A 4-term energy level scheme for the high-spin ferrous hemoproteins: evidence for the 5E eta, and 5B2 terms as the ground multiplets in hemoproteins with a histidine and a cysteine protein-derived heme ligand, respectively

Spectroscopic features of cytochrome P450 reaction intermediates

Cytochromes P450 constitute a broad class of heme monooxygenase enzymes with more than 11,500 isozymes which have been identified in organisms from all biological kingdoms [1]. These enzymes are responsible for catalyzing dozens chemical oxidative transformations such as hydroxylation, epoxidation, N-demethylation, etc., with very broad range of substrates [2,3]. Historically these enzymes received their name from 'pigment 450' due to the unusual position of the Soret band in UV-vis absorption spectra of the reduced CO-saturated state [4,5]. Despite detailed biochemical characterization of many isozymes, as well as later discoveries of other 'P450-like heme enzymes' such as nitric oxide synthase and chloroperoxidase, the phenomenological term 'cytochrome P450' is still commonly used as indicating an essential spectroscopic feature of the functionally active protein which is now known to be due to the presence of a thiolate ligand to the heme iron [6]. Heme proteins with an imidazole ligand such as myoglobin and hemoglobin as well as an inactive form of P450 are characterized by Soret maxima at 420nm [7]. This historical perspective highlights the importance of spectroscopic methods for biochemical studies in general, and especially for heme enzymes, where the presence of the heme iron and porphyrin macrocycle provides rich variety of specific spectroscopic markers available for monitoring chemical transformations and transitions between active intermediates of catalytic cycle.

Electronic Coupling between Heme Electron-Transfer Centers and Its Decay with Distance Depends Strongly on Relative Orientation

A method for calculating the electron-transfer matrix element V(RP) using density functional theory Kohn-Sham orbitals is presented and applied to heme dimers of varying relative orientation. The electronic coupling decays with increased iron separation according to V(RP) = V(0)(RP)exp(-beta r/2) with a distance dependence parameter beta approximately 2 A(-1) for hemes with parallel porphyrins and either 1.1 or 4.0 A(-1) when the porphyrin planes are perpendicular, depending on the alignment of the iron d(pi) orbital. These findings are used to interpret the observed orientation of the hemes in tetraheme redox proteins such as Flavocytochrome c(3) fumarate reductase (Ifc(3), PDB code 1QJD) of Shewanella frigidimarina, another flavocytochrome from the same bacterium (Fcc(3), 1E39) and a small tetraheme cytochrome of Shewanella oneidensis strain MR1 (1M1P). Our results show that shifting and rotating the hemes controls the adiabaticity of the three electron hopping steps.

Characterization of electronic structure and properties of a Bis(histidine) heme model complex

Ferric and ferrous hemes, such as those present in electron transfer proteins, often have low-lying spin states that are very close in energy. To explore the relationship between spin state, geometry, and cytochrome electron transfer, we investigate, using density functional theory, the relative energies, electronic structure, and optimized geometries for a high- and low-spin ferric and ferrous heme model complex. Our model consists of an iron-porphyrin axially ligated by two imidazoles, which model the interaction of a heme with histidine residues. Using the B3LYP hybrid functional, we found that, in the ferric model heme complex, the doublet is lower in energy than the sextet by 8.4 kcal/mol and the singlet ferrous heme is 6.7 kcal/mol more stable than the quintet. The difference between the high-spin ferric and ferrous model heme energies yields an adiabatic electron affinity (AEA) of 5.24 eV, and the low-spin AEA is 5.17 eV. Both values are large enough to ensure electron trapping, and electronic structure analysis indicates that the iron d(pi) orbital is involved in the electron transfer between hemes. Mössbauer parameters calculated to verify the B3LYP electronic structure correlate very well with experimental values. Isotropic hyperfine coupling constants for the ligand nitrogen atoms were also evaluated. The optimized geometries of the ferric and ferrous hemes are consistent with structures from X-ray crystallography and reveal that the iron-imidazole distances are significantly longer in the high-spin hemes, which suggests that the protein environment, modeled here by the imidazoles, plays an important role in regulating the spin state. Iron-imidazole dissociation energies, force constants, and harmonic frequencies were calculated for the ferric and ferrous low-spin and high-spin hemes. In both the ferric and the ferrous cases, a single imidazole ligand is more easily dissociated from the high-spin hemes.

