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The structure of sitting-atop complexes of metalloporphyrins studied by theoretical methods

Shen, Yong LU and Ryde, Ulf LU (2004) In Journal of Inorganic Biochemistry 98(5). p.878-895
Abstract
The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are still protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and extended X-ray absorption fine structure (EXAFS) data has been reported for Cu2+ in acetonitrile. We have used density functional calculations to obtain reasonable models for SAT complexes of porphyrins with Mg2+, Fe2+, and Cu2+. The results show that there are many possible SAT complexes with 1-5 solvent molecules, one or two metal ions, and cis or trans protonation of the porphyrin ring. Many of these have similar energies and their relative stabilities vary with the... (More)
The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are still protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and extended X-ray absorption fine structure (EXAFS) data has been reported for Cu2+ in acetonitrile. We have used density functional calculations to obtain reasonable models for SAT complexes of porphyrins with Mg2+, Fe2+, and Cu2+. The results show that there are many possible SAT complexes with 1-5 solvent molecules, one or two metal ions, and cis or trans protonation of the porphyrin ring. Many of these have similar energies and their relative stabilities vary with the metal ion. A complex with two cis pyrrolenine nitrogens atoms and 2-4 solvent molecules coordinated to Cu2+ fits the NMR and EXAFS data best. However, we cannot fully exclude the possibility that what is observed is rather a mixture of a doubly protonated porphyrin and the copper porphyrin. Mg2+ has a lower affinity for porphyrin and stronger affinity for water, so a complex with five water molecules and only one bond to porphyrin seems to be most stable. For Fe2+, a cis structure with two first-sphere water molecules and four interactions to the porphyrin seems to be most likely. (C) 2004 Elsevier Inc. All rights reserved. (Less)
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author
organization
publishing date
type
Contribution to journal
publication status
published
subject
keywords
Sitting-atop complex, Haem, Chlorophyll, Porphyrin distortion, Ligand exchange, Density functional theory
in
Journal of Inorganic Biochemistry
volume
98
issue
5
pages
878 - 895
publisher
Elsevier
external identifiers
  • wos:000221678300025
  • pmid:15134934
  • scopus:2342556417
ISSN
1873-3344
DOI
10.1016/j.jinorgbio.2004.01.004
language
English
LU publication?
yes
id
8a032a0b-47f0-47d4-bd4b-942044f05afd (old id 139663)
date added to LUP
2007-07-09 14:15:12
date last changed
2017-07-30 04:33:37
@article{8a032a0b-47f0-47d4-bd4b-942044f05afd,
  abstract     = {The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are still protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and extended X-ray absorption fine structure (EXAFS) data has been reported for Cu2+ in acetonitrile. We have used density functional calculations to obtain reasonable models for SAT complexes of porphyrins with Mg2+, Fe2+, and Cu2+. The results show that there are many possible SAT complexes with 1-5 solvent molecules, one or two metal ions, and cis or trans protonation of the porphyrin ring. Many of these have similar energies and their relative stabilities vary with the metal ion. A complex with two cis pyrrolenine nitrogens atoms and 2-4 solvent molecules coordinated to Cu2+ fits the NMR and EXAFS data best. However, we cannot fully exclude the possibility that what is observed is rather a mixture of a doubly protonated porphyrin and the copper porphyrin. Mg2+ has a lower affinity for porphyrin and stronger affinity for water, so a complex with five water molecules and only one bond to porphyrin seems to be most stable. For Fe2+, a cis structure with two first-sphere water molecules and four interactions to the porphyrin seems to be most likely. (C) 2004 Elsevier Inc. All rights reserved.},
  author       = {Shen, Yong and Ryde, Ulf},
  issn         = {1873-3344},
  keyword      = {Sitting-atop complex,Haem,Chlorophyll,Porphyrin distortion,Ligand exchange,Density functional theory},
  language     = {eng},
  number       = {5},
  pages        = {878--895},
  publisher    = {Elsevier},
  series       = {Journal of Inorganic Biochemistry},
  title        = {The structure of sitting-atop complexes of metalloporphyrins studied by theoretical methods},
  url          = {http://dx.doi.org/10.1016/j.jinorgbio.2004.01.004},
  volume       = {98},
  year         = {2004},
}