposition of airborne particles that contain combustion products (2,3). In addition to the abiotic source of aromatic compounds, there is little doubt that the rapid industrialization over the last century has increased the deposition of man-made aromatic hydrocarbons into the environment. Environmental contamination by anthropogenic sources results from the isolation, processing, combustion and disposal of fossil fuel. The production of value-added petroleum products such as polymers, plastics, pesticides, solvents, explosives and even pharmaceuticals can lead to release of aromatic hydrocarbons into the environment.

Screening of unsubstituted cyclic compounds as inhibitors of monoamine oxidases

A number of unsubstituted aromatic hydrocarbons, azaheterocycles, oxaheterocycles and cyclic ketones were screened for their inhibitory potency towards monoamine oxidases (MAO; EC 1.4.3.4.) A and B. Fair activities ( ic 50 10–100 μM) and selectivities were found for, e.g. naphthalene, anthracene, phenanthrene, isoquinoline and acridine. The most active inhibitors are oxygen-containing compounds (e.g. coumarin, flavone, dibenzofuran, xanthene, thioxanthone and acridone), with xanthone emerging as a potent ( ic 50 0.8 μM) and reversible MAO-A inhibitor. All tested inhibitors seem to act in a reversible and time-independent manner.

On the composition of dissolved and particle-associated fossil fuel residues in Mediterranean surface water

Gas chromatography-mass spectrometry (GC-MS) analyses of organic concentrates from eastern Mediterranean surface water show that dissolved fossil fuel residues are dominated by unsubstituted aromatic hydrocarbons and heterocyclic aromatics and thus resemble products of incomplete combustion. Deviating from this interpretation, preferential photochemical oxidation of alkyl-substituted aromatics is suggested as the principal causative mechanism, although, especially near land-based sources. the influence of combustion-derived unsubstituted aromatics is not defined. Total concentrations of dissolved aromatics as determined by GC-FID (flame ionization detection) and GC-MS from a limited data subset were in reasonable agreement with UV fluorescence measurements of dissolved/dispersed oil residues. The same preponderance of unsubstituted aromatics over their alkyl derivatives was found in extracts of the particulate phase, but concentrations were small relative to alkanes and/or alkenes of recent marine biosynthetic origin. CPI (carbon preference index) values of higher molecular weight n-alkanes indicated varying contributions of plant waxes to the particulate hydrocarbon pool. CPI values of corresponding dissolved n-alkanes were found to be near unity, indicating fossil fuel origin. Despite differences in higher molecular weight n-alkane CPI values and the predominance of recent biosynthetic aliphatics, the similarity of dissolved and particulate aromatic fractions suggests limited adsorption by particles of fossil fuel derived hydrocarbons and heterocyclics.

Comparison of supercritical carbon dioxide and supercritical propane as mobile phases in supercritical fluid chromatography

Comparisons of supercritical propane and supercritical carbon dioxide eluents were made on bare silica and octadecylsilane-derivatized silica using substituted and unsubstituted aromatic hydrocarbons as test solutes. The greater elution strength of carbon dioxide relative to propane for these solutes on underivatized silica is indicated to be largely the result of more effective competitive adsorption by carbon dioxide rather than enhanced mobile phase solubility. The effects of the addition of methanol modifier on solute retention were more pronounced in propane for both polar and non-polar solutes. An increase in the retention of polar aromatics on silica was observed with increasing density in methanol modified propane. This is apparently the result of a concomitant increase in the availability of stationary phase adsorption sites.

Adsorption isotherms on silica for methanol and 1-hexanol modifiers from supercritical carbon dioxide

Adsorption isotherms were determined for methanol and 1-hexanol on silica from supercritical carbon dioxide at four temperatures and three mobile phase densities. Maximum stationary phase concentrations were extrapolated from linear least squares fits of the data to the Langmuir equation for monolayer adsorption. From these results, maximum surface area coverages were calculated using the mean molecular area of each modifier. The maximum, molar stationary phase concentration of methanol was found to exceed that of 1-hexanol under all experimental conditions; however, in each case the surface area coverage by hexanol was calculated to be larger. The capacity factors of several substituted and unsubstituted aromatic hydrocarbons were determined in 0–1% (w/v) methanol modifier carbon dioxide. From the linearity of capacity factor versus modifier concentration plots, the ability of the solutes to compete with methanol for active column sites was determined. Unsubstituted aromatic hydrocarbons do not appear to compete with the modifier for direct adsorption onto modifier-sorbing active sites.

Kinetic analysis of aromatic photocyanation: naphthalene, biphenyl, and phenanthrene

Nucleophilic photocyanation of the unsubstituted aromatic hydrocarbons, naphthalene, biphenyl, and phenanthrene, in both dry and aqueous acetonitrile can be described by a mechanism involving two photoinduced transients. The primary step of the mechanism is the formation of a transient ionic complex through triplet excimer of aromatic hydrocarbon or, when an electron acceptor is present, through triplet exciplex. The attack of cyanide ion on the transient complex yields the cyanated radical, ArHCN˙, which in aqueous acetonitrile reacts with itself to yield cyano- and dihydrocyano-products or, in dry acetonitrile after being attacked by an electron, is oxidized to the cyano-product. The rate constants for the formation of radical ions and the attack of cyanide ion are calculated from the experimental results.

Relationships between carcinogenicity, mutagenicity, and theoretical reactivity indices for polycyclic aromatic hydrocarbons

Statistical correlations have been examined between two experimental carcinogenicity indices, a mutagenicity index, and several theoretical reactivity indices for a sample of polycyclic aromatic hydrocarbons displaying a considerable range of carcinogenic activities. Salmonella/microsome mutagenicity assay results show no quantitative correlation with either lball indices (r = 0.08) or a mouse-skin tumor index (r = −0.11) for this sample, whereas the two carcinogenicity indices show reasonable agreement (r = 0.84). The theoretical indices, which represent chemical reactivities for several putative carcinogenic “activation” steps, correlate with the cancer indices ( r ∼ 0.6–0.9). but not the mutagenicity index, suggesting a usefulness in cancer prescreening procedures. Similar results are found for a sample of 24 unsubstituted aromatic hydrocarbons. Available hamster V79 cell assay results show good correlations with the carcinogenicity indices (r = 0.87–0.99).