| Abstract
| - The diastereomeric spiroiminodihydantoin (Sp) lesions are oxidation products of guanine and 8-oxo-7,8-dihydroguanine (8-oxoG) and have generated considerable interest because of their stereochemistry-dependent mutagenic properties in vivo (Henderson, P. T., et al. (2003) Biochemistry 42, 9257−9262).However, the absolute configurations of the two diastereomers have not yet been elucidated, and suchinformation may prove valuable for understanding relationships between biological function and structureat the DNA level (Jia, L., Shafirovich, V., Shapiro, R., Geacintov, N. E., and Broyde, S. (2005)Biochemistry 44, 13342−13353). We have synthesized the two chiral Sp nucleobases by hydrolysis ofthe nucleosides denoted by dSp1 and dSp2 according to their elution order in HPLC experiments usinga Hypercarb column, and determined their absolute configurations using a combination of experimentallymeasured optical rotatory dispersion (ORD) spectra in aqueous solutions and computed ORD specificrotations using density functional theory (DFT). Recent developments have shown that DFT methodsare now sufficiently robust for predicting ORD values of chiral molecules (Polavarapu, P. L. (2002)Chirality14, 768−781). The nucleobases Sp1 and Sp2 exhibit experimentally measured CD and ORDspectra that are very close to those of the respective precursor nucleosides dSp1 and dSp2 in shape andsign. The first nucleoside stereoisomer (dSp1) to elute from a typical Hypercarb HPLC column has (−)-S, while the second (dSp2) has (+)-R absolute configuration. The R and S assignments are applicable tothe amino tautomeric forms in each case.
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