Study of iron porphyrins

A basic tenet of bioinorganic chemistry is that the structure and function of large biomolecules may be mimicked using simpler inorganic complexes to model the active sites. Obviously to fully understand the mechanisms of heme protein function, a study of iron porphyrins must be undertaken in which the characteristics of the metal (spin state, oxidation state and coordination number) and the steric and electronic effects of the porphyrin and other ligands are systematically varied. 

Choi, I.-K., Y. Liu, D. Feng, K.-J. Paeng, and M.D. Ryan (1991). Electrochemical and spectroscopic studies of iron porphyrin nitrosyls and their reduction products. Irwrg. Chem. 30, 1832-1839. 

Much of the mechanistic information available on olefin epoxidation with 0 has been obtained from studies of iron porphyrin complexes as... 

The initial application of EXAFS spectroscopy to thiolate-hgated heme systems was a combined study of iron porphyrin model complexes, ferric cytochrome P-450, and ferric chloroperoxidase by Cramer et al. [109]. This study reported the development of methods of EXAFS spectral analysis for iron porphyrin model complexes. Analysis of the EXAFS spectral data for a variety of structurally defined model iron porphyrins demonstrated that a fit of the EXAFS spectra using 3 waves (Fe-Np, Fe-Ca, and Fe-X J led to determination of axial Fe-X distances with an accuracy of better than 0.025 A. Fe-Np bond distances were determined with even greater accuracy. 

H NMR spectroscopy studies of iron(IIl) a-alkyl and o-aryl porphyrins have been very important in elucidating spin states. Alkyl and most aryl complexes with simple porphyrin ligands (OEP, TPP, or TTP) are low spin, S — I /2 species. NMR spectra for the tetraarylporphyrin derivatives show upheld resonances for the porphyrin pyrrole protons (ca. — 18 to —35 ppm), and alternating upfield and downfield hyperfine shifts for the axial alkyl or aryl resonances. For -alkyl complexes, the a-protons show dramatic downfield shifts (to ca. 600 ppm), upfield shifts for the /3-protons (—25 to — 160 ppm) and downfield shifts for the y-protons (12 ppm). The cr-protons of alkyliron porphyrins are not usually detected as a result of their large downfield shift and broad resonance. These protons were first detected by deuterium NMR in the dcuterated complexes Fe(TPP)CD3 (532 ppm) and Fe(TPP)CD2CDi (562, -117 ppm). ... 

While it is apparent that there are several pathways for relaxation of the excited state of iron porphyrins, the exact steps involved in these different pathways are still to a great extent unknown. It is the purpose of this study to explore the role of spin state and axial ligands in the decay process of the excited states of iron porphyrins. 

There have been very few reports of the Raman spectra of spin-equilibrium complexes. In one experiment the presence of both high-spin and low-spin isomers of an iron(II) Schiff base complex was observed by the resonance Raman spectra of the imine region (11). The temperature dependence of the spectra was recorded for both solid and solution samples. Recently differences were described in the resonance Raman spectra of four- and six-coordinate nickel(II) porphyrin complexes which undergo coordination-spin equilibria. These studies are extensions of a considerable literature on spin state effects on the Raman spectra of iron porphyrins and hemes. There are apparently no reports of attempts to use time-resolved Raman spectra for dynamics experiments. 

Studies with synthetic P450 enzyme models have already been mentioned to contribute to the understanding of the mechanisms of P450 action and to provide information on the possible structure and electronic nature of iron porphyrins as intermediates of the catalytic cycle. 

Extensive RR studies on iron porphyrins and heme proteins (9) have shown that all porphyrin core frequencies give negative linear correlations with the core size of the Fe center, and the slopes of these correlations are roughly... 

Rovira C, Kune K, Hutter J et al (1997) Equilibrium geometries and electronic structure of iron-porphyrin complexes a density functional study. J Phys Chem A 101 8914—8925... 

Magnetic circular dichroism (MCD) is a technique for studying magnetic-field induced perturbations in the energy levels of a system. It provides additional information about the nature of the energy levels that cause a transition in optical spectroscopy. Recent reviews of iron porphyrin MCD have been written by Dawson and Dooley [10] and Cheesman et al. [29]. 

The rich low-valent chemistry of iron porphyrins has been studied over many years. For example, [Fc°TPP]2 generated electrochemically reacts with R4N+ electrolyte cation to give [RFe TPP]. 79 The reactions of reduced porphyrins have also been studied by DVCA 80 The oxidation of iron(III) complexes of novel pentadentate pendant macrocyclic ligands were investigated and shown to polymerize in the presence of superoxide (02-).81... 

As a possible in-vitro model for one-electron transfer in photosynthesis, the photochemical reaction between hemin and Chla in pyridine solution has been studied, and it has been shown that the relatively slow reduction and oxidation of iron porphyrins can be accelerated by the presence of Chla by an order of magnitude and that light further increases the rate or reaction. Chla probably forms a rather stable complex with hemin, as is shown by fluorescence quenching experiments [Brody (17)]. 

Wayland, B. B., and Olson, L. W. (1974). Spectroscopic studies and bonding model for nitric oxide complexes of iron porphyrins. /. Am. Chem. Soc. 96, 6037-6041. 

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