Magnetic Circular Dichroism Intensity Research Articles

MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S >/= (1)/(2). Applications to S = (1)/(2) and S = (5)/(2).

The magnetic circular dichroism (MCD) properties of a spin-allowed transition from an orbitally nondegenerate ground state manifold A to an orbitally nondegenerate excited state manifold J in the presence of spin-orbit coupling (SOC) are derived for any S >/= (1)/(2). Three physically distinct mechanisms are identified that lead to MCD intensity and depend on SOC between excited states which leads to a sum rule and SOC between the ground state and other excited states that leads to deviations from the sum rule. The model is valid for any symmetry of the magnetic coupling tensors and arbitrary transition polarizations. The S = (1)/(2) case is analytically solved, and the determination of linear polarizations from MCD saturation magnetization data is discussed. For all mechanisms the MCD intensity is proportional to the spin-expectation values of the ground state sublevels which are conveniently generated from a spin-Hamiltonian (SH). For Kramers systems with large zero-field splittings (ZFSs) this allows the contribution from each Kramers doublet to the total MCD intensity to be related through their effective g-values, therefore significantly reducing the number of parameters required to analyze experimental data. The behavior of high-spin systems is discussed in the limits of weak, intermediate, and strong ZFS relative to the Zeeman energy. The model remains valid in the important case of intermediate ZFS where the ground state sublevels may cross as a function of applied magnetic field and there are significant off-axis contributions to the MCD intensity due to a change of the electron spin quantization axis. The model permits calculation of MCD C-term signs from molecular wave functions, and explicit expressions are derived in terms of MOs for S = (1)/(2) and S = (5)/(2). Two examples from the literature are analyzed to demonstrate how the C-term signs can be evaluated by a graphical method that gives insight into their physical origin.

Interaction of photosynthetic pigments with various organic solvents: Magnetic circular dichroism approach and application to chlorosomes

Magnetic circular dichroism (MCD) and absorption spectra have been measured on three intact photosynthetic pigments with the chlorin ring as macrocycle: chlorophyll a, bacteriochlorophyll c and d, in various hydrophilic organic solvents. The MCD intensity of a Q y(0-0) transition for the Mg chlorin derivative was sensitive to the coordination state of the central Mg atom by the solvent molecules. The coordination number has been characterized in terms of the relationship between the ratio of Q y(0-0) MCD intensity to its dipole strength ( B/ D) and the difference in energies of Q x(0-0) and Q y(0-0) transitions. This relationship depends not only on the coordination number of the magnesium (Mg) atom but also on the coordination interaction of the solvent molecules to the Mg atom, and can clarify the spectroscopic change of chlorosomes by alcohol treatment. We propose that the correlation between the MCD intensity of Q y(0-0) transition and the energy difference can be used as a new measure for determining the coordination number of the Mg atom and for estimating the interaction strength of the Mg atom with solvent molecules.

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 π→π*, 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 π→π* transition with one spin-forbidden CT transition of an 6 A 1→ 4 E 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 6 A 1→ 4 A 1 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.

Interpretation of x-ray circular dichroism: Multiple-scattering theory approach

The relationship between x-ray-absorption magnetic circular dichroism (MCD) and the magnetic properties of a metal is investigated within the framework of spin-polarized relativistic multiple-scattering theory. The MCD sum rules discovered by Thole et al. which relate the energy-integrated MCD intensities to the spin and orbital magnetic moments of the solid, are rederived. Several rules that relate a MCD spectrum to the $l$-projected spin and orbital magnetization densities of empty states of the solid, are also obtained. The application of these rules is discussed.

Magnetic Circular Dichroism Spectroscopic Studies of Mononuclear Non-Heme Ferrous Model Complexes. Correlation of Excited- and Ground-State Electronic Structure with Geometry

