The chemical reactivities of the optically-pure fjord-region syn- and anti-benzo[g]chrysene 11,12-dihydrodiol 13,14-epoxides (B[g]CDEs) toward DNA in vitro have been compared with those of the optically-pure fjord-region syn- and anti-benzo[c]phenanthrene 3,4-dihydrodiol 1,2-epoxides (B[c]PhDEs), using the standard 32P-postlabeling assay. The (+)-anti-, (+)-syn-, and (-)-syn-isomers of the two sets of diol epoxides showed similar extents of reaction with DNA, but the (-)-anti-B[c]PhDE was 2.5 times more reactive toward DNA than the corresponding B[g]CDE isomer and was the most reactive of the eight diol epoxides studied. When the reactions of the B[g]CDEs with DNA were analyzed by the nuclease P1-enhanced method of 32P-postlabeling, the observed adduct levels were between 3 and 10 times lower than were obtained using the standard method of 32P-postlabeling. By analyzing by TLC and HPLC the 32P-postlabeled products of the reactions of the diol epoxides with synthetic polynucleotides, the relative reactions of the B[g]CDEs and B[c]PhDEs with guanine and adenine bases in DNA were determined. All four B[g]CDE isomers reacted with adenine residues in similar proportions to those seen for the B[c]PhDE isomers. Thus, the presence of an additional benzene ring on the benzo[c]phenanthrene structure, distant from the fjord region, does not radically alter the reactivity or base selectivity of the fjord-region diol epoxides, except in the case of the (-)-anti-isomer of benzo[g]chrysene. The reasons for the lower reactivity of this isomer compared with that of the corresponding isomer of benzo[c]phenanthrene are unclear.(ABSTRACT TRUNCATED AT 250 WORDS)