Xm. Xie et al., Origins of conformational differences between cis and trans DNA adducts derived from enantiomeric anti-benzo[a]pyrene diol epoxides, CHEM RES T, 12(7), 1999, pp. 597-609
The two enantiomeric metabolites of the carcinogen precursor benzo[a]pyrene
, (+)- and (-)-anti-BPDE [(7R,8S)-dihydroxy-(9S,10R)-epoxy-7,8,9,10-tetrahy
drobenzo[a]pyrene and the corresponding 7S,8R,9R,10S enantiomer, respective
ly], bind predominantly to the exocyclic amino groups of dG residues in dou
ble-stranded DNA by either cis or trans addition to yield four stereoisomer
ically distinct [BP]-N-2-dG adducts. Both the 10S (+)-trans and 10R (-)-tra
ns adducts assume minor groove conformations in normal, full duplexes, but
with opposite 5' or 3' orientations, respectively, relative to the modified
strand. In contrast, the 10R (+)-cis and 10S (-)-cis adducts assume opposi
tely oriented base-displaced intercalative conformations in normal duplexes
, with the inserted pyrenyl residues pointing toward the major groove in th
e (+)-cis isomer and toward the minor groove in the (-)-cis isomer. A BPDE-
modified nucleoside is a small system which can be studied by computational
methods with a very thorough survey of the potential energy surface. To in
vestigate conformational differences between cis and trans adducts, and to
elucidate origins governing the opposite orientations of these (+)- and (-)
-diol epoxide adducts, we have carried out extensive investigations of the
(+)- and (-)-trans-anti- and (+)- and (-)-cis-anti-[BP]-N-2-dG deoxynucleos
ide adduct pairs. We report results for the (+)- and (-)-cis-anti pair, and
compare them with the (+)- and (-)-trans-anti adducts. We created 373 248
different conformers for each adduct, which uniformly sampled at 5 degrees
intervals the possible rotamers about three flexible torsion angles governi
ng base (chi) and carcinogen (alpha' and beta') orientations, and computed
each of their energies. The potential energy surface of the molecule was th
en mapped from these results. While four potential energy wells or structur
al domains are found for the (Jr)-trans adduct and four for the (-)-trans a
dduct, only two of these four domains are favored for each of the two cis a
dducts. In both cis and trans adducts, the (+)/(-) pairs of each structural
domain are nearly mirror images. The most favored of the domains in both c
is and trans adducts is observed experimentally in the duplexes containing
each of these [BP]-N-2-dG lesions. The opposite orientations in both cis an
d trans adducts stem from steric crowding at the benzylic ring, engendered
when a (+) stereoisomer is rotated into the analogous conformation of its (
-) partner, and vice versa. Furthermore, the key role of the difference in
absolute configuration between trans and cis adducts at the hydroxyls of C9
and C8 in governing conformational preferences and flexibility is delineat
ed. Cis adducts are less conformationally flexible than trans adducts becau
se they are inherently more sterically crowded, with C9-OH and C8-OH on the
same side of the benzylic ring as guanine and sugar, while they are on the
opposite side of the benzylic ring in the trans adducts. Consequently, the
cis adducts inherently favor less the minor groove position adopted by tra
ns adducts in DNA duplexes because the C9-OH and C8-OH are directed inward
into the minor groove in the cis adducts. In the trans adducts, the C9-OH a
nd C8-OH are directed outward, away from the interior of the minor groove.
Observed differential processing of these four adducts by replication, repa
ir, and transcription enzymes may well stem from their differing conformati
onal preferences.