Magnetic Circular Dichroism Bands Research Articles

Spectroscopic studies on lacquer peroxidase from Rhus vernicifera and its derivatives

In order to probe the active site of lacquer peroxidase (LAP) from Rhus vernicifera, electronic absorption and magnetic circular dichroism (MCD) studies of the enzyme with and without various exogenous ligands (NO, CO, and CN −) have been carried out at room temperature. The absorption and MCD spectra of native LAP are quite similar to those of horseradish peroxidase (HRP), indicating that the enzyme has a heme with the fifth and the sixth coordination sites occupied by histidine imidazole and an exogenously exchangeable ligand (H 2O), respectively. Moreover, the superhyperfine structure of the electron paramagnetic resonance (EPR) signal of NO-ferrous LAP provides strong support for the fifth imidazole ligand. A comparison of the Soret MCD band of native LAP with that of Hg(II)-treated LAP demonstrated that an equilibrium mixture of high-spin heme (major species) and low-spin heme (minor species) is present in the enzyme at room temperature. All of the derivatives of NO, CO, and CN − exhibit electronic and MCD spectra of low-spin complexes; similar spectral features are found for the corresponding derivatives of HRP and myoglobin (Mb). The treatment of LAP with H 2O 2 gives rise to stable compound II at room temperature, while compound I is too unstable to be spectroscopically observed.

Investigation of the absorption of Ti3+in LiNbO3by means of optical detection of magnetic resonance (ODMR)

Abstract The broad band absorption in the visible and near infrared region of LiNbO3:Ti3+ is examined by measurement of the magnetic circular dichroism (MCD). By the method of ODMR we show that this MCD must be attributed to Ti3+. Comparing MCD and absorption bands we find that Ti accounts for most part of the absorption in our LiNbO3:Ti3+ samples. The possible nature of the transitions underlying absorption and MCD is discussed.

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.

A theoretical model of the intensity of the near-infrared porphyrin-to-iron charge-transfer transitions in low-spin iron(III) haemoproteins. A correlation between the intensity of the magnetic circular dichroism bands and the rhombic distortion parameter of iron

A theoretical model is presented of the source of the intensity of the porphyrin-to-iron charge-transfer (c.t. ) band of low-spin iron(III) haems in proteins and model complexes. The intensity mechanism involves intensity borrowing via the ‘transfer term’ from the intense visible and Soret bands of the haem spectrum. The experimental evidence suggests that about 5% of the intensity is borrowed. The intensity of the magnetic circular dichroism (m.c.d) spectrum of the c.t. bands at 4.2 K depends upon the ground-state gz value and upon the product ab where a and b are the coefficients of the iron (III) ion d-orbitals 3dxz and 3dyz′ respectively, in the ground-state Kramers doublet. This leads to a dependence of the m.c.d. C-term intensity upon the rhombic distortion, V, experienced by the iron (III) ion. IfV is zero then a=b, and the hole in the t2g5d subshell is equally divided between the two orbitals. The c.t. band is x,y- polarized and the m.c.d, intensity a maximum value. As V tends to a large value either a or b drops to zero, and the hole becomes localized in one of the dxz,yz orbitals. This results in a c.t. transition which is predominantly x- or y-polarized. The m.c.d. intensity drops to zero since a pair of perpendicularly polarized transitions is needed for the absorption of circularly polarized light. Experimental evidence drawn from measurements on a wide range of low-spin iron (III) haemoproteins has been used to substantiate the model. This study shows that the intensity of the near-i.r m.c.d. transition can be a useful qualitative indicator of the magnitude of the rhombic crystal-field component at the iron(III) ion. This in turn allows some conclusions to be drawn about the relative orientation of the axial ligands to iron(III) in bis (histidine) co-ordinated haems.

