Synthesis and solid-state molecular structures of nitrosoalkane complexes of iron porphyrins containing methanol, pyridine, and 1-methylimidazole ligands

https://doi.org/10.1016/j.jinorgbio.2004.04.008Get rights and content

Abstract

Nitrosoalkanes belong to the family of C-nitroso compounds and are known to bind to the iron center in heme proteins. We have prepared and characterized a series of new nitrosoalkane heme model complexes of the form (por)Fe(RNO)(L) (por=porphyrinato dianion; R=isopropyl; L=MeOH, pyridine, 1-methylimidazole) by infrared and 1H NMR spectroscopy and X-ray crystallography. Within the set of octaethylporphyrinato (OEP) compounds, the infrared stretching frequencies of the NO groups decrease in the order (OEP)Fe(iPrNO)(MeOH) · MeOH (1433 cm−1) > (OEP)Fe(iPrNO)(py) (1429 cm−1) > (OEP)Fe(iPrNO)(1-MeIm) (1423 cm−1), reflecting the increased backdonation of electron density in the 1-methylimidazole derivative. The molecular structures of the compounds as determined by crystallography reveal N-binding of the nitrosoalkane ligands to the formally ferrous metal centers.

Introduction

The nitroso group of nitrosoalkanes (RNO; R=alkyl) is isoelectronic with dioxygen. Nitrosoalkanes belong to the general class of C-nitroso compounds, and the binding of nitrosoalkanes and/or nitrosoarenes to heme iron has been demonstrated in heme proteins such as myoglobin (Mb), hemoglobin (Hb), leghemoglobin, soluble guanylyl cyclase (sGC), NO synthase, catalase, prostaglandin H synthase, microperoxidase, and cytochrome P450 [1]. Importantly, heme–RNO bond formation has been implicated in the severe inhibition of the catalytic functions of hepatic microsomal cytochrome P450. Such heme–RNO compounds can be produced from the oxidative metabolism of amine-containing drugs or by reduction of nitroorganics.

Excellent work by Mansuy and coworkers [2], [3] has shown that nitrosoalkane complexes of Mb can be prepared from treatment of met Mb with nitroalkanes in the presence of dithionite. Related nitrosoarene Mb complexes have been reported [4], [5], [6]. We recently determined the molecular structure of Mb(EtNO) by X-ray crystallography [7]. We showed that the nitrosoethane ligand was attached to the iron center through its N-atom (Fig. 1), although distal pocket residues also helped orient the nitrosoethane ligand to its final orientation with respect to the normal to the heme plane via a hydrogen bond between the nitroso O-atom and the distal His64 residue. This was the first reported structure of a nitrosoalkane complex of a heme protein. Also, it is surprising that to date there is only one reported crystal structure of a nitrosoalkane complex of a heme model, namely that of (TPP)Fe(iPrNO)(iPrNH2) which contains a trans amine ligand [8].

We have characterized by X-ray crystallography a series of new iron porphyrin complexes of the form (por)Fe(iPrNO)(L) (L=MeOH, py, 1-MeIm). The latter imidazole complexes more clearly represent the axial coordination by histidine in biomolecules such as Mb, Hb, and sGC.

Section snippets

Experimental

All reactions were performed under an atmosphere of pre-purified nitrogen using standard Schlenk glassware and/or in an Innovative Technology Labmaster 100 Dry Box. Solutions for spectral studies were also prepared under a nitrogen atmosphere. Solvents were distilled from appropriate drying agents under nitrogen just prior to use: CH2Cl2 and hexane (CaH2).

Syntheses

Mansuy and coworkers [8] previously reported that nitrosoalkane complexes (por)Fe(RNO)(L) (L=Lewis base) formed from the reaction of the ferric porphyrin precursors (por)FeCl with alkylhydroxylamines (RNHOH) in the presence of excess L. In these studies, it was proposed that RNHOH reduces the ferric porphyrin to ferrous (the two-electron oxidation of RNHOH then gives RNO). We have employed this methodology to synthesize several new nitrosopropane complexes of iron porphyrins containing

Conclusion

In summary, we have prepared several new nitrosoalkane compounds of iron porphyrins and we have shown by spectroscopy and by X-ray crystallography that trans axial ligands have an effect on axial ligand orientations and the extent of π-backbonding to the RNO ligands. Combined with our recent crystallographic results for Mb(EtNO), we are now in a position to examine electronic and distal pocket influences on Fe–RNO geometry and reactivity in a series of non-protein and protein environments.

Supplementary material

CCDC 231891–231895 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Abbreviations

    por

    porphyrinato dianion

    TPP

    5,10,15,20-tetraphenylporphyrinato dianion

    TTP

    5,10,15,20-tetra-p-tolylporphyrinato dianion

    OEP

    octaethylporphyrinato dianion

Acknowledgements

We thank the National Institutes of Health (Grant No. GM 64476) for financial support of this work. C.D.S. is grateful to the University of Oklahoma Undergraduate Research Opportunities Program for a research award.

References (29)

  • J. Lee et al.

    Chem. Rev.

    (2002)
  • D. Mansuy et al.

    J. Am. Chem. Soc.

    (1977)
  • D. Mansuy et al.

    Eur. J. Biochem.

    (1977)
  • Q.H. Gibson

    Biochem. J.

    (1960)
  • D. Mansuy et al.

    Eur. J. Biochem.

    (1978)
  • J.M. Fukuto et al.

    Biochemistry

    (1986)
  • D.M. Copeland et al.

    Proteins: Struct. Func. Genet.

    (2003)
  • D. Mansuy et al.

    J. Am. Chem. Soc.

    (1983)
  • A.I. Ryer et al.

    J. Am. Chem. Soc.

    (1951)
  • K.M. Kadish et al.

    Electrochemistry of metalloporphyrins in nonaqueous media

  • J.W. Buchler et al.

    Struct. Bonding

    (1978)
  • N. Godbout et al.

    J. Am. Chem. Soc.

    (1999)
  • L.-S. Wang et al.

    Chem. Commun.

    (1996)
  • L. Cheng et al.

    Inorg. Chem.

    (2001)

    • Cited by (0)

    View full text
    Copyright © 2004 Elsevier Inc. All rights reserved.