Magnetic Circular Dichroism Intensity Research Articles

Spectral changes upon subunit association in valency hybrid hemoglobins

The absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of valency hybrid hemoglobins and their constituents α + and β chains for α 2 + β 2, α and β + chains for α 2 β 2 +: + denotes ferric heme) were measured in the Soret region for F −, H 2O, N 3 − and CN − derivatives. Absorption and MCD spectra of valency hybrid hemoglobins were very similar to the arithmetic mean of respective spectra of their corresponding component chains in all derivatives. The Soret MCD intensity around 408 nm for various complexes of valency hybrid hemoglobins seems to reflect the spin state of ferric chains. Upon ferric and deoxy ferrous subunit association to make the deoxy valency hybrid hemoglobins, only the high-spin forms bound with F − and H 2O of α 2 + β 2 displayed a blue shift in the peak position around 430 nm and those of α 2 β 2 + an increase in intensity around 430 nm. The blue shift and the increase in intensity were considered to be caused by the structural changes in deoxy β chains of α 2 + β 2 and deoxy α chains of αβ 2 +, respectively. These spectral changes were interpreted on the basis of their oxygen-equilibrium properties. In contrast to absorption and MCD spectra, the CD spectra of valency hybrid hemoglobins were markedly different frm the simple addition of those of their component chains in all derivatives examined. The large part of CD spectral changes upon subunit association were interpreted as changes in the heme vicinity accompanied by formation of the α 1 β 1 subunit contact.

Low temperature MCD study of the species formed by photolysis of horseradish peroxidase compound I

During the reaction of the enzyme horseradish peroxidase (HRP) with hydrogen peroxide, a highly oxidized species is formed which is known as ▪ compound I. The electronic structure of HRP compound I is considered to involve an Fe(IV) porphyrin π-cation radical [1]. Previous studies of horseradish peroxidase compound I have shown that light accelerates the spontaneous conversion of compound I to compound II at room temperature, yielding, finally, the native enzyme [2]. However, photolysis at low temperatures ( i.e. those less than the glassing temperature of the solvent) produces a photochemical product with optical properties which are close to, but not identical to, those of compound II [2–4]. Although, magnetic circular dichroism (MCD) spectroscopy is closely related to optical absorption spectroscopy, the data obtained provide considerably more information about the ground and excited state electronic configurations than is obtained from the absorption spectrum alone. In this paper we describe MCD data recorded between 4 K and 50 K following the photolysis of HRP compound I at 80 K. Figure 1 shows the 4 K and K MCD spectra of the HRP compound I photochemical product which was prepared by an exhaustive photolysis of compound I at 80 K. The 50 K spectrum is very similar to the MCD spectra of HRP compound II which have been previously reported for 127 K and 208 K [5]. Both sets of spectra in the visible region contain features which are quite unlike those of a ferric heme with either a S = 5 2 or 1 2 spin state. However, like the MCD spectra of ferric hemes, the spectrum of the photochemical species is very temperature dependent and is dominated Faraday C terms. Fig. 2 shows the temperature dependence of the peak-to-trough intensity for the major features in the B or Soret regions and the Q or α band regions of the MCD spectrum of the heme in the photochemical product. The almost linear ▪ relationship between the intensity and the inverse of the absolute temperature suggest that the ground state in the iron porphyrin species which is formed photochemically is obtained degenerate. The non-linearity of this relationship at very low temperatures and the high magnetic field (4.58 T) used in this study most likely arises from simple Boltzmann saturation effects. Various structures have been suggested for the product of the photolysis of HRP compound I [2, 4, 6, 7,]. The close similarity between the optical absorption and MCD spectra of the photolysis product and HRP compound II suggests that the heme retains the S = 1 Fe(IV) porphyrin electronic structure following photolysis and that the porphyrin is reduced from the 17 π-electron cation radical of compound I to the stable 18 π electron configuration as in compound II [8, 9]. In addition, the strong temperature dependence of the MCD intensity observed in the spectra of the photochemical product indicates that significant coupling takes place between the paramagnetic iron and the diamagnetic porphyrin π system.

Magnetic circular dichroism studies on the electronic configuration of catalase compounds I and II.

Absorption and magnetic circular dichroism spectra of native catalase, compound I and compound II have been measured and the data compared with that observed previously for horseradish peroxidase. The native catalase data at pH 6.9 are characteristic of a high-spin ferric porphyrin and are similar to the data reported for the ferric myoglobin and ferric horseradish peroxidase at pH 7. Oxidation of native catalase by peroxoacetic acid forms the compound I species that is identified by its low absorbance in the 400 nm Soret region and a series of overlapping bands between 450 nm and 680 nm. The magnetic circular dichroism spectra of compound I of catalase closely resembles that previously obtained for horseradish peroxidase compound I. These results indicate that the ground state of the heme pi-system is the same in both catalase and horseradish peroxidase compound I species. The compound II data show that the ratio of the magnetic circular dichroism intensity for the Soret to alpha A terms is 0.5 which means that there is a redistribution of angular momentum between the pi* excited states that give rise to the Soret and alpha-bands compared with the horseradish peroxidase compound II data where the ratio of the analogous A term intensities has a value of about 3. In addition, the magnetic CD spectra of both the catalase and horseradish peroxidase compound II species are reminiscent of typical metalloporphyrin spectra which lack charge-transfer transitions and where the metal d-orbitals are decoupled from the porphyrin pi-system.

