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Alkali metals (Li, Na, and K)in methyl phosphodiester hydrolysis

机译：甲基磷酸二酯水解中的碱金属（Li，Na和K）

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The phosphodiester linkage central to biological systems has been modeled by methyl phosphodiester (MPDE) in various theoretical and experimental studies. Under physiological conditions, hydrolysis of the phosphodiester is negligible, however this process can be catalyzed in the presence of metal ions. To understand the role of alkali metals in MPDE hydrolysis and, in particular, how it influences the reaction pathway and the associated energetics, density functional calculations employing the 6-31 + G(d,p) basis set have been carried out. Different pathways that include the reactant, intermediates and the products have been investigated for MPDE hydrolysis catalyzed by one or two lithium ions, characterized as stationary point geometries on the potential energy surface. The pathways A and B incorporate a single lithium ion bonded to different oxygens of the diester functionality. In pathway C, a six-membered ring was noticed wherein the nucleophile bridges two lithium ions interacting with different oxygens of the phosphoryl group. Furthermore, in the pathway (D) incorporating two lithium ions, one of the lithium ions interacts with the hydroxyl group and another with the methoxy oxygen; both metal ions are coordinated by the same phosphoryl oxygen. In addition to this, yet another pathway (E), where the metal ions are bound to different oxygens of the phosphoryl group, has also been dealt with. The calculations have shown that the A and B pathways lead to a single step reaction. A three-step mechanism including the nucleophilic (hydroxyl) attack, rotation of a methyl group and, finally, departure of the methoxy group has been predicted for the D and E profiles. Both D and E pathways are favored equally (with a marginal difference of 0.3 kJ mol~(-1) in their activation energies) in the gas phase and a transition state corresponding to nucleophilic attack with an energy barrier of 32.5 kJ mol~(-1) was located when hthium was used. A penta-coordinated phosphorous intermediate on the potential energy surface was characterized along these pathways. MPDE hydrolysis yielded a lower energy barrier for lithium than those for the remaining alkali metal ions. This agrees well with the experimentally observed trend for the hydrolysis rates: Li > Na > K. Self consistent reaction field (SCRF) calculations reveal the lower energy barrier between the reactant and the transition state for the nucleophilic attack in nonpolar solvents. The extent of bond formation'(or cleavage) in different stationary point structures along the reaction path as estimated from the electron density at the bond critical point in the molecular electron density topography, has proven useful in distinguishing the associative or dissociative reaction pathways.
机译：在各种理论和实验研究中，已通过甲基磷酸二酯（MPDE）对生物系统中心的磷酸二酯键进行了建模。在生理条件下，磷酸二酯的水解可以忽略不计，但是该过程可以在金属离子存在下催化。为了了解碱金属在MPDE水解中的作用，特别是它如何影响反应路径和相关的能级，已进行了使用6-31 + G（d，p）基集的密度泛函计算。对于一种或两种锂离子催化的MPDE水解，已研究了包括反应物，中间体和产物的不同途径，其特征为势能表面上的固定点几何形状。途径A和B掺入结合到二酯官能度的不同氧上的单个锂离子。在途径C中，注意到六元环，其中亲核试剂桥接两个与磷酰基的不同氧相互作用的锂离子。此外，在包含两个锂离子的途径（D）中，一个锂离子与羟基相互作用，另一个与甲氧基氧相互作用;两种金属离子均由相同的磷酰氧配位。除此之外，还处理了另一种途径（E），其中金属离子与磷酰基的不同氧键合。计算表明，A和B途径导致一步反应。对于D和E图谱，预测了包括亲核（羟基）攻击，甲基旋转以及最后甲氧基离开的三步机制。气相中D和E途径均受到同等偏爱（其活化能的边际差异为0.3 kJ mol〜（-1）），并且对应于亲核攻击的过渡态的能垒为32.5 kJ mol〜（-） 1）在使用h时定位。沿着这些途径表征了势能表面上的五配位磷中间体。 MPDE水解对锂的能垒比对其余碱金属离子的能垒低。这与实验观察到的水解速率趋势非常吻合：Li> Na>K。自洽反应场（SCRF）计算显示，在非极性溶剂中，亲核进攻的反应物和过渡态之间的能垒较低。由分子电子密度拓扑图中的键临界点处的电子密度估计，沿反应路径在不同固定点结构中的键形成程度（或裂解）已被证明可用于区分缔合或解离反应途径。

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