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

Shen, YongLund University,Lunds universitet,Kemiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Department of Chemistry,Departments at LTH,Faculty of Engineering, LTH (author)

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.

NATURVETENSKAP Biologi Biokemi och molekylärbiologi hsv//swe
NATURAL SCIENCES Biological Sciences Biochemistry and Molecular Biology hsv//eng
Sitting-atop complex
Haem
Chlorophyll
Porphyrin distortion
Ligand exchange
Density functional theory

Ryde, UlfLund University,Lunds universitet,Beräkningskemi,Enheten för fysikalisk och teoretisk kemi,Kemiska institutionen,Institutioner vid LTH,Lunds Tekniska Högskola,Computational Chemistry,Physical and theoretical chemistry,Department of Chemistry,Departments at LTH,Faculty of Engineering, LTH(Swepub:lu)teok-ury (author)
Kemiska institutionenInstitutioner vid LTH (creator_code:org_t)

In:Journal of Inorganic Biochemistry: Elsevier BV98:5, s. 878-8951873-33440162-0134

About the subject

NATURAL SCIENCES: NATURAL SCIENCES; and Biological Scien ...; and Biochemistry and ...

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