Solvent effect on MCD of Fe(III) heme complexes: Magnetic circular dichroism spectra of five-coordinated high-spin iron(III) protoporphyrin-IX-dimethylester in the visible region and their environmental effect. A characterization of the visible electronic transitions in Fe(III) high-spin porphyrins

Magnetic circular dichroism (MCD) spectra were observed to characterize the nature of the visible bands for high-spin Fe(III) protoheme derivatives with p-nitrothiophenolate, p-nitrophenolate, and methoxy anion as the fifth ligands in several solvents. The visible MCD bands for p-nitrophenolate heme were very sensitive to the solvent polarity, but that those for p-nitrothiophenolate heme and methoxy heme were not dependent on solvent polarity. Thus, both the two visible MCD band positions and the magnitudes were dependent on E Tr value (a solvent polarity parameter) in the former complex, but not in the latter two complexes. The results are consistent with previously proposed electronic structures of high-spin Fe(III) heme complexes.

ChemInform Abstract: Visible and Magnetic Circular Dichroism Studies on Cobalt(II)‐Substituted Rhus vernicifera Laccase.

Abstract The visible and magnetic circular dichroism (MCD) spectra of Co(II) derivatives of Rhus vernicifera laccase are reported. Anaerobic incorporation of 1 g-atom of Co(II) into apolaccase gave bands at 528(ϵ = 248), 558 (254) and 589 nm (shoulder) attributable to dd transitions. The MCD spectrum in the corresponding region is similar to that of Co(II)-substituted hemocyanin, indicating that the Co(II) ion incorporated into apolaccase is tetrahedral. On increasing the amount of Co(II) ion acting on the apolaccase, both the intensities of the absorption and the MCD spectra increased, and 2 g-atoms of tetrahedral Co(II) ion were introduced into the apolaccase. Very similar absorption and MCD spectra were obtained when laccase whose type I copper site was occupied by Hg(II) and both type II and type III copper sites were vacant (TlHg apolaccase) was treated with Co(II); this clearly supports the hypothesis that Co(II) cannot be incorporated into a type I copper site but may possibly be incorporated into a type III copper site. A tetrahedral Co(II) ion was also introduced into a type II copper site of type II copper-depleted (T2D) laccase, although its MCD bands were shifted ca. 20 nm to the longer wavelength region from the MCD bands due to tetrahedral Co(II) ion incorporated into type III copper site(s). The present study demonstrate that a tetrahedral Co(II) ion is introduced into type II or type III copper site(s) of laccase.

Visible and magnetic circular dichroism studies on cobalt(II)-substituted rhus vernicifera laccase

The visible and magnetic circular dichroism (MCD) spectra of Co(II) derivatives of Rhus vernicifera laccase are reported. Anaerobic incorporation of 1 g-atom of Co(II) into apolaccase gave bands at 528(ϵ = 248), 558 (254) and 589 nm (shoulder) attributable to dd transitions. The MCD spectrum in the corresponding region is similar to that of Co(II)-substituted hemocyanin, indicating that the Co(II) ion incorporated into apolaccase is tetrahedral. On increasing the amount of Co(II) ion acting on the apolaccase, both the intensities of the absorption and the MCD spectra increased, and 2 g-atoms of tetrahedral Co(II) ion were introduced into the apolaccase. Very similar absorption and MCD spectra were obtained when laccase whose type I copper site was occupied by Hg(II) and both type II and type III copper sites were vacant (TlHg apolaccase) was treated with Co(II); this clearly supports the hypothesis that Co(II) cannot be incorporated into a type I copper site but may possibly be incorporated into a type III copper site. A tetrahedral Co(II) ion was also introduced into a type II copper site of type II copper-depleted (T2D) laccase, although its MCD bands were shifted ca. 20 nm to the longer wavelength region from the MCD bands due to tetrahedral Co(II) ion incorporated into type III copper site(s). The present study demonstrate that a tetrahedral Co(II) ion is introduced into type II or type III copper site(s) of laccase.

