11-Methylbenzo[a]pyrene: bay region distortions.

Substitution of a methyl group in the 11-position of benzo[a]pyrene (B[a]P) enhances its carcinogenicity. An X-ray crystallographic determination of the three-dimensional structure of 11-methylbenzo[a]pyrene (11-MeB[a]P) shows that steric overcrowding in the bay region is relieved somewhat by distortions of the bay-region bond angles in the plane of the ring system. A comparison with the structure of 7,12-dimethylbenz[a]anthracene (DMBA), which shows out-of-plane distortions to relieve such strain, shows that, in general, H...H intramolecular interactions between neighboring rings in a polycyclic aromatic hydrocarbon are the primary determinants of the nature of the molecular distortions as a result of steric overcrowding (mainly in-plane for 11-MeB[a]P and mainly out-of-plane for DMBA). The 11-MeB[a]P molecule exhibits considerable flexibility as evidenced by slightly different conformations in the two molecules found in the asymmetric unit of the crystal. One molecule is fairly flat with bond angle distortions in the bay region, while the other is slightly buckled as a result of some twist (15 degrees) in the bay region. Computer modeling indicates that steric overcrowding as a result of the bay-region 11-methyl group may affect the conformation of the ring that bears the diol and epoxide groups in the anti-diolepoxide. The nature of this distortion may, in turn, provide a clue to the reason for the greater carcinogenicity of B[a]P when methylated at the 11-position in the non-benzo bay region site. In addition, the 11-methyl group, because of its bulk, may affect the orientation of the polycyclic hydrocarbon as it lies between the nucleic-acid bases when covalently bound to DNA.

Chemical reactivities and mutagenicities of a series of chloromethylbenzo[a]pyrenes.

The chemical and mutagenic properties of a series of chloromethylbenzo[a]pyrenes (chloromethyl-BaP) (chloromethyl groups in position 1-, 4-, 5-, 6-, 10-, 11- or 12-) were studied in order to address the question of the importance of arylmethyl carbocations as possible ultimate carcinogens of methylated polycyclic aromatic hydrocarbons (PAH). The rates of solvolysis of the series of chloromethyl-BaP in 50% aqueous acetone decrease in the order: 6 greater than 1 much greater than 4 greater than 12 greater than 5 greater than 10 greater than 11. There is a rough correlation (r = -0.80, P less than 0.05) between rates of solvolysis and the carbon chemical shifts of the methylene carbons. There is a good correlation (r = 0.98, P less than 0.001) between the rates of solvolysis and the gas phase stabilities of the carbocations, (M+ -35), obtained from mass spectral analysis. The mutagenicities of the series of chloromethyl-BaP in the Ames assay with strains TA98 and TA100 showed strong to very strong mutagenicities for the 4-, 5-, 10-, 11- and 12-isomers and weak mutagenicities for the 1- and 6-isomers. The corresponding hydroxymethyl-BaP were not mutagenic. The mutagenicities of some of the chloromethyl-BaP are among the highest reported for direct-acting (not requiring microsomal activation) mutagens in the Ames assay.

Comparative tumor-initiating activity of methylated benzo(a)pyrene derivatives in mouse skin.

The abilities of various mono and dimethyl derivatives of benzo(a)pyrene (BP) to initiate skin tumors in mice were determined by using a two-stage system of tumorigenesis. 11-Methylbenzo(a)pyrene was found to be approximately 3 times more active as a tumor initiator than was the parent hydrocarbon; 1-methyl benzo(a)pyrene was about twice as active as was BP. Substitution of a methyl group in positions 7, 8, 9, or 10 of BP, which would be involved in a bay-region diol-epoxide, completely counteracts the tumor-initiating ability of BP. 3-, 4-, and 12-methyl-benzo(a)pyrenes and activity equivalent to that of BP, whereas 2-, 5-, and 6-methylbenzo(a)pyrenes, as well as 1,2-, 4,5-, 1,6-, and 3,6-dimethylbenzo(a)pyrenes, were all less active than BP. The concepts of steric inhibition of metabolic activation and stereospecific activation are suggested to explain the tumor-initiating activities of various methylated derivatives.

Reactivity-selectivity properties of reactions of carcinogenic electrophiles and nucleosides: influence of pH on site selectivity.

6-Chloromethylbenzo[a]pyrene (6-CMBP) labeled with 13C in the chloromethyl group was used as a model for those carcinogens which form essentially free carbocations. Using 13C-NMR to identify products, the selectivity with which this electrophile modifies nucleosides was investigated. At pH 7, guanosine and deoxyguanosine are the most nucleophilic nucleosides toward the carbocation generated by solvolysis of 6-CMBP. Attack at N-7 predominates over attack at N-2. At higher pH, the nucleophilicity of guanosine and deoxyguanosines increases markedly. In addition, the site of modification changes to N-1 with secondary modification at O-6. The pH dependence of the rate of this reaction implicates a group with pK-value approx. 8.7 which was assigned to the hydrogen on N-1. The presence of a methyl group on the N-7 position of guanosine lowers this pK-value to approx. 7.2. Consequently, N7-methylguanosine shows the high nucleophilicity at physiological pH that guanosine has at high pH. These observations lead to the suggestion of a one base : two-site model for chemical carcinogenesis.