Mononuclear non-heme iron enzymes catalyze a variety of biologically important reactions involving dioxygen, and yet, the non-heme ferrous active sites have been difficult to study by most spectroscopic methods. A combination of near-infrared (NIR) magnetic circular dichroism (MCD) and variable-temperature, variable-field (VTVH) MCD spectroscopies has been applied to 24 structurally defined mononuclear non-heme ferrous model complexes to rigorously correlate spectral data with geometric and electronic structure. While general trends for the excited-state splittings have been predicted by ligand field theory, these predictions are now evaluated by systematically studying the NIR MCD spectra of a series of high-spin (S = 2) ferrous models with a wide range of coordination numbers and geometries. VTVH MCD spectroscopy is used to probe ground-state electronic structure, and a complete MCD intensity expression for non-Kramers systems that includes z-polarization, ℬ-terms, and excited states has been derived. This expression has been applied to these model complexes to determine signs of the zero-field splitting and to obtain ground-state spin-Hamiltonian parameters, which can be related to ground-state ligand field splittings. These experimental ground-state data are used to develop the information content available from VTVH MCD, in particular the ability to probe specific metal−ligand bonding interactions for different coordination environments. The excited-state ligand field data are used to construct a set of spectroscopic guidelines which, combined with the ground-state information, allow one to clearly determine the coordination number and geometry of an unknown ferrous center, with the exception of only a few ambiguous cases. Additionally, the MCD data provide insight into the origin of the MCD 𝒞-term intensities and signs for low-symmetry ferrous centers. The results obtained through these model studies now provide the basis for investigating ferrous active sites of non-heme iron enzymes to probe the geometric and electronic structure of a site with respect to oxygen reactivity and understanding how differences in structure correlate with differences in reactivity.

Assignment of conformation-sensitive near-infrared bands in high-spin ferrous haemoproteins.

An assignment of the near-infrared bands in the 600–800 nm spectral region observed in magnetic circular dichroism (MCD) spectra of high-spin ferrous haemoproteins is presented. The assignment is based on a relative energy level scheme for iron d-electrons, a comparison of predicted and measured temperature dependences of MCD intensity, a sign of MCD bands and a group theoretical analysis of allowed transitions. The proposed assignment is consistent with the ∼15-nm red shift of the ∼760 nm band on breakage of the Fe-His bond in deoxy-myoglobin at low pH, with low-temperature photolysis experiments available for CO complexes of several haemoproteins. In accordance with the observations, the intensity of the MCD bands for proteins with a sulphur anion of cysteine as proximal haemligand (cytochrome P450 and chloroperoxidase) is predicted to be diminished by at least one order of magnitude compared to that for proteins with an imidazole of a histidine as a protein-derived haemligand (i.e. myoglobin, haemoglobin and horseradish peroxidase).

The 3d–4f exchange interaction, X-ray second-order optical processes and the magnetic circular dichroism (MCD) spin sum rule in rare earths

We discuss the effect of the 3d–4f exchange interaction in rare earths by choosing two subjects on the basis of an atomic model including the full multiplet. One subject is the selection rules in resonant 4f photoemission spectroscopy (PES), 4f→3d X-ray emission spectroscopy (XES) and inverse photoemission spectroscopy (IPES) at the 3d threshold. The other is the applicability of the magnetic circular dichroism (MCD) spin sum rule in 3d→4f X-ray absorption (XAS). The interplay between the diagonal term of the exchange interaction and the 3d core spin–orbital interaction is shown to cause the contribution of the higher-energy final states of PES and XES, which are not accessible at off-resonance, to the process passing through the intermediate state in the higher-energy region at each of the 3d 5/2 and 3d 3/2 thresholds. Furthermore it is shown to cause a suppression of the contribution of the higher-energy final states of IPES seen at off-resonance for the process passing through the intermediate state with the lower-energy region at each of the 3d 5/2 and 3d 3/2 thresholds. We show, on the other hand, that the off-diagonal term of the exchange interaction causes a large amount of 3d 5/2 –3d 3/2 mixing in less-than-half filled 4f shells, but not in the more-than-half filled cases, in the Hund's rule ground state. This means that the use of the so-called M 4 and M 5 edge MCD intensities in the MCD spin sum rule formula, which is the only possible choice in its application, is meaningless in less-than-half filled 4f shells at least, although the M 4 and M 5 edges are apparently separated from each other.

Spectroscopic and Electrochemical Studies on the FOUR-α- Helices Bundle Cytochrome C

Abstract The cyclic voltammogram of the cytochrome c' from Achromobacter xylosoxidans NCIB 11015 displayed a quasi-reversible electron-transfer process and the observed half-wave peak potential was estimated to be E1/2 = 184 mV vs NHE. The redox potential of cyt c'(Fe3+ / Fe2+) decreases with increasing buffer concentration, implying that the oxidized state of the protein is stabilized by counter anion shielding of the positive charges on the protein surface. The intensities of the magnetic circular dichroism (MCD) spectra of the protein at λ = 402 and 418 nm decrease with increasing ionic strength. The decreasing of the MCD intensity at high ionic strength indicates the increasing of the high-spin state species. These results suggest that the structural change of the solvent exposed charged amino acid residues linking to outside of the protein might allow the regulation of the electrontransfer reaction of cytochrome c'.

