Magnetic circular dichroism study of porphyrin π cation radical species

The Dimerisation of Phthalocyanines

In this review, we have shown that the dimerisation of phthalocyanine compounds, notably here the sulphonated aluminium phthalocyanines, is dependent upon concentration, on the medium in which the dye is dissolved, and upon pH. Complex equilibria between various monomer and dimer species are observed as a function of pH, and the probable structures of the dimers elucidated by semi-empirical and ab initio calculations. The formation of a red-shifted dimer leads to the quenching of monomer singlet state in concentrated solution, in reverse micelles, and in lipid vesicles, and this behaviour can account for the fluorescence intensity distributions and decay characteristics of phthalocyanine dyes in living cells as a function of irradiation time.

Charge Transfer of Tetraphenylporphyrin Zinc Complexes Incorporated in a Poly(2-vinylpyridine) Matrix

An indium tin oxide (ITO) electrode was coated by a poly(2-vinylpyridine) (P-2VP) film incorporating tetraphenylporphyrin zinc ( ZnTPP ) as a redox center. A cyclic voltammogram (CV) exhibited a couple of redox waves at 0.7 V (vs Ag / AgCl ). The potential-step chronoamperospectrometry (PSCAS) measurement was carried out on the modified electrode immersed in a 0.1 M LiClO 4 aqueous solution at pH 3.5 under argon atmosphere. Assuming a random distribution of the redox species in the macromolecular structure, the charge transfer distance between the adjacent ZnTPP complexes was obtained as 1.08 nm. The charge hopping distance between the redox centers (1.01 nm) and the bounded motion distance (0.07 nm after 2h) were obtained according to their different response time in the electrochemical measurement. Considering the contributions from both the charge hopping and bounded motion, the second order rate constant of electron hopping was 4.1 × 104 M−1s−1.

Symmetry states of metalloporphyrin π-cation radicals, models for peroxidase compound I

Oxoferryl porphyrin π-cation radical active sites of compound I intermediates which are found in enzymes such as peroxidases and catalases have been extensively modeled by oxidized synthetic metalloporphyrins. The electronic symmetry states of these compounds were initially assigned on the basis of electronic absorption data. In recent years new experimental and theoretical results have become available which have led to a re-evaluation and modification of the original assignments. A historical perspective of these developments is provided in the context of recent NMR, resonance Raman, and other spectroscopic data and theoretical calculations for the synthetic models and enzymatic systems.

Theoretical Analysis of the Structural and Electronic Properties of Metalloporphyrin π-Cation Radicals

A method for analyzing the A(1u)/A(2u) contents of metalloporphyrin pi-cation radicals is developed and applied to a series of unsubstituted planar metalloporphines (MPs) (M=Cr, Mn, Fe, Co, Ni, Cu, and Zn). The structures and electronic properties of the MPs and their cation radicals were calculated by density functional theory (DFT) and subsequently analyzed. It was found that the MPs with small core sizes have a tendency to form A(1u)-type radicals, while the MPs with large core size have a preference for an A(2u)-type. Neither of these pure-state species, however, is stable under the D(4)(h) symmetry, and both radical cation types are subject to pseudo-Jahn-Teller (pJT) distortion. The pJT distortion leads to structures with lower symmetry and states that have mixed character with respect to the A(1u) and A(2u) components. The degree of mixing could be estimated by employing orbital projection technique or a complementary spin density decomposition. Both techniques produce very similar results, pointing out that the frontier orbital, which becomes empty upon electron removal, plays a critical role in determining electronic properties.

Application of MCD spectroscopy and TD-DFT to a highly non-planar porphyrinoid ring system. New insights on red-shifted porphyrinoid spectral bands

