Porphyrins magnetic circular dichroism

Compounds 1,2,3,5,10,11,12,13,14 were dissolved in EPIP (diethyl ether, petroleum ether, isopropanol 5 5 2)whereas compounds 4,6,7,8,9,15 were dissolved in THF-DE (tetrahydrofurane, diethyl ether 1 1). These solvent mixtures can be frozen as glassy samples at 77 K. The absorption spectra were recorded on a standard spectrophotometer SF-10 or Beckman-5270. The measurements of fluorescence excitation and emission spectra were made with the aid of a spectrofluorometer SLM-4800 with automatic correction of spectral response. Fluorescence lifetimes were measured with the aid of a pulse fluorometer PRA-3000. Magnetic circular dichroism (MCD) measurements were carried out in a 8 kG magnetic field using a JASCO J-20 circular dichrometer. Triplet state formation was observed for investigated compounds at the experimental set up, whose detailed description can be found in our paper (27). The optical experiments were carried out with a porphyrin concentration of 4.10- - 4.10 mol.l". In NMR investigations (Bruker WM-360) we used higher concentrations ( 5.10" raol.l ) and dried solvents (CDCl, C 2 and toluene-d0). 

Alternatively, the porphyrin ring can be constructed starting from [16]annulene. In the first step, two electrons are added to form the corresponding [16]annulene dianion, which is transformed into porphyrin Cl") by adding bridges and heteroatoms. Unlike the structure derived from [18]annulene, the dianion-based model has a fourfold symmetry, and was considered suitable for the description of metal complexes [23] (see Sect. 2.3.2). In yet another approach [24], based on the so-called perimeter model, the porphyrin macrocycle is derived from the [20]annulene dication (I "). Both the [16]- and [20]annulene models were employed to describe electronic absorption spectra and magnetic circular dichroism of porphyrinoids [24, 25],... 

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]. 

Ghosh et al. reported that the XPS (X-ray photoelectron spectra) of porphyrins distinguishes the unprotonated (N Is peak at 398.6 eV) from the protonated central nitrogens (N Is at 400.7 eV) in octakis(AUV-diethy lcar-boxamido)porphyrin (93JOC6932). The observation of a pair of MCD (magnetic circular dichroism) bands of opposite sign within the transition of monoacetylporphyrins is probably related to the existence of an equilibrium between tautomers (83TL2433). 

Optical rotatory dispersion and circular dichroism (83, 84) can often be of great value, and the spectra are particularly sensitive to the conformation of the protein. Much work remains to be done in this field before the results can be definitely interpreted in terms of the electronic structure of the metal. Magneto-optical rotation and magnetic circular dichroism (MOR and MCD), which are beginning to be applied to porphyrins and haemoproteins, offer much greater promise (30, 85). 

Iron-porphyrins, synthesis and coordination properties of 86PAC1493. Magnetic circular dichroism spectra of -substituted porphyrins 88-ACR95. 

Magnetic circular dichroism (MCD) is independent of, and thus complementary to, the natural CD associated with chirality of nuclear structure or solvation. Closely related to the Zeeman effect, MCD is most often associated with orbital and spin degeneracies in chromophores. Chemical applications are thus typically foimd in systems where a chromophore of high symmetry is present metal complexes, porphyrins and other aromatics, and haem proteins are... 

Mack, J. and Stillman, M.J. (2003) Electronic structure of metal phthalocyanine and porphyrin complexes from analysis of the UV-visible absorption and magnetic circular dichroism spectra and... 

M. Sono, A.M. Bracete, A.M. Huff, M. Ikeda-Saito, and J.H. Dawson, Evidence that a formyl-substituted iron porphyrin is the prosthetic group of myeloperoxidase magnetic circular dichroism similarity of the peroxidase to Spirographis heme-reconstituted myoglobin, Proc. Natl. Acad. Sci. USA 88 11148 (1991). 

Praneeth VK, Neese F, Lehnert N (2005) Spin density distribution in five- and six-coordinate iron (II)-porphyrin NO complexes evidenced by magnetic circular dichroism spectroscopy. Inorg Chem 44 2570-2572... 

Dawson JH and Dooley DM (1989) Magnetic circular dichroism spectroscopy of iron porphyrins and heme proteins. In Lever ABP and Gray, HB (eds) Iron Porphyrins, Part III, pp. 1-133. New York V. V. H. Publishers. 

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