Benzo(j)fluoranthene

Benzo 'j''luoranthene (B''j''F) is an organic compound with the chemical formula C₂₀H₁₂. Classified as a polycyclic aromatic hydrocarbon (PAH), it is a colourless solid that is poorly soluble in most solvents. Impure samples can appear off white. Closely related isomeric compounds include benzo 'a''luoranthene (B''a''F), bendo 'b''luoranthene (B''b''F), benzo 'e''luoranthene (B''e''F), and benzo 'k''luoranthene (B''k''F). B''j''F is present in fossil fuels and is released during incomplete combustion of organic matter. It has been traced in the smoke of cigarettes, exhaust from gasoline engines, emissions from the combustion of various types of coal and emissions from oil heating, as well as an impurity in some oils such as soybean oil.

Structure and synthesis

B''j''F consists of two naphthalene-like structures which are fused by a cyclopentane structure. This cyclopentane is not included in the aromaticity of the molecule. B''j''F can be obtained when either 2-(1-chloroethenyl)benzo henanthrene or 6-(1-chloroethenyl)chrysene is treated by flash vacuum thermolysis (FVT) at high temperatures (above 900 °C) followed by ring rearrangements (ring contraction/expansion) to selectively yield B''j''F. Benzo 'k''luoranthene may also be converted via similar processes to B''j''F by FVT at temperatures of at least 1100 °C (6% yield) or at least 1200 °C (11% yield) with 38% mass recovery.

Reactivity

B''j''F can be functionalized by means of electrophilic aromatic substitution. In the body it is metabolized into phenols (3,4,6 or 10 hydroxy), dihydrodiols (4,5 and 9,10) and 4,5-dione (fig. 1).

Mechanism of action

B''j''F is categorized by the IARC as possibly carcinogenic to human beings, like many other PAHs, on the basis of sufficient evidence in animals. For example, B''j''F is active as a tumor initiator on mouse skin and is carcinogenic in both mouse skin and in rat lungs. Recently, B''j''F was also found to induce tumors in newborn mouse lung and liver. The mechanism of actions of B''j''F is similar to other PAHs. The diolepoxide mechanism involves formation of stable and unstable DNA adducts, mainly at G and A, which can lead to mutations in proto-oncogenes (RAS) and tumour-suppressor genes (P53). Many polycyclic aromatic hydrocarbon diolepoxides and their precursor diols and epoxides are tumorigenic in animals. The radical cation mechanism involves generation of unstable adducts at G and A, leading to apurinic sites and mutations in HRAS. Orthoquinone formation could lead to stable and unstable DNA adducts and generation of reactive oxygen species, inducing mutations in P53.

Toxicity

PAHs

One of the earliest connection between PAHs, combustion, and cancer was established by Cook and co-workers with the isolation of the carcinogen benzo 'a''yrene from coal tar extract. Benzo 'a''yrene now has been well characterized in toxicology reports and is a known potent carcinogen. Benzo 'a''yrene requires metabolic activation to become, ultimately, BPDE ((±)-''anti''-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo 'a''yrene) which binds to the DNA to form a covalent trans adducts at the N2 position of guanine. Hereafter binding to DNA at cancer hotspots, especially in the P53 tumour suppressor gene at codons: 157, 248 and 273 (figure 3), it has the possibility of inducing lung cancer. Structural similarity of PAHs contributes to the similarity in metabolism, biotransformation and toxicology. Benzo 'a''yrene has been extensively reviewed and is used as a model for the toxicology and metabolism of other PAHs.

Benzo 'j''luoranthene

Specific studies on B''j''F showed that it exhibits mutagenic toxicity in ''S. typhimurium'' TA98 and TA1000 under the presence of microsomal activation. B''j''F can form DNA-adducts, covalently binding of chemicals to DNA can result in strand breaks and DNA damage, which ultimately leads to mutations. In mice studies B''j''F induced tumorigenic activity on the skin, lung adenomas and liver adenomas/hepatomas. Lung implantation of B''j''F also induced lung epidermoid carcinomas in 3-month-old female rats. Tail vein injection of B''j''F also causes covalently binding to mouse hemoglobin and serum proteins, with binding to serum proteins being 10-fold higher than to hemoglobin.Agency for Toxic Substances and Disease Registry (ATSDR). (1995) "Toxicological profile for Polycyclic Aromatic Hydrocarbons (PAHs)" Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service.

References

{{PAHs

Polycyclic aromatic hydrocarbons
IARC Group 2B carcinogens