The present study was aimed at gaining further insights into stereochemical and conformational features of the 4R and 4S diastereomers of spiroiminodihydantoin 2'-deoxyribonucleosides that have been shown to be the predominant singlet oxygen oxidation products of 2'-deoxyguanosine in aqueous solutions. It may be added that spiroiminodihydantoin derivatives are efficiently generated by one-electron and singlet oxygen oxidation of the 8-oxo-7,8-dihydroguanine moiety of several nucleic acid components including nucleosides, nucleotides, and oligonucleotides. The reported structural data on the pair of diastereomeric spiroiminodihydantoin 2'-deoxyribonucleosides 1 and 2 are mostly inferred from extensive (1)H and (13)C NMR analyses including two-dimensional nuclear Overhauser effect measurements performed in both D(2)O and dimethyl sulfoxide. This approach that has been shown previously to be suitable to assign the stereochemistry of the base moiety of oxidized pyrimidine nucleosides was completed by molecular modeling and quantum mechanics studies. Thus, application of these two complementary approaches together with the consideration of the results of a recent relevant quantum mechanic study has allowed the assignment of the absolute stereoconfiguration of the C-4 carbon of diastereomers 1 and 2. In addition, information is provided on the conformational features of the 2-deoxyribose moiety and the orientation of the base around the N-glycosidic bond of both 2'-deoxyribonucleosides 1 and 2.

A dumbbell double-stranded DNA decamer tethered with a hexaethylene glycol linker moiety (DDSDPEG), with a nick in the centre of one strand, has been synthesised. The standard NMR methods, E.COSY, TOCSY, NOESY and HMQC, were used to measure (1)H, (31)P and T:(1) spectral parameters. Molecular modelling using rMD-simulated annealing was used to compute the structure. Scalar couplings and dipolar contacts show that the molecule adopts a right-handed B-DNA helix in 38 mM phosphate buffer at pH 7. Its high melting temperature confirms the good base stacking and stability of the duplex. This is partly attributed to the presence of the PEG(6) linker at both ends of the duplex that restricts the dynamics of the stem pentamers and thus stabilises the oligonucleotide. The inspection of the global parameters shows that the linker does not distort the B-DNA geometry. The computed structure suggests that the presence of the nick is not disturbing the overall tertiary structure, base pair geometry or duplex base pairing to a substantial extent. The nick has, however, a noticeable impact on the local geometry at the nick site, indicated clearly by NMR analysis and reflected in the conformational parameters of the computed structure. The (1)H spectra also show much sharper resonances in the presence of K(+) indicating that conformational heterogeneity of DDSDPEG is reduced in the presence of potassium as compared to sodium or caesium ions. At the same time the (1)H resonances have longer T:(1) times. This parameter is suggested as a sensitive gauge of stabilisation.