Moment analysis for absorption and magnetic circular dichroism bands of atomic P←S transitions: Application to matrix-isolated chromium

Equations useful for the moment analysis of magnetic circular dichroism (MCD) and absorption bands of P←S electronic transitions of atoms or ions for any spin multiplicity have been derived. In particular, it is shown that the first through third MCD moments, together with the zeroth and second absorption moments may be employed to obtain such electronic and vibronic parameters as (1) the excited state spin–orbit coupling constant, (2) the excited state orbital g factor, and (3) the contributions of the cubic and noncubic metal/lattice cage modes to the spectral absorption bandwidths. The MCD and absorption spectra of Cr atoms isolated in krypton and xenon matrices are presented. Forbidden transitions between the two allowed 7P←7S bands are observed. Their appearance is ascribed to second-order spin–orbit coupling to the near-lying 7P states. The moment equations are applied in an appropriate manner to the observed allowed bands. Excited state spin–orbit splitting (−14 cm−1:Cr/Kr and −56 cm−1:Cr/Xe) are found to be significantly reduced from the gas phase value (+194 cm−1) for the z7P←7S transition. For the y 7P←S band, an enhancement (+275 cm−1:Cr/Kr) and a reduction (+171 cm−1:Cr/Xe) from the gas phase value of 206 cm−1 are observed. It is further shown that the Jahn–Teller effect is strongly operative in the z 7P←7S Cr band in both matrices.

The electronic and magnetic circular dichroism spectra of matrix-isolated scandium atoms and the scandium dimer

The electronic and magnetic circular dichroism (MCD) spectra of scandium atoms isolated in argon, krypton and xenon matrices have been measured and the bands assigned. Some aspects of the assignment present problems and the resulting matrix shifts are rather irregular. Magnetization studies of the above systems are also reported and the data show that there are particularly strong guest—host interactions in the case of Sc/Xe. Furthermore, they suggest that there is significant guest—host interaction in the ground electronic state. Computer simulation of the magnetization curves and the MCD spectra, using a crystal field model, enables some tentative suggestions concerning the nature of the matrix sites to be made. All sites show an axial character. MCD bands of a scandium dimer have been observed. The form and magnetization properties of one band support a 5Σ ground state for the molecule.

Absorption and Magnetic Circular Dichroism Spectra of CsI:Tl+3

AbstractThe temperature dependence of the optical absorption and magnetic circular dichroism (MCD) of CsI crystals doped with Tl+ is measured between liquid helium temperature and room temperature. The absorption spectra are more complicated than usual for impurity ions with the s2 ground‐state configuration and show five strong bands on the long‐wavelength side of the absorption edge. Three of these bands are assigned to internal transitions of the Tl+ ion and the other two to charge‐transfer transitions. The first and second moments of the first two absorption bands, as well as the first and third moments of thecorresponding MCD bands, are determined from spectra measured below 140 K. For these bands at higher temperatures and for the remaining bands strong overlapping precludes a meaningfull determination of the higher moments. Coupling constants and g‐values are obtained from the moments, and the origin of the spectral bands is discussed.

Magnetic Circular Dichroism Studies of Bipyridyls

Abstract The magnetic circular dichroism (MCD) and UV absorption spectra of 2,2′- and 4,4′-bipyridyl in cyclohexane and sulfuric acid were measured at room temperature. The MCD spectrum of 2,2′-bipyridyl is different from that of 4,4′-bipyridyl, reflecting the difference in molecular symmetry between them. The absorption and MCD spectral profiles of 2,2′-bipyridyl in cyclohexane are different from those of 2,2′-bipyridyl in mineral acids, in this system protonation taking place on the nitrogen atoms to give a mono- or di-cation with low symmetry. On the contrary, the MCD spectra of 4,4′-bipyridyl in cyclohexane and in acidic media at pH 0.98 exhibit a similar spectral profile to each other due to the D2 symmetry being retained in 4,4′-bipyridyl. Theoretical calculations were carried out on the basis of the PPP-SCF-CI method to get a reasonable explanation for the observed MCD spectra. The assignments of the MCD bands of these compounds are given and their dihedral angles are estimated.

