Determination of Absolute Configurations of 4-Hydroxyequilenin-Cytosine and -Adenine Adducts by Optical Rotatory Dispersion, Electronic Circular Dichroism, Density Functional Theory Calculations, and Mass Spectrometry

Abstract

Estrogen components of some hormone replacement formulations have been implicated in the initiation of breast cancer. Some of these formulations contain equine estrogens such as equilin and equilenin that are metabolized to the genotoxic catechol 4-hydroxyequilenin (4-OHEN). Auto-oxidation generates the o-quinone form that reacts with dC and dA in oligodeoxynucleotides to form unusual stable cyclic bulky adducts, with four different stereoisomers identified for each base adduct. The dC and dA adducts have the same unsaturated bicyclo[3.3.1]nonane type linkage site with identical stereochemical characteristics. Stereochemical effects may play an important part in the biological consequences of the formation of 4-OHEN-DNA adducts, and the assignment of the absolute configurations of the stereoisomeric 4-OHEN-dC and -dA adducts is therefore needed to understand structure-function relationships. We used density functional theory (DFT) to compute the specific optical rotations and electronic circular dichroism (ECD) spectra of the four 4-OHEN-C stereoisomers, and the results were compared with experimentally measured optical rotatory dispersion (ORD) and ECD spectra. The predicted ORD curves for the four stereoisomeric base adducts reproduced the shapes and signs of experimental spectra in the transparent spectral region. The stereochemistry of the C3' atom was determined by comparison of the calculated and experimental ORD and ECD spectra, and the stereochemistry of C2' was determined by mass spectrometric methods. Combining the ORD and mass spectrometry data, the absolute configurations of the four 4-OHEN-C and the stereochemically identical -dC adducts have been identified. The molecular architecture of the linkage site at the 4-OHEN-C/A and 4-OHEN-dC/dA is identical, and it is shown that the deoxyribose group does not substantially contribute to the optical activities. The absolute configurations of the 4-OHEN-dA adducts were thus deduced by comparing the experimental ORD with computed ORD values of 4-OHEN-A adducts.