Molecular structures and optical properties of aggregated forms of chlorophylls analyzed by means of magnetic circular dichroism

Magnetic circular dichroism (MCD) spectra were measured on chlorophylls in different aggregation states formed in organic solvents and in pigment-protein complexes. An index which reflects the magnetic dipole contribution of a specific transition is introduced, that is, a ratio of MCD intensity of Faraday B-term ( B) to that of dipole strength ( D) ( B/D). The absolute value of the index is higher in monomeric chlorophyll a and that in monomeric bacteriochlorophyll a. This index changed depending on the aggregation state of bacteriochlorophyll; in the case of the dimer of bacteriochlorophyll a and bacteriochlorophyll d the value was nearly the half of that of respective monomer, and it decreases further for a higher aggregate of bacteriochlorophyll d. Based on this index, a molecular structure of the primary electron donor in reaction center complexes is discussed.

A ligand field model for MCD spectra of biological cupric complexes

A ligand field calculation of magnetic circular dichroism (MCD) spectra is described that provides new insights into the information contained in electronic spectra of copper sites in metalloenzymes and synthetic analogs. The ligand field model uses metal-centered p- and f-orbitals to model sigma, pi LMCT mixing mechanism for intensity, allowing the basic features of optical absorption, MCD, and electron paramagnetic resonance spectra to be simultaneously computed from a single set of parameters and the crystallographically determined ligand coordinates. We have used the model to predict changes in spectra resulting from the transformation of electronic wavefunctions under systematic variation in geometry in pentacoordinate ML5 complexes. The effectiveness of the calculation is demonstrated for two synthetic copper model compounds and a galactose oxidase enzyme complex representing limiting coordination geometries. This analysis permits immediate recognition of characteristic patterns of MCD intensity and correlation with geometry. A complementarity principle between MCD and CD spectra of transition metal complexes is discussed.

Spectroscopic and mutagenesis studies on the CuA centre from the cytochrome-c oxidase complex of Paracoccus denitrificans.

Cytochrome-c oxidase contains an unusual copper centre (CuA) located in subunit II. This centre mediates one-electron transfer from cytochrome c to low-spin heme a. Recent spectroscopic and biochemical studies have shown that this centre is a valence delocalised dinuclear [Cu(+1.5)-Cu(+1.5)] centre. We have measured the absorption, EPR and variable-temperature magnetic circular dichroism spectra of the CuA-binding domain isolated from Paracoccus denitrificans cytochrome aa3. The EPR spectrum showed the following signals: gparallel = 2.18; gperpendicular = 2.03. gparallel exhibited a seven-line hyperfine splitting pattern, with an intensity ratio showing that the single unpaired electron interacted equally with two copper nuclei. The magnetic circular dichroism spectrum was identical to those from CuA in bovine heart cytochrome-c oxidase and centre A of nitrous-oxide reductase, showing the close structural similarity between the three centres. To identify the ligands of CuA, all the conserved putative ligands in the P. denitrificans CuA domain were substituted. Only five residues, Cys244, Cys248, His209, His252, and Met255, were required for correct assembly of the CuA centre. Replacement of Met255 caused protein misfolding. Hence, methionine may have a structural role for the folding of the protein rather than being a CuA ligand. Given that both copper ions must have identical coordination geometries, the number of possible structures is limited. Two models are proposed: one involves the thiolate side-chains of Cys244 and Cys248 bridging a pair of copper ions with one histidine coordinating each copper ion, and the other has terminal ligation of each copper ion by one cysteine and one histidine residue. In both models, the metal-metal distance can be sufficiently short to permit direct d-orbital overlap of the copper ions. The magnetic circular dichroism transitions at 475 nm and 525 nm are assigned to thiolate-to-copper charge-transfer processes polarised perpendicular to one another, although the magnetic circular dichroism intensities show that the excited states were heavily mixed with copper d-orbitals. These intensities can be interpreted in the thiolate bridged model in terms of transitions within a Cu2(SR)2 rhomb. In the model involving terminal cysteine ligation, exciton coupling of two thiolate-to-copper charge-transfer transitions of similar energy, polarised along the Cu-S bonds, would contribute two transitions perpendicular to one another. This requires that the cysteine ligands have a cis orientation relative to one another.(ABSTRACT TRUNCATED AT 400 WORDS)

Optical study of the magnetic phase transition in Cd1-xMnxTe

Magnetic circular dichroism (MCD) was used to investigate the magnetic phase transition in Cd1-xMnxTe (0.3 < × ≤ 1). The composition dependence of the transition temperature was estimated from the temperature dependence of MCD intensity. A comparison with theoretically predicted magnetic phase diagrams implies that Cd1-xMnxTe becomes an antiferromagnet for x ≥ 0.7.

