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Reaction mechanism of porphyrin metallation studied by theoretical methods

Shen, Yong LU and Ryde, Ulf LU (2005) In Chemistry: A European Journal 11(5). p.1549-1564
Abstract
We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ 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... (More)
We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ 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 thermoneutral ( +/- 20 kJ mol(-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. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
in
Chemistry: A European Journal
volume
11
issue
5
pages
1549 - 1564
publisher
John Wiley & Sons
external identifiers
  • pmid:15662683
  • wos:000227262000019
  • scopus:14844323140
ISSN
1521-3765
DOI
10.1002/chem.200400298
language
English
LU publication?
yes
id
084faa45-fe15-44b1-88ce-86b0d0d1f40c (old id 152739)
date added to LUP
2007-07-17 13:56:55
date last changed
2017-07-30 04:37:34
@article{084faa45-fe15-44b1-88ce-86b0d0d1f40c,
  abstract     = {We have studied the reaction mechanism for the insertion of Mg2+ and Fe2+ 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 thermoneutral ( +/- 20 kJ mol(-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.},
  author       = {Shen, Yong and Ryde, Ulf},
  issn         = {1521-3765},
  language     = {eng},
  number       = {5},
  pages        = {1549--1564},
  publisher    = {John Wiley & Sons},
  series       = {Chemistry: A European Journal},
  title        = {Reaction mechanism of porphyrin metallation studied by theoretical methods},
  url          = {http://dx.doi.org/10.1002/chem.200400298},
  volume       = {11},
  year         = {2005},
}