Stone M J, Nedderman A N, Williams D H, Lin P K, Brown D M
University Chemical Laboratory Cambridge, U.K.
J Mol Biol. 1991 Dec 5;222(3):711-23. doi: 10.1016/0022-2836(91)90507-3.
In order to reach a more detailed understanding of the mechanism of the mutagenic action of methoxyamine and of N4-methoxycytidine and its 2'-deoxyribo-analogue, the solution structures of the self-complementary octanucleotide, d(CGAATTCG) and its analogues, d(CGAATCCG), d(CGAATMCG) and d(CGAATPCG) (designated 8mer-AT, 8mer-AC, 8mer-AM, and 8mer-AP, respectively), were investigated by 1H nuclear magnetic resonance spectroscopy; M is N4-methoxycytosine (mo4C) and P is an analogue, the bicyclic dihydropyrimido[4,5-c][1,2]oxazin-7-one, in which the N-O bond is held in the anti configuration with respect to N3 of the cytosine ring. Correlated spectroscopy and nuclear Overhauser spectroscopy allowed assignment of the base, anomeric and H2'/H2" protons in 8mers-AT, -AM and -AP, and showed that all three had features consistent with a regular B-DNA duplex structure. Duplex-to-coil transition temperatures were determined to be 52(+/- 2) degrees C (8mer-AT), 51(+/- 2) degrees C (8mer-AP), 32(+/- 2) degrees C (8mer-AM); on the chemical shift timescale, the melting transition was fast for 8mer-AT and 8mer-AP, but slow for 8mer-AM. Imino proton spectra were indicative of Watson-Crick base-pairing in 8mers-AT, -AP and -AM. The 8mer-AP duplex had a structure and melting characteristics virtually identical with those of the 8mer-AT duplex. The preferred syn configuration of the methoxyl group in M had a destabilising effect on the 8mer-AM duplex. At low temperatures, the A.M base-pair was in fast equilibrium between Watson-Crick and wobble configurations, with the methoxyl function anti-oriented, but the melting transition was accompanied by isomerization of the methoxyl group to the syn conformation. This syn-anti isomerization was the rate-determining step in the duplex-to-coil transition. The 8mer-AC oligomer did not form a stable duplex.
为了更详细地了解甲氧基胺、N4-甲氧基胞苷及其2'-脱氧核糖类似物的诱变作用机制，通过1H核磁共振光谱研究了自互补八聚体d(CGAATTCG)及其类似物d(CGAATCCG)、d(CGAATMCG)和d(CGAATPCG)（分别命名为8mer-AT、8mer-AC、8mer-AM和8mer-AP）的溶液结构；M为N4-甲氧基胞嘧啶（mo4C），P为一种类似物，即双环二氢嘧啶并[4,5-c][1,2]恶嗪-7-酮，其中N-O键相对于胞嘧啶环的N3处于反式构型。相关光谱和核Overhauser光谱确定了8mer-AT、-AM和-AP中碱基、异头质子和H2'/H2"质子的归属，并表明这三种结构均具有与规则B-DNA双链结构一致的特征。双链到单链的转变温度分别为52（±2）℃（8mer-AT）、51（±2）℃（8mer-AP）、32（±2）℃（8mer-AM）；在化学位移时间尺度上，8mer-AT和8mer-AP的熔解转变很快，但8mer-AM的熔解转变很慢。亚氨基质子光谱表明8mer-AT、-AP和-AM中存在沃森-克里克碱基配对。8mer-AP双链的结构和熔解特性与8mer-AT双链几乎相同。M中甲氧基的优选顺式构型对8mer-AM双链有去稳定作用。在低温下，A.M碱基对在沃森-克里克构型和摆动构型之间快速平衡，甲氧基功能为反式取向，但熔解转变伴随着甲氧基异构化为顺式构象。这种顺-反异构化是双链到单链转变的速率决定步骤。8mer-AC寡聚物未形成稳定的双链。
