磷酸基团与二价反离子的31P-NMR化学位移的分子起源
Molecular origin of 31P-NMR chemical shifts of phosphate groups with bivalent counter ions
AI总结：
研究揭示了磷酸基团与二价反离子相互作用对31P-NMR化学位移的影响机制，通过理论与实验结合的方法，阐明了接触离子对形成对σiso(31P)的平均位移及温度依赖性。
摘要：
磷酸基团与反离子的静电相互作用对DNA或RNA的结构、功能和反应性具有关键影响。我们对水溶液中的二甲基磷酸盐（DMP-）进行了联合实验-理论研究，这是糖磷酸骨架的已知模型系统。利用31P-NMR光谱作为磷酸-离子结合的探针，Mg²+和Ca²+含量的变化表现出对31P化学位移（δiso(31P)）的系统屏蔽作用，具有适度的温度依赖性。使用增强采样分子动力学（MD）和从头计算（GIAO-DF-LMP2）水平的理论来揭示微观机制。对DMP-离子几何构型的配位簇及其第一水合壳层进行了模拟，证明了（i）核屏蔽常数σiso(31P)变化的空间收敛性，（ii）分子内几何起源的短时间尺度σiso(31P)波动，以及（iii）与Mg²+或Ca²+离子形成接触离子对时σiso(31P)的平均位移约为3-5ppm。对不同离子含量和温度下δiso(31P)的定量分析使我们能够提取温度依赖的接触离子对物种分数，表明溶剂分离或自由离子对是能量更优的物种。这些结果为改进磷酸离子力场设定了边界条件，并建立了导致δiso(31P)变化的相互作用基础。
英文摘要：
The electrostatic interactions of phosphate groups and counter ions critically affect the structure, function and reactivity of DNA or RNA. We present a joint experimental-theoretical investigation of dimethyl phosphate (DMP-) in aqueous solution, an established model system of the sugar-phosphate backbone. Utilizing 31P-NMR spectroscopy as probe of phosphate-ion association, variations of Mg2+ and Ca2+ content exhibit a systematic shielding of the 31P chemical shift (δiso(31P)) with moderate temperature dependence. Enhanced sampling molecular dynamics (MD) and ab initio (GIAO-DF-LMP2) level of theory are used to reveal the microscopic mechanism. Simulations are performed for a configurational ensemble of DMP-ion geometries and their first solvation shells, demonstrating (i) the spatial convergence of changes of the nuclear shielding constant σiso(31P), (ii) the intramolecular geometric origin of short-timescale σiso(31P) fluctuations and (iii) an average shift of σiso(31P) of about 3-5 ppm upon contact ion pair formation with Mg2+ or Ca2+ ions. A quantitative analysis of δiso(31P) for varying ion content and temperature allows us to extract the temperature-dependent fraction of the contact ion pair species, indicating that solvent separated or free ion pairs are the energetically preferred species. The results impose boundary conditions for improvements of phosphate ion force fields and establish the interactions underlying the changes of δiso(31P).
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