A comparison of the heme electronic states in equilibrium and nonequilibrium protein conformations of high-spin ferrous hemoproteins low temperature magnetic circular dichroism studies

The visible and near infrared magnetic circular dichroism (MCD) spectra of equilibrium high-spin ferrous derivatives of myoglobin, hemoglobin, horseradish peroxidase and mitochondrial cytochrome c oxidase at 15 K are compared with those of the corresponding proteins in nonequilibrium conformations produced by low-temperature photodissociation of CO-complexes of these proteins as well as of O 2-complexes of myoglobin and hemoglobin. Over all the spectral region (450–800 nm) the intensities of MCD bands of hemoproteins studied in equilibrium conformation are shown to be strongly temperature-dependent, including a negative band at ca. 630 nm and positive bands at ca. 690 nm and at ca. 760 nm. In contrast to the absorption spectra, the low-temperature MCD spectra of high-spin ferrous hemoproteins differ significantly, reflecting the peculiarities in the heme iron coordination sphere which are created by a protein conformation. The MCD spectra reveal clearly the structural changes in the heme environment which occur on ligand binding. On the basis of assignment of d → d and charge-transfer transitions in the near infrared region the correlation is suggested between the wavelength position of the MCD band at approx. 690 nm and the value of iron out-of-plane displacement as well as between the location of the band at approx. 760 nm and the Fe- N ϵ , (proximal histidine) bond strength (length) in equilibrium and nonequilibrium conformations of the hemoproteins studied. The high sensitivity of low-temperature MCD spectra to geometry at heme iron is discussed.

Magnetic Circular Dichroism and Molecular Orbital Studies on the Intramolecular Charge‐Transfer Transitions in the Bridged Tropylium Cations

AbstractThe magnetic circular dichroism (MCD) spectra of the bridged tropylium cations have been measured at room temperature. Very weak MCD bands have been observed in the longest wavelength region and assigned to the intramolecular charge‐transfer (CT) transitions arising from the electronic excitations from the highest occupied MO(HOMO) of the remote chromophore to the lowest vacant MO(LVMO) of the tropylium cation. In the shorter wavelength region, four distinct MCD bands with alternating sign have been detected, clarifying four π→π* transitions in the perturbed tropylium moiety.

Magnetic Circular Dichroism Studies of 4,4′-Disubstituted Biphenyls

Abstract The magnetic circular dichroism (MCD) and ultraviolet (UV) absorption spectra of 4,4′-disubstituted biphenyls were measured. The MCD spectra showed diverse spectral patterns reflecting the natures of the substituent groups. These observed MCD spectra were reproduced by the theoretical calculations based on the Pariser-Parr-Pople-SCF-CI method. The dihedral angles of 4,4′-disubstituted biphenyls were estimated to be about 20° by comparing the observed MCD spectra with the calculated ones. The assignments for the MCD bands of 4,4′-disubstituted biphenyls were given on the basis of these calculations. The substituent effects of biphenyls were interpreted by the use of the coefficients of the configuration interactions.

Absorption and magnetic circular dichroism spectra of matrix isolated Ta atoms

The absorption and magnetic circular dichroism (MCD) spectra of Ta atoms isolated in an argon matrix at 14 K have been measured. Over sixty-five MCD bands have been observed and used together with gas phase intensity and energy pattern data to reassign the matrix spectrum. The observed matrix shift (average: 765 cm −1) to the blue is smaller than determined by previous workers. From this study it is suggested that the assignment of matrix-isolated atomic species can be considerably aided by the determination of their magnetic circular dichroism spectra.

Spectroscopic studies of ascorbate oxidase. Electronic structure of the blue copper sites.

Low-temperature electronic absorption and room temperature circular dichroism (CD) and magnetic circular dichroism (MCD) spectra are reported for ascorbate oxidase. Bands attributable to d-d electronic transitions in the type 1 (blue) coppers (2B2 ground state) have been observed at 5800, 10000, and 12000 cm-1 (1725, 1000, and 835 nm). The three bands are assigned to the transitions 2B2 leads to 2E, 2B2 leads to 2B1, and 2B2 leads to 2A1, respectively, in the slightly flattened tetrahedral blue copper centers. The ligand field theoretical treatment predicts a type 1 copper reduction potential of 417 mV. The MCD spectrum of ascorbate oxidase contains one major band at 14000 cm-1 (710 nm) and a very weak feature at 13 200 cm-1 (550 nm). The intensities of the CD and MCD bands are consistent with two or three type 1 coppers in each enzyme molecule, and the near-infrared spectroscopic results suggest that the electronic structures of the blue coppers are closely similar.

