Linear dichroism properties and orientations of different ultraviolet transition moments of benzo[a]pyrene derivatives bound noncovalently and covalently to DNA

Conformational studies of the (+)-trans, (-)-trans, (+)-cis, and (-)-cis adducts of anti-benzo[a]pyrene diolepoxide to N2-dG in duplex oligonucleotides using polyacrylamide gel electrophoresis and low-temperature fluorescence spectroscopy

Using polyacrylamide gel electrophoresis (PAGE) and low-temperature, laser-induced fluorescence line narrowing (FLN) and non-line narrowing (NLN) spectroscopic methods, the conformational characteristics of stereochemically defined and site-specific adducts derived from the binding of 7 beta,8 alpha-dihydroxy-9 alpha,10 alpha-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (anti-BPDE, a metabolite of the environmental carcinogen benzo[a]pyrene), to DNA were studied. The focus of these studies was on the four stereochemically distinct anti-BPDE modified duplexes 5'-d(CCATCGCTACC).(GGTAGCGATGG), where G denotes the lesion site derived from trans or cis addition of the exocyclic amino group of guanine to the C10 position of either (+) or (-)-anti-BPDE. PAGE experiments under non-denaturing conditions showed that the (+)-trans adduct causes a significantly greater retardation in the electrophoretic mobility than the other three adducts, probably the result of important adduct-induced distortions of the duplex structure. Low-temperature fluorescence studies in frozen aqueous buffer matrices showed that the (+)-trans adduct adopts primarily an external conformation with only minor interactions with the helix, but a smaller fraction (approximately 25%) appears to exists in a partially base-stacked conformation. The (-)-trans adduct exists almost exclusively (approximately 97%) in an external conformation. Both cis adducts were found to be intercalated; strong electron-phonon coupling observed in their FLN spectra provided additional evidence for significant pi-pi stacking interactions between the pyrenyl residues and the bases. FLN spectroscopy is shown to be suitable for distinguishing between trans and cis adducts, but lesions with either (+)- or (-)-trans, or (+)- or (-)-cis stereochemical characteristics showed very similar vibrational patterns.(ABSTRACT TRUNCATED AT 250 WORDS)

Stereochemistry-dependent bending in oligonucleotide duplexes induced by site-specific covalent benzo[a]pyrene diol epoxide-guanine lesions

The apparent persistence length of enzymatically linearized pIBI30 plasmid DNA molecules approximately 2300 bp long, as measured by a hydrodynamic linear flow dichroism method, is markedly decreased after covalent binding of the highly tumorigenic benzo[a]pyrene metabolite 7R,8S-dihydroxy-9S,10R-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+)-anti-BPDE]. In striking contrast, the binding of the non-tumorigenic, mirror-image 7S,8R,9R,10S enantiomer [(-)-anti-BPDE] to DNA has no measurable effect on its alignment in hydrodynamic flow gradients (< or = 2.2% of the DNA bases modified). In order to relate this effect to BPDE-nucleotide lesions of defined stereochemistry, the bending induced by site-specifically placed and stereochemically defined (+)- and (-)-anti-BPDE-N2-dG lesions in an 11mer deoxyoligonucleotide duplex was studied by ligation and gel electrophoresis methods. Out of the four stereochemically isomeric anti-BPDE-N2-deoxyguanosyl (dG) adducts with either (+)-trans, (-)-trans, (+)-cis, and (-)-cis adduct stereochemistry, only the (+)-trans adduct gives rise to prominent bends or flexible hinge joints in the modified oligonucleotide duplexes. Since both anti-BPDE enantiomers are known to bind preferentially to dG (> or = 85%), these observations can account for the differences in persistence lengths of DNA modified with either (+)-anti-BPDE or the chiral (-)-anti-BPDE isomer.

Differences in unwinding of supercoiled DNA induced by the two enantiomers of anti-benzo[a]pyrene diol epoxide

