化学位移估算研究ATP构象随溶液pH值的变化

利用Johnson和Bovey的理论和方法计算了不同扭曲角χ(O4′－C1′－N9－C4)的ATP(5′-三磷酸腺苷)分子中糖环质子H1′和H2′由于环流效应引起的化学位移.H1′的化学位移与扭曲角χ有较强的依赖关系,反映了ATP在溶液中细微的构象变化.将计算结果与实验结果比较,证明在本文讨论的pH值范围(1～10)内,Mg2＋加入后,ATP的扭曲角χ在230～360°范围内变化.随溶液的pH值减小,ATP分子的构象由trans 构象通过-gauche构象转变为cis构象. 从而证明在酸性条件下, ATP倾向于以cis构象存在,而在碱性条件下trans构象更为稳定,从另一方面支持了在酸性条件下N1参与配位而在碱性条件下N7参与配位的结论.在讨论中也考虑了由pH变化所引起的环流强度的变化.

pH Value Dependent Conformation of ATP Studied by Chemical Shift Calculation

The chemical shifts induced by the ring current was calculated for H1′ and H2′ of ATP with different torsion angle χ (O4′－C1′－N9－C4) based on the method of Johnson and Bovey. The chemical shift of H1′ showed relatively strong dependence on the χ angle, which reflected fine conformation changes with pH. By comparing the calculated results with the experimental data, it is shown that in the pH range (1～10) studied in this paper and in the presence of Mg2＋,the torsion angle χ of ATP varies in the small range from 230° to 360°.As the pH of the solution is decreased, the conformation of the ATP molecule changes from trans to cis through -gauche form. At lower pH, the ATP molecule is more likely to take the cis conformation, while at higher pH, the trans conformation is more stable. This supports the conclusion that N1 binds the metal ion at lower pH while the binding site becomes N7 when pH is neutral. In the discussion, the change of the ring current strength induced by pH variation was also taken into account.