The first magnetic circular dichroism (MCD) spectra are reported for tetraphenyltetraacenaphthoporphyrin (TPTANP). The impact on the electronic structure of steric interactions between the fused acenaphthalene rings and the meso-tetraphenyl substituents is explored based on an analysis of the optical spectra of the Zn(II) complex (ZnTPTANP) and the free base dication species ([H4TPTANP]2+). In the case of ZnTPTANP, significant folding of the porphyrinoid ligand induces a highly unusual MCD-sign reversal providing the first direct spectroscopic evidence of ligand nonplanarity. Density functional theory (DFT) geometry optimizations for a wide range of Zn(II) porphyrinoids based on the B3LYP functional and TD-DFT calculations of the associated UV-visible absorption spectra are reported, allowing a complete assessment of the MCD data. TPTANP complexes are found to fall into a class of cyclic polyenes, termed as soft MCD chromophores by Michl (J. Pure Appl. Chem. 1980, 52, 1549.), since the signs of the Faraday A1 terms observed in the MCD spectrum are highly sensitive to slight structural changes. The origin of an unusually large red shift of the main B (or Soret) band of MTPTANP (the most red shifted ever reported for fused-ring-expanded metal porphines) and of similar red shifts observed in the spectra of other peripherally crowded porphyrinoid complexes is also explored and explained on this basis.

A spectroscopic study of rat liver and Scylla serrata crab metallothioneins

The absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of native rat liver and crab (Scylla serrata) Cd,Zn-metallothionein have been measured and the data are compared. The MCD data indicate that there are close similarities in the geometries of the cadmium-binding sites in both of these proteins; however, the CD spectra are quite different for the rat liver and crab proteins. The CD spectrum for the crab metallothionein is unlike any previously reported for a cadmium-containing metallothionein. This suggests that the CD spectrum is sensitive to the different bridging pattern used in the binding sites in the crab compared with the rat-liver metallothionein. Cadmium binding to the metal-free metallothionein is demonstrated for both proteins and it is shown that there are only minor structural differences between the native and remetallated proteins. The structural changes that occur near to the cadmium-binding sites during cadmium loading to the native proteins have been followed using absorption and CD spectroscopy. Marked changes are observed in the CD spectrum which can be associated with a two-phase reaction: initially Zn2+ is displaced by the Cd2+, then at higher concentrations of Cd2+ the tetrahedral geometry of the Cd2+-binding sites is lost as more Cd2+ is bound using the same thiolate groups. While this latter reaction results in considerable change to the CD spectrum, only minor changes are observed in the absorption spectrum. A significant red shift is observed in the S leads to Cd charge transfer transition region of the MCD spectrum (230-270 nm) following both cadmium loading of native rat liver, Cd,Zn-metallothionein and the metallation of metal-free metallothionein with cadmium. There are two contributions to this effect in Cd,Zn-metallothionein: (i) there is a S leads to Zn band underlying the S leads to Cd band; and (ii) the occupation of zinc sites by cadmium changes the energy of the S leads to Cd transition.

The temperature dependence of the MCD spectrum of horseradish peroxidase compound I

The magnetic circular dichroism spectrum of the compound I species of horseradish peroxidase, which contains an iron (IV) porphyrin pi-cation radical complex, has been measured between 273 K and 4.2 K. The spectrum is temperature independent between 273 K and 30 K. However, very strong temperature dependence is observed below 30 K. These data do not appear to fit the temperature dependence expected for the presence of a simple MCD C term, or combination of C terms, but suggest that an increase in the coupling between the S = 1 iron (IV), and the S = 1/2 porphyrin pi-cation radical occurs forming a degenerate ground state. This increase in coupling below 30 K may be the result of a phase change in the protein which in turn affects the electronic structure of the heme group.

Evidence for heme pi cation radical species in compound I of horseradish peroxidase and catalase

Magnetic circular dichroism spectra are reported for the compound I species of beef liver catalase (hydrogen-peroxide: hydrogen-peroxide oxidoreductase, EC 1.11.1.6) and horseradish peroxidase (donor: hydrogen-peroxide oxidoreductase, EC 1.11.1.7) and the pi cation radical derivatives of porphyrins that have been suggested as models of the electronic configuration of the heme in the compound I species of these enzymes. Comparison of the magnetic circular dichroism spectra of the compound I species with the spectra of [Co(octaethylporphyrin)]2Br and [Co(octaethylporphyrin)]2ClO4 indicates that in both the intermediate enzyme species the heme has been oxidized to a pi cation radical. While there is a clear distinction between the magnetic circular dichroism spectra of the 2A2u porphyrin [Co(III)octaethylporphyrin]2ClO4, and the 2A1u porphyrin, [Co(III)octaethylporphyrin]2Br, such specific differences are not observed in the spectra of the two enzymes. Analysis of our data suggests that the ground states in the two enzymes are far more similar than the ground states in the two model porphyrins.