NMR and computational studies of stereoisomeric equine estrogen-derived DNA cytidine adducts in oligonucleotide duplexes: opposite orientations of diastereomeric forms.

The equine estrogens equilin (EQ) and equilenin (EN) are the active components in the widely prescribed hormone replacement therapy formulation Premarin. Metabolic activation of EQ and EN generates the catechol 4-hydroxyequilenin (4-OHEN) that autoxidizes to the reactive o-quinone form in aerated aqueous solutions. The o-quinones react predominantly with C, and to a lesser extent with A and G, to form premutagenic cyclic covalent DNA adducts in vitro and in vivo. To obtain insights into the structural properties of these biologically important DNA lesions, we have synthesized site-specifically modified oligonucleotides containing the stereoisomeric 1'S,2'R,3'R-4-OHEN-C3 and 1'R,2'S,3'S-4-OHEN-C4 adducts derived from the reaction of 4-OHEN with the C in the oligonucleotide 5'-GGTAGCGATGG in aqueous solution. A combined NMR and computational approach was utilized to determine the conformational characteristics of the two major 4-OHEN-C3 and 4-OHEN-C4 stereoisomeric adducts formed in this oligonucleotide hybridized with its complementary strand. In both cases, the modified C adopts an anti glycosidic bond conformation; the equilenin distal ring protrudes into the minor groove while its two proximal hydroxyl groups are exposed on the major groove side of the DNA duplex. The bulky 4-OHEN-C adduct distorts the duplex within the central GC*G portion, but Watson-Crick pairing is maintained adjacent to C* in both stereoisomeric adducts. For the 4-OHEN-C3 adduct, the equilenin rings are oriented toward the 5'-end of the modified strand, while in 4-OHEN-C4 the equilenin is 3'-directed. Correspondingly, the distortions of the double-helical structures are more pronounced on the 5'- or the 3'-side of the lesion, respectively. These differences in stereoisomeric adduct conformations may play a role in the processing of these lesions in cellular environments.