Conformational analysis of a 4-hydroxyequilenin guanine adduct using density functional theory

Equilenin, a component of the drug Premarin (Wyeth), can be metabolized to a quinonoid, 4-hydroxyequilenin (4-OHEN). 4-OHEN can react with 2′-deoxynucleosides to form unusual cyclic adducts, among which 4-hydroxyequilenin-2′-deoxyguanosine (4-OHEN-dG) is the major product under physiological conditions. The structure and stereochemistry of one stereoisomer, 4-OHEN-dG1, has been obtained previously using electrospray mass spectrometry and NMR methods [Shen et al. (1997) J. Am. Chem. Soc. 119, 11126-11127]; however, details of the conformations around the linkage site have not yet been investigated. The objective of this paper was to determine the conformation at the five-membered ring linkage site for this adduct. We have carried out a computational investigation involving high level quantum mechanical geometry optimization using density functional theory (DFT) for the 4-hydroxyequilenin-guanine adduct (4-OHEN-G1). Our results reveal that there are three conformational families which differ in the puckering of the five-membered ring at the linkage site and in the cyclohexene-type A ring conformation. The overall structures of all three families are "V"-shaped; however, two are quite compact while the third is more open. The lowest energy structure contains a half chair-type cyclohexene A ring, while two structures whose energies are ∼3-4 kcal/mol higher are boat-type. Since the Watson-Crick hydrogen bonding edge of the modified guanine is obstructed by the formation of this bulky nonplanar adduct, it likely would reside in a groove of the DNA double helix.