Syntheses via the phosphotriester method are described for DNA oligonucleotides containing O4-ethylated thymine (e4Thy) and adenine (Ade) bases, d(e4TpA), d(Ape4T), and d(Ape4TpA), for their unmodified analogs, d(TpA), d(ApT), and d(ApTpA), and for d(Tpe4TpT). 1H NMR chemical shift data obtained from 10 to 70 °C at 300 and 500 MHz show that these molecules form right-handed minihelices at low temperature in aqueous solution; the presence of the O4-ethyl group does not seem to have a drastic effect on the stacking geometry of the thymine base. The presence of right-handed stacking is confirmed by circular dichroic data obtained over a similar temperature range. Coupling constants for the sugar ring indicate that the e4Tp unit of d(e4TpA) does not show the shift towards the 3′-endo (N) pucker noted for the corresponding unit of d(e4TpT). A prominent quasimolecular ion peak [M – H]− is observed for the modified and unmodified molecules in the spectra obtained by Fast Atom Bombardment mass spectrometry (FAB-MS) operating in the negative ion mode, indicating that the labile O4-ethyl group is sufficiently stable to withstand the ionization process used in this method. In addition, a number of fragment ion peaks such as [O4-ethylthymine – H]− are observed that reveal the potential of FAB-MS for characterizing DNA oligomers modified by alkylating agents. Keywords: oligonucleotides, O-alkylation, stereochemistry, NMR, mass spectrometry.
通过
磷酸三酯法合成了含有O4-乙基化胸腺
嘧啶(e
4Thy)和
腺嘌呤(Ade)碱基的DNA寡核苷酸,即d(e
4TpA)、d(Ape
4T)和d(Ape
4TpA),以及它们未修饰的类似物,即d(TpA)、d(ApT)和d(ApTpA),以及d(Tpe
4TpT)。在300和500
MHz下从10到70°C获得的
1H NMR
化学位移数据显示,这些分子在
水溶液中在低温下形成右旋微螺旋;O4-乙基基团的存在似乎对胸腺
嘧啶碱基的堆叠几何没有显著影响。通过在类似温度范围内获得的圆二色数据确认了右旋堆叠的存在。糖环的耦合常数表明d(e
4TpA)的e
4Tp单元不像d(e
4TpT)的相应单元那样向3'-内褶叠(N)发生移位。在负离子模式下通过快速原子轰击质谱(FAB-MS)获得的光谱中观察到修改和未修改分子的显著准分子离子峰[M – H]
−,表明易失性的O4-乙基基团足够稳定,可以承受该方法中使用的电离过程。此外,观察到了一些片段离子峰,如[O4-乙基胸腺
嘧啶 – H]
−,揭示了FAB-MS用于表征被烷化剂修改的DNA寡聚物的潜力。关键词:寡核苷酸,O-烷基化,立体
化学,NMR,质谱。