Magnetic circular dichroism spectroscopy of zinc(II) tetraphenylporphyrin-ligand complexes: The effect of the axial ligand on spectral properties

The magnetic circular dichroism (MCD) spectra in the UV—visible spectral region (300–700 nm) of an extensive set of zinc tetraphenylporphyrin (ZnTPP) complexes with oxygen, nitrogen and sulfur donor axial ligands are reported. Because zinc porphyrins do not change oxidation or spin states and only bind one axial ligand, this study evaluates the effect of the axial ligand on the MCD spectral properties. The three types of axial ligand complexes can be discriminated by examination of the MCD band positions and intensities for the Soret, beta and alpha transitions of each ZnTPP adduct.

4f-5dExchange Interaction and Magnetic Circular Dichroism in L2,3Absorption for Ferromagnetic Rare Earths

We give an interpretation for the observed systematic trend of magnetic circular dichroism (MCD) in L 2,3 photoabsorption of rare-earth (RE) atoms from Ce to Tm in ferro- or ferrimagnetic RE 2 Co 17 assuming the 2 p →5 d electric dipole transition. We assume that the 4 f state of trivalent RE ions is in the polarized Hund-ground-state. We then take into account the spin and orbital magnetic moments induced in the RE 5 d state through the intraatomic 4 f -5 d exchange interaction approximated by a molecular field with assuming a band width of several eV and one tenth filling of the 5 d band. The integrated MCD intensity calculated with a naive one-electron picture is shown to correspond well to the observed trend. The 5 d orbital moment estimated both from the present model and from a MCD sum rule applied to the experiment is of the order of ∼ 0.1 µ B , which is comparable to the 5 d spin moment.

Formation of high spin cytochrome a in isolated cytochrome c oxidase.

The effect of temperature on the electronic structure of cytochromes a and a3 has been determined at two different concentrations of the solubilizing detergent, N-dodecyl beta-D-maltoside. In each case, an incubation temperature of 40 degrees C resulted in a 20-30% increase in the 430-nm conformer of cytochrome a3 when compared with a 20 degrees C control, but the higher detergent concentration led to additional formation of a new charge transfer band at 630 nm. Induction of this new feature was correlated with a loss of alpha-band intensity, a blue shift in the Soret absorbance, and a decrease in the Soret magnetic circular dichroism. The absorbance changes were associated with isosbestic wavelengths at 613, 581, 563, 453, and 416 nm, indicating a two-state electronic conversion. The loss of magnetic circular dichroism intensity and the lack of an isosbestic point between 630 and 655 nm were consistent with a selective effect on the electronic structure of ferricytochrome a. Comparison of these spectral changes with the model heme a studies of Carter and Palmer (Carter, K., and Palmer, G. (1982) J. Biol. Chem. 257, 13507-13514) provides evidence for a low to high spin transition in cytochrome a in which one of the axial Fe-N bonds is either broken or sterically strained. This selective effect on cytochrome a and the associated thermal resistance of the a3-CuB domain are discussed in terms of the 4-helix bundle motif that has been recently proposed for the catalytic site of cytochrome c oxidase.

Magnetic circular dichroism of zinc-blende-phase MnTe.

Temperature dependence of the magneto-optic effect, i.e., magnetic-circular-dichroism (MCD) spectra, of metastable zinc-blende MnTe films has been investigated. MCD peaks from the \ensuremath{\Gamma} point of the Brillouin zone as well as from the Mn d-d intra-atomic transitions were observed. Below 70 K the energy of the \ensuremath{\Gamma} peak showed a distinct magnetic blueshift accompanying a rapid increase of the MCD intensity. These anomalies reflect the first-order phase transition into the type-III antiferromagnetic-ordering state of a frustrated Heisenberg antiferromagnet. In the antiferromagnetic-ordering state, a larger magneto-optic effects as compared with that in the paramagnetic state was observed. This large magneto-optic effect in the antiferromagnetic-ordering state is related with the sharpening of the optical-absorption spectra.

