Polarization effects on the reversible and covalent DNA binding of bay- and K-region metabolites of benzo [a] pyrene and benz [a] anthracene

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Abstract

Ab initio SCF calculations with a 4–31G basis set have been employed to obtain polarization energies for the interaction of a positive point charge of 0.6 eu at various positions on planes 2.0 Å above the aromatic rings of epoxide containing metabolites of benzo [a] pyrene (BP) and benz [a] anthracene (BA) that exhibit varying carcinogenic activities. The metabolites examined are the bay-region diolepoxides, (±)-trans-7,8-dihydroxy-anti-9,10-epoxy-7,8,9,10-tetrahydro-BP (BPDE) and (±)-trans-3,4-dihydroxy-anti-1,2-epoxy-1,2,3,4-tetrahydro-BA (BADE), and the K-region epoxides, BP-4,5-oxide (BPO) and BA-5,6-oxide (BAO). The average values of the polarization energies that are calculated when the charge is located above each of the aromatic carbon atoms of the π systems are 20.91, 20.55, 19.42, and 18.17 kcal/mol for BADE, BPDE, BPO, and BAO, respectively. These results are consistent with the finding that, for a given parent hydrocarbon, association constants for reversible binding to DNA are larger for bay-region metabolites than for K-region metabolites. The relationship between average polarization energies of bay- vs. K-region metabolites and DNA association constants is consistent with the conclusion that van der Waals forces strongly influence the reversible binding of these metabolites to DNA. For BPDE and BPO, polarization energy contour maps have been constructed by carrying out additional calculations that yielded polarization energies at a total of 484 grid points on planes 2.0 Å above the aromatic rings of the metabolites. A comparison of the maps for BPDE and BPO demonstrates that, in the region near the benzylic C atom at the BPDE reaction center, contours of equal polarization energy are 1.0 to 2.0 kcal/mol greater than are contours at corresponding positions near benzylic C atoms at the BPO reaction center. The larger polarization energy contours at the BPDE reaction center, compared to the BPO reaction center, are consistent with the greater chemical reactivity of BPDE compared to BPO. © 1993 John Wiley & Sons, Inc.

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