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 hyb...