A b i n i t i o study of the electronic magnetic circular dichroism spectrum in acetylene: The B̃←X̃ and 1 1B2←X̃ transitions

This study utilizes a newly implemented method based on first-order perturbation theory for calculating the electronic magnetic circular dichroism (MCD) spectra in molecules. As an initial application, the MCD band maxima have been calculated for the B̃(1 1Bu)←X̃(1 1Ag) and 1 1B2←X̃(1 1A1) electronic transitions in the trans and cis bent conformations (respectively) of acetylene. The band intensity is assumed to come entirely from the ℬ0 term in the MCD equations of Stephens, which explicitly includes a first-order perturbation correction to the two electronic states involved in the transition. The wave functions are determined using ab initio quantum chemical techniques including state averaged CASSCF and multireference CI. There has been speculation that the 1 1B2←X̃ band system might overlap the B̃←X̃, and be part of the reason for the diffuse nature of the spectrum in the 185–170 nm region. This study considers this claim. The current calculations predict MCD band maxima for the 0–0 and 1–0 vibrational bands in the B̃←X̃ to be Δεmax0=−3.48 and −5.82, respectively, while experiment gives −0.8 and −1.6. This is 103 times larger than the largest band maximum [Δεmax0(3–0)=−0.0037] calculated for the 1 1B2←X̃ transition. This study also finds the absorption oscillator strengths for the B̃←X̃ to be a factor of 60 larger than that found in the 1 1B2←X̃. Thus while these results do predict both the B̃←X̃ and 1 1B2←X̃ transitions to lie in the same spectral region, they do not support the hypothesis that the 1 1B2←X̃ is a major contributor to either the absorption or MCD intensity in the 185–170 nm region.

Soft-x-ray magnetic circular dichroism at the L2,3 edges of nickel.

Magnetic circular dichroism (MCD) has been observed at the ${\mathit{L}}_{2,3}$ absorption edges of ferromagnetic nickel by use of circularly-polarized soft-x-ray synchrotron radiation. The MCD intensity ratio between the ${\mathit{L}}_{2}$ and ${\mathit{L}}_{3}$ edges is found to differ appreciably from that predicted by a simple exchange-split-valence-band model. Fine MCD features, imperceptible in the absorption spectra, are also observed, and a tentative interpretation is given. This work, demonstrating the feasibility of MCD measurements in the soft-x-ray region, provides a new approach to study 3d and 4f ferromagnetic systems with their respective dipole-permitted 2p\ensuremath{\rightarrow}3d and 3d\ensuremath{\rightarrow}4f transitions.

Ab initio study of the electric dipole transition moment for the C̃←X̃ electronic transition in acetylene: theoretical predictions of the absorption and magnetic circular dichroism intensities

The electric dipole transition moment has been calculated a priori for the C̃ (1 1Π u)← X̃(1 1Π + g) electronic transition as a function of R(CC), using ab initio quantum chemical techniques including state averaged, complete active space MCSCF and multireference CI. The transition moment is then averaged over the vibrational wavefunctions for the CC stretch, and theoretical predictions are made for the adsorption oscillator strength and the magnetic circular dichroism (MCD) intensities, 〈Δϵ⦔ 1, for the lowest four vibrational bands originating from v=O in X̃.

Low-temperature EPR and near-infrared MCD studies of highly anisotropic low-spin ferrihaem complexes

The low-temperature EPR spectra and near-infrared magnetic circular dichroism (MCD) spectra are reported of the bis complexes of imidazole, 1-methylimidazole, 4-methylimidazole, 1,2-dimethylimidazole and 2-methylimidazole with Fe(III) octaethylporphyrin (OEP) in the mixed organic solvent dichloromethane/ diethyl ether. It is shown that the latter two have highly anisotropic EPR spectra characteristic of the lowspin ferric state. The optical charge-transfer bands have an unusually high MCD intensity, with a narrow linewidth. It is proposed that this feature is typical of bis-histidine-ligated haem in cytochromes with sterically strained coordination and may be used as diagnostic of such a conformation. The EPR and near-infra-red MCD spectra of the bis-butylamine complex of Fe(III(OEP are also reported. It is shown that bisamine-ligated haem may be clearly distinguished from sterically hindered bis-imidazole (bis-histidine) by near-infrared MCD spectroscopy whereas the assignment is uncertain using EPR spectroscopy alone.