Covalent binding of benzo[a]pyrene 7,8-dihydrodiol 9,10-epoxides to DNA: molecular structures, induced mutations and biological consequences.

Optical spectroscopic techniques have been used to characterize adducts formed upon reaction of the (+)- and (-)-enantiomers of 7R,8S-dihydroxy 9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE) to DNA or synthetic oligonucleotides. The reaction yields preferentially adducts in which the exocyclic aminogroup of deoxyguanosine is bound to the C10 position of the diol epoxide either cis (BPDEc-N2-G adduct) or trans (BPDEt-N2-G adduct) relative to the hydroxyl group at the C9 position. The BPDEc-N2-G and BPDEt-N2-G adducts fall into the categories of type I and type II complexes, respectively. Two-dimensional NMR in conjunction with energy minimization computation have provided detailed information on the solution structure of single adducts localized in oligonucleotides. The results demonstrate that the pyrenyl chromophores of both the (+)- and (-)-BPDEt-N2-G adduct are located in a widened minor groove and directed towards the 5'-end [(+)-BPDEt-N2-G] or the 3'-end [(-)-BPDEt-N2-G] of the modified strand. The chromophore of the (+)-BPDEc-N2-G adduct is quasi-intercalated into the oligonucleotide and associated with a displacement of the deoxyguanosine ring into the minor groove. Replication of racemic or (+)-anti-BPDE modified DNA in mammalian cells leads predominantly to single point mutations of transversion type (GC-->TA). The mutagenic specificity however, appears to be determined by the base sequence context and local conformation at the adduct site. Cooperative adduct formation at certain base sequences is suggested by excimer fluorescence, most probably derived from two closely located (+)-BPDEt-N2-G adducts in adjacent base pairs on opposite DNA-strands.