Optical detection of spin polarization in single-molecule magnets [Mn(12)O(12)(O(2)CR)(16)(H(2)O)(4)]

A magneto-optical study has been undertaken of the mixed-valence single-molecule magnet [Mn(IV)(4)Mn(III)(8)O(12)L(16)] in which the ligands, L, are acetate (Mn(12)Ac) or the long-chain carboxylic acid, C(14)H(29)COOH (Mn(12)C(15)), that confers better solubility in organic solvents. Thin polymer films of these compounds in poly(methyl methacrylate) (PMM) have been cast by solvent evaporation to provide samples suitable for variable-temperature and field magnetic circular dichroism (MCD) studies. The absorption spectra in isotropic light are featureless, whereas the low-temperature MCD spectra contain resolved peaks, both positive and negative. MCD magnetization curves measured at temperatures above 4.2 K have established a ground-state spin of S = 10 and an axial zero-field parameter, D, of -0.61 K, similar to that determined for single crystals of Mn(12)Ac. By studying at a variety of optical wavelengths, the polarization ratios of the optical transitions relative to the unique axis of the zero-field distortion have been determined. The MCD magnetization curves measured at 4.2 K between 0 and 5 T for the case of Mn(12)C(15) in the PMM film can be fitted only on the assumption of nonrandom distribution of molecular z-axes arising from stresses in the polymer film during the process of casting. MCD-detected hysteresis curves measured in both frozen solution and PMM films, below the blocking temperature of approximately 3 K, show a high retention of spin polarization after reduction to zero of a polarizing magnetic field. This generates intense zero-field circular dichroism (CD) with maximum intensity for xy-polarized optical transitions whose sign depends on the direction of the original polarizing field. The optical polarization and the selection rules for MCD select a subset of molecular orientations with respect to the direction of field. Thus, the magnetically induced CD provides a highly sensitive and rapid optical method of reading the spin polarization of molecular magnets.

The energy level scheme for the ferryl heme in compound II of the peroxidase-catalase family as determined from analysis of low-temperature magnetic circular dichroism

The expressions for temperature-dependent magnetic circular dichroism (MCD) of the ferryl heme (Fe(4+)Por, S=1), which is a model of an intermediate product of the catalytic cycle of heme enzymes (compound II), have been derived in the framework of a two-term model. Theoretical predictions for the temperature and magnetic field dependence of MCD intensity of the ferryl heme are compared with those of the high-spin and low-spin ferric heme. Analysis of reported MCD spectra of myoglobin peroxide [Foot et al., Biochem. J. 2651 (1989) 515-522] and compound II of horseradish peroxidase [Browett et al., J. Am. Chem. Soc. 110 (1987) 3633-3640] has shown the presence in the samples of approximately 1% of a low-spin ferric component, which, however, should be taken into account in simulating observed temperature dependences of MCD intensity. The values of two adjustable parameters are estimated from the fit of the observed and simulated plots of MCD intensity against the reciprocal of the absolute temperature. One of them, the energy gap between the ground and excited terms, predetermines the axial zero-field splitting. The other parameter is correlated with the energy of splitting of excited quartets arising from either the porphyrin pi-->pi* transition or the spin-allowed charge-transfer transition.

Determination of zero-field splitting and evidence for the presence of charge-transfer transitions in the Soret region of high-spin ferric hemoproteins obtained from an analysis of low-temperature magnetic circular dichroism

Theoretical expressions for magnetic circular dichroism (MCD) of the porphyrin pi-->pi*, spin-allowed charge transfer (CT) and spin-forbidden d-d or CT transitions in high-spin ferric heme are derived. The transitions can be discriminated by their MCD to absorption ratio and/or temperature dependence of MCD intensity. An analysis of the Soret MCD of fluoride complexes of myoglobin (Mb), hemoglobin (Hb) and horseradish peroxidase (HRP), recorded at temperatures from 290 down to 2 K, is given. It is shown that the Soret MCD of HRPF can be well described by overlapping of the pi-->pi* transition with one spin-forbidden CT transition of an 6A1-->4E type. In the case of MbF and HbF it is necessary to assume the presence in the Soret region of the second spin-forbidden CT transition, most probably of an 6A1-->4A1 type. The parameters of transitions have been extracted from a non-linear least-squares fitting procedure. The best fit values of parameter D of the zero-field splitting of the ground manifold for HbF (6.1 cm(-1)) and MbF (6.4 cm(-1)) agree well with those obtained by other methods. The D value for HRPF (8.3 cm(-1)) is obtained for the first time.