Magnetic circular dichroism and molecular orbital studies on bicyclo[6,2,0] decapentaene

The magnetic circular dichroism (MCD) spectrum of bicyclo[6,2,0] decapentaene has revealed four skeletal π → π * electronic transitions in the visible and ultraviolet region. The four MCD bands are assigned to the B 2 ← A 1, A 1 ← A 1. B 2 ← A 1 and A 1 ← A 1 electronic transitions in increasing order of energy.

Visible and near-infrared magnetic circular dichroism spectra of high spin iron(III) complexes of protoporphyrin-IX-dimethylester. Characterization of charge-transfer bands of high spin heme [Fe(III)

Magnetic circular dichroism (MCD) spectra for several five-coordinated heme [Fe(III)] complexes were observed from the near-ultraviolet to the near-infrared (IR) regions (300 ∼ 1200 nm) to characterize the nature of the MCD bands especially in the visible and near-infrared regions (500 ∼ 1200 nm). The MCD spectra reveal that the wavelengths and magnitudes of the visible and near-IR MCD bands depend critically on the nature of the axial ligand. The dependence can be explained on the basis of a qualitative molecular orbital theory by using the previously proposed assignment that these bands are attributable to the in-plane porphyrin π to metal charge-transfer transitions. The observed effects of solvent and environment on the MCD patterns and magnitudes of the model heme complexes give some implications about the heme environments of native hemoproteins.

Magnetic resonance raman optical activity of ferrocytochrome e at different laser wavelengths

Magnetic field induced resonance Raman circular intensity differential spectra are reported for the two argon ion laser wavelengths (514.5 and 501.7 nm) that fall within the β electronic absorption band of ferrocytochrome c. The signs and magnitudes of the effects are completely different in the two sets of spectra, and might be related to the slope of the corresponding magnetic circular dichroism band envelope at the excitation wavelength.

Electronic state of heme in cytochrome oxidase. I. Magnetic circular dichroism of the isolated enzyme and its derivatives

Magnetic circular dichroism (MCD) spectra have been recorded for beef heart cytochrome oxidase and a number of its inhibitor complexes. The resting enzyme exhibits a derivate shape Faraday C term in the Soret region, characteristic of low spin ferric heme, which accounts for 50% of the total oxidase heme a. The remaining heme a (50%) is assigned to the high spin state. MCD temperature studies, comparison with the MCD spectra of heme a-imidazole model compounds, and ligand binding (cyanide, formate) studies are consistent with these spin state assignments in the oxidized enzyme. Furthermore, the ligand binding properties and correlations between optical and MCD parameters indicate that in the resting enzyme the low spin heme a is due solely to cytochrome a3+ and the high spin heme a to cytochrome a33+. The Soret MCD of the reduced protein is interpreted as th sum of two MCD curves: an intense, asymmetric MCD band very similar to that exhibited by deoxymyoglobin which we assign to paramagnetic high spin cytochrome a3(2+) and a weaker, more symmetric MCD contribution, which is attributed to diamagnetic low spin cytochrome a2+. Temperature studies of the Soret MCD intensity support this proposed spin state heterogeneity. Ligand binding (CO, CN-) to the reduced protein eliminates the intense MCD associated with high spin cytochrome a3(2+); however, the band associated with cytochrome a2+ is observed under these conditions as well as in a number of inhibitor complexes (cyanide, formate, sulfide, azide) of the partially reduced protein. The MCD spectra of oxidized, reduced, and inhibitor-complexed cytochrome oxidase show no evidence for heme-heme interaction via spectral parameters. This conclusion is used in conjunction with the fact that ferric, high spin heme exhibits weak MCD intensity to calculate the MCD spectra for the individual cytochromes of the oxidase as well as the spectra for some inhibitor complexes of cytochrome a3. The results are most simply interpreted using the model we have recently proposed to account for the electronic and magnetic properties of cytochrome (Palmer, G., Babcock, F.T., and Vcikery, L.E. (1976) Proc. Natl. Acad. Sci. U. S. A. 73, 2206-2210).