The unwinding of supercoiled phi X174 RFI DNA induced by the tumorigenic (+) and non-tumorigenic (-) enantiomers of trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) has been investigated by agarose slab-gel and ethidium titration tube gel electrophoresis. The differences in adduct conformations were verified by flow linear dichroism techniques. Both enantiomers cause a reversible unwinding by the formation of noncovalent intercalative complexes. The effects of covalently bound BPDE residues on the electrophoretic mobilities of the RF I DNA form in agarose gels were investigated in detail in the range of binding ratios rb approximately 0.0-0.06 (covalently bound BPDE residues/nucleotide). In this range of rb values, there is a striking difference in the mobilities of (+)-BPDE- and (-)-BPDE-adducted phi X174 DNA in agarose slab-gels, the covalently bound (+)-BPDE residues causing a significantly greater retardation than (-)-BPDE residues. Increasing the level of covalent adducts beyond rb approximately 0.06 in the case of the (+)-BPDE enantiomer, leads to further unwinding and a minimum in the mobilities (corresponding to comigration of the nicked form and the covalently closed relaxed modified form) at rb 0.10 +/- 0.01; at still higher rb values, rewinding of the modified DNA in the opposite sense is observed. From the minimum in the mobility, a mean unwinding angle (per BPDE residue) of theta = 12 +/- 1.5 degrees is determined, which is in good agreement the value of theta = 11 +/- 1.8 degrees obtained by the tube gel titration method. Using this latter method, values of theta = 6.8 +/- 1.7 degrees for (-)-BPDE-phi X174 adducts are observed. It is concluded that agarose slab gel techniques are not suitable for determining unwinding angles for (-)-BPDE-modified phi X174 DNA because the alterations in the tertiary structures for rb < 0.06 are too small to cause sufficiently large changes in the electrophoretic mobilities. The major trans (+)-BPDE-N2-guanosine covalent adduct is situated at external binding sites and the mechanisms of unwinding are therefore different from those relevant to noncovalent intercalative BPDE-DNA complexes or to classical intercalating drug molecules; a flexible hinge joint and a widening of the minor groove at the site of the lesion may account for the observed unwinding effects. The more heterogeneous (-)-BPDE-nucleoside adducts (involving cis and trans N2-guanosine, and adenosine adducts) are less effective in causing unwinding of supercoiled DNA for reasons which remain to be elucidated.

Influence of benzo[a]pyrene diol epoxide chirality on solution conformations of DNA covalent adducts: the (-)-trans-anti-[BP]G.C adduct structure and comparison with the (+)-trans-anti-[BP]G.C enantiomer

Benzo[a]pyrene (BP) is an environmental genotoxin, which, following metabolic activation to 7,8-diol 9,10-epoxide (BPDE) derivatives, forms covalent adducts with cellular DNA. A major fraction of adducts are derived from the binding of N2 of guanine to the C10 position of BPDE. The mutagenic and carcinogenic potentials of these adducts are strongly dependent on the chirality at the four asymmetric benzylic carbon atoms. We report below on the combined NMR-energy minimization refinement characterization of the solution conformation of (-)-trans-anti-[BP]G positioned opposite C and flanked by G.C base pairs in the d(C1-C2-A3-T4-C5-[BP]G6-C7-T8-A9-C10-C11).d(G12-G13-T14++ +-A15-G16-C17- G18-A19-T20-G21-G22) duplex. Two-dimensional NMR techniques were applied to assign the exchangeable and non-exchangeable protons of the benzo[a]pyrenyl moiety and the nucleic acid in the modified duplex. These results establish Watson-Crick base pair alignment at the [BP]G6.C17 modification site, as well as the flanking C5.G18 and C7.G16 pairs within a regular right-handed helix. The solution structure of the (-)-trans-anti-[BP]G.C 11-mer duplex has been determined by incorporating intramolecular and intermolecular proton-proton distances defined by lower and upper bounds deduced from NOE buildup curves as constraints in energy minimization computations. The BP ring spans both strands of the duplex in the minor groove and is directed toward the 3'-end of the modified strand in the refined structure. One face of the BP ring of [BP]G6 stacks over the C17 residue across from it on the partner strand while the other face is exposed to solvent.(ABSTRACT TRUNCATED AT 250 WORDS)

Covalent bonding of bay-region diol epoxides to nucleic acids

Although the solution chemistry of diol epoxides is now fairly well understood, a great deal remains to be elucidated regarding their reaction in the presence of DNA. Not only DNA but also small molecules are capable of sequestering diol epoxides in aqueous solutions with equilibrium constants on the order of 10(2)-10(4) M-1. In the case of DNA, at least two major families of complexes are presently recognized, possibly the result of groove binding vs. intercalation. As is the case for diol epoxides free in solution, the complexed diol epoxides undergo solvolysis to tetraols and in some cases possibly to keto diols as well. Fractionation between covalent bonding and solvolysis from within the complex(s) is determined more by the nature of the parent hydrocarbon from which the diol epoxide is derived than any other factor. Studies of a wide variety of alkylating and arylating agents have show that practically every potentially nucleophilic site on DNA can serve as a target for modification. In the case of the diol epoxides, practically all of the modification occurs at the exocyclic amino groups of the purine bases. In contrast to the diol epoxides, other epoxides such as those derived from aflatoxin B1, vinyl chloride, propylene, 9-vinylanthracene, and styrene preferentially bind to the aromatic ring nitrogens N-7 in guanine and N-3 in adenine (cf. Chadha et al., 1989). Molecular modeling as well as the spectroscopic evidence suggests that the hydrocarbon portion of the diol epoxides lies in the minor groove of DNA when bound to the exocyclic 2-amino group of guanine and in the major groove when bound to the exocyclic 6-amino group of adenine. Detailed conformational analysis of adducted DNA should prove to be extremely valuable in developing mechanistic models for the enzymatic processing of chemically altered DNA. At present, the critical lesion or lesions responsible for induction of neoplasia remains obscured by the large number of apparently noncritical adducts which form when polycyclic hydrocarbon diol epoxides bond to DNA.