Orientation dependence of magnetic circular dichroism in tin-doped yttrium iron garnet

Measurements are reported of optical absorption and magnetic circular dichroism (MCD), in the range of wavelength 1.0-2.7 mu m, made on tin-doped YIG with the specimen orientations (100), (110) and (111). These effects are studied at room and liquid-nitrogen temperatures. The transitions at 1.15, 1.65 and 2.00 mu m, are clearly observed in the low-temperature absorption spectrum as well as in the MCD measurements. The MCD intensities at 1.15 mu m are proportional to 1:0.85:1.03 at room temperature and to 1:0.87:1.15 at 77K respectively for the (100), (110) and (111) samples. They are also consistent with the hypothesis assumed in the theory of Leoni and Tucciarone (1972) if the differences in doping concentration among the three samples are taken into consideration.

Magnetic circular dichroism studies of bovine liver catalase

Absorption, circular dichroism (CD) and magnetic circular dichroism (MCD) spectra of beef liver catalase at pH 5.0 and 6.9, and its complexes with NaF, KCNO, NaCNS, NaN 3 and NaCN, have been measured between 250 nm and 700 nm at room temperature. The pH 6.9 native catalase MCD shows the presence of several additional transitions not resolved in the absorption spectrum. While these bands can be seen in the spectra of all the derivatives, with the exception of the cyanide, their relative intensities changes considerably between complexes. Of special interest in the MCD of ferric hemes is the signal intensity at about 400 nm and 620 nm. The data indicate that the MCD intensity at 620 nm increases as the high spin iron porphyrin fraction increases, reaching a maximum with the fluoride complex. The 430 nm band intensity increases as the proportion of low spin iron increases, reaching a maximum with the cyanide complex. The MCD spectra also indicate clearly the existence of spin mixtures in the complexes with CNO −, CNS − and N 3 −, where both the 430 nm and 620 nm bands have appreciable intensity. It is significant that despite almost identical absorption spectra the CNS − complex has a higher fraction of low spin iron than either the CNO − or the N 3 − species. The differences between the pH 5 and 6.9 MCD spectra of the native catalase suggest that the environment of the heme centre is sensitive to protonation.

Molecular weight dependence of magnetic circular dichroism for polystyrenes in solution

AbstractThe molecular weight dependence of the magnetic circular dichroism (MCD) for polystyrenes (PS) has been studied in various solvents by using the normalized dipole‐strength D*. The normalized MCD intensity B* is not so much affected by the molecular weight as the normalized dipole strength D+. The ratio of the Faraday parameter to the dipole strength, B*/D*, decreases as the molecular weight increases to 4,0.103 and is constant above it. The increase of the normalized D* with the increase of molecular weight is interpreted in terms of the closer approach of chromophores. These results indicate that MCD for polystyrenes is influenced by the interaction of chromophores in a polymer chain: the decrease of B*/D* comes from the increase of the interaction of chromophores. This tendency is enhanced by solvent effects on MCD.

Magnetic and natural circular dichroism of metalloporphyrin complexes of human and rabbit hemopexin.

Magnetic circular dichroism (MCD) spectra of several metalloporphyrin complexes of rabbit and human serum hemopexins in the spectral region of 300 to 650 nm and natural circular dichroism (CD) in the 300 to 450 nm region are reported. The MCD spectra of the heme (iron-protoporphyrin IX) complexes of both proteins were essentially identical suggesting similar iron coordination. The Soret region MCD spectrum of ferriheme . hemopexin has a shape and amplitude typical of other completely low spin (S = /sup 1///sub 2/) ferric hemeproteins, and the temperature dependence of the MCD intensity indicates that it is composed predominantly of Faraday C-type, terms. The visible region MCD spectrum of this complex closely resembles those characteristic of cytochrome b/sub 5/ and other bisimidazole-coordinated heme derivatives. Under aerobic conditions, heme . hemopexin is in the fully oxidized state. The ferroheme . hemopexins also exhibit MCD spectra similar to that of ferrocytochrome b/sub 5/, consistent with a low spin state and histidyl side-chain coordination of the heme iorn in the reduced as well as in the oxidized state. The deuteroheme derivatives of rabbit hemopexin exhibit MCD spectra similar to those of the heme complex except for the expected slight differences in wavelength extrema, indicatingmore » that the vinyl side chains of protoporphyrin have little influence on the coordination. In contrast, the natural CD spectra of the heme complexes of rabbit and human hemopexin do not resemble the CD of cytochrome b/sub 5/, reflecting differences in the crevice regions of the different hemeproteins. Furthermore, the CD spectra of the ferroheme complexes of rabbit and human hemopexin point to differences in the local environments of the heme chromophores.« less

The electronic state of heme in cytochrome oxidase II. Oxidation-reduction potential interactions and heme iron spin state behavior observed in reductive titrations.

Magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), and optical absorption spectroscopies have been used to monitor the concentrations of oxidized and reduced heme and copper during stoichiometric reductive titrations of purified beef heart cytochrome oxidase. The MCD data are deconvoluted to obtain the concentrations of reduced cytochromes a and a3 during the titrations; analysis of the EPR spectra provides complementary data on the concentrations of the EPR-detectable species. For the native enzyme in the absence of exogenous ligands, cytochromes a and a3 are reduced to approximately the same extent at all points in the titration. The reduction of the EPR-detectable copper, on the other hand, initially lags the reduction of the two cytochromes but in the final stages of the titration is completely reduced prior to either cytochrome a or a3. These non-Nernstian titration results are interpreted to indicate that the primary mode of heme-heme interaction in cytochrome oxidase involves shifts in oxidation-reduction potential for each of the two cytochromes such that a change in oxidation state for one of the hemes lowers the oxidation-reduction potential of the second heme by approximately 135 mV. In these titrations high spin species are detected which account for 0.25 spin/oxidase maximally. Evidence is presented to indicate that at least some of these signals can be attributed to cytochrome