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首页> 外文期刊>Chemistry: A European journal >Reaction Mechanism of Porphyrin Metallation Studied by Theoretical Methods
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Reaction Mechanism of Porphyrin Metallation Studied by Theoretical Methods

机译：理论方法研究卟啉金属化反应机理

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We have studied the reaction mechanism for the insertion of Mg~(2+) and Fe~(2+) into a porphyrin ring with density functional calculations with large basis set and including solvation,zero-point and thermal effects.We have followed the reaction from the outer-sphere complex,in which the metal is coordinated with six water molecules and the porphyrin is doubly protonated,until the metal ion is inserted into the deprotonated porphyrin ring with only one water ligand remaining.This reaction involves the stepwise displacement of five water molecules and the removal of two protons from the porphyrin ring.In addition,a step seems to be necessary in which a porphyrin pyrrolenine nitrogen atom changes its interaction from a hydrogen bond to a metal-bound solvent molecule to a direct coordination to the metal ion.If the protons are taken up by a neutral imidazole molecule,the deprotonation reactions are exothermic with minimal barriers.However,with a water molecule as an acceptor,they are endothermic.The ligand exchange reactions were approximately thermo-neutral (+-20 kJmol~(-1),with one exception) with barriers of up to 72 kJ mol~(-1) for Mg and 51 kJ mol~(-1) for Fe.For Mg,the highest barrier was found for the formation of the first bond to the porphyrin ring.For Fe,a higher barrier was found for the formation of the second bond to the porphyrin ring,but this barrier is probably lower in solution.No evidence was found for an initial pre-equilibrium between a planar and a distorted porphyrin ring.Instead,the porphyrin becomes more and more distorted as the number of metal-porphyrin bonds increase (by up to 191 kJ mol~(-1)).This strain is released when the porphyrin becomes deprotonated and the metal moves into the ring plane.Implications of these findings for the chelatase enzymes are discussed.
机译：我们利用密度泛函计算（包括溶剂化，零位和热效应），通过大的密度泛函计算研究了Mg〜（2+）和Fe〜（2+）插入卟啉环的反应机理。来自外层络合物的反应，其中金属与六个水分子配位并且卟啉被双重质子化，直到金属离子被插入到去质子化的卟啉环中而只剩下一个水配体。五个水分子并从卟啉环上除去两个质子。此外，似乎有必要采取以下步骤，使卟啉吡咯啉氮原子从氢键变为与金属结合的溶剂分子，从而将其相互作用改变为与氢键的直接配位。如果质子被中性的咪唑分子吸收，则去质子反应是放热的，具有最小的障碍。但是，以水分子为受体的内质子配体交换反应约为热中性（+ -20 kJmol〜（-1），一个例外），对于Mg和51 kJ mol〜（-1）的势垒分别高达72 kJ mol〜（-1）。对于Fe，对于Mg，发现与卟啉环的第一键形成的最高障碍。对于Fe，对于Porphyrin环的第二键形成的阻挡层较高，但该障碍可能较低。没有发现关于平面和扭曲的卟啉环之间的初始预平衡的证据，相反，随着金属-卟啉键数的增加（高达191 kJ mol〜（-1），卟啉变得越来越失真。 ））。当卟啉去质子化并且金属移入环平面时，该菌株释放。讨论了这些发现对螯合酶的意义。

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