Unsubstituted PAH

Human exposure to complex mixtures of polycyclic aromatic hydrocarbons (PAH) occurs through inhalation of tobacco smoke and polluted indoor or outdoor air, through ingestion of certain foods and polluted water, and by dermal contact with soots, tars, and oils CO. Methylated PAH are always components of these mixtures and in some cases, as in tobacco smoke and in emissions from certain fuel processes, their concentrations can be in the same range as some unsubstituted PAH. The estimated emission of methylated PAH from mobile sources in the U.S. in 1979 was approximately 1700 metric tons (2). The occurrence of methylated and unsubstituted PAH has been recently reviewed (1, 2). In addition to their environmental occurrence, methylated PAH are among the most important model compounds in experimental carcinogenesis. 7,12-Dimethylbenz[a]anthracene, one of... 

For unsubstituted PAH, such as benzo[a]pyrene (BP), pyridinium or acetoxy derivatives are formed by direct attack of pyridine or acetate ion, respectively, on the radical cation at C-6, the position of maximum charge density (Scheme 1). This is followed by a second one-electron oxidation of the resulting radical and loss of a proton to yield the 6-substituted derivative. For methyl-substituted PAH in which the maximum charge density of the radical cation adjacent to the methyl group is appreciable, as in 6-methylbenzo[a]-pyrene (6-methylBP) (Scheme 2), loss of a methyl proton yields a benzylic radical. This reactive species is rapidly oxidized by iodine or MnJ to a benzylic carbonium ion with subsequent trapping by pyridine or acetate ion, respectively. 

Van Cauwenberghe (1985) proposed two different mechanisms for the degradation of unsubstituted PAHs... 

Metabolic activation of PAHs consists of an oxidation of the rings of unsubstituted PAHs. These oxidations are carried out by mixed function oxidases of the liver which contain cytochromes P450 and P448 and require reduced nicotine adenine dinucleotide and oxygen. In this oxidation, an epoxide intermediate is formed which has been shown to have the requisite chemical reactivity to bind covalently with DNA and histones and to serve as the ultimate carcinogenic form of PAH. Administration of 3-MC to rats increased hepatic nuclear proteins and caused a turnover of protein of the endoplasmic reticulum. Studies of " C amino acid incorporation showed that 3-MC causes increased protein synthesis and reduced degradation of protein. 

The discussion so far has treated only those reaction intermediates that can be formed from a parent unsubstituted PAH. We also wish to present the treatment for cases in which a delocalized it intermediate is formed from methyl-substituted derivatives. For example, in considering the formation of an arylmethyl cation, thermocyclic arguments (5, 6) have to be implemented to estimate the difference in Ec>(0 between the arylmethyl cation and the fragments before the union, that is, the neutral even PAH and the CH2 + ion. The localized positive charge of the CH2+ ion and the condition qr = 0 in the neutral even PAH leads to the following ... 

Preformed aromatic hydrocarbons appear to serve as templates for the formation of PAHs thus, burning benzene leads to a far greater yield of PAHs than burning aliphatic hydrocarbons. Unsubstituted PAHs are typical of high-temperature combustion processes, whereas lower-temperature flames afford a large number of PAHs substituted with alkyl (primarily methyl) groups (Hites,... 

Rgure 3 Concentrations of PAHs in a sediment core from Kitimat Arm, Douglas Channel, British Columbia. O - total unsubstituted PAHs A - unsubstituted perylene - phenan-threne x -fluoranthrene <>-2-methylphenanthrene. (Adapted from Simpson CD, Harrington CF, Cullen WR, Bright DA, and Reimer KJ (1998) Polycyclic aromatic hydrocarbon contamination in marine sediments near Kitimat, British Columbia. Environmental Science and Technology 32 3266-3272.)... 

The way these carcinogens produce cancer is now fairly well understood. Unsubstituted PAHs are not very soluble, so to eliminate hydrocarbons, the body usually oxidizes them to render them more water soluble and then excretes them in urine. The metabolic oxidation products seem to be the real culprits in causing cancer. For example, one of the most potent carcinogens of this type is benzo[a]pyrene. Enzymatic oxidation converts it to the diol-epoxide shown below. The diol-epoxide reacts with cellular DNA, causing mutations... 

PAHs are ubiquitous in the marine environment and occur at their highest concentrations near urban centers of human populations (LaFIamme and Hites 1978). They are derived from petroleum sources, combustion products, and naturaT synthesis by organisms (LaFIamme and Hites 1978). Taken together, PAHs are considered a signature of human activity as a result of industrial processes however, localized accumulations may result from natural events such as forest fires, volcanism, and petroleum seeps. It is generally believed that if substituted PAHs, including alkyl derivatives, predominate, the source of the PAHs is petroleum based. If, however, the unsubstituted PAHs (especially the HPAHs) predominate, then it is more... 

EMAP). The analyte list of these studies (Table 1) includes 18 individual unsubstituted PAHs and 16 groups with alkyl substitutions (34 PAHs). EPA designated this list of 34 PAHs as total PAH and based their ESG for PAH mixtures on the following expression of the additivity of Type I narcotic chemicals ... 

Figure 3). Surficial sediment (0-2 cm) was collected at ten stations and to -0.5 m at one station. Aliquots of IW separated from sediment (16) were processed directly (total concentrations) or passed through untreated C-18 cartridges (DOM-bound concentrations). The freely-dissolved concentrations were determined as the difference between the total and bound concentrations (16J 8). De-watered sediment was sonicated in acetonitrile. Extracts were cleaned on treated C-18 columns (36), and quantified by mass spectrometer in the single ion monitoring mode Twenty-three PAHs and 22 PCB congeners were quantified. The PAHs consisted of 18 unsubstituted PAHs (see Table I) and 5 alkylated PAHs (1 and 2 methyl-, 2,6 dimethyl- and 2,3,5 trimethyl-naphthalene, and 1 methyl-phenanthrene). Prior to extraction all samples were amended with 6 deuterated PAH and 4 PCB compounds in a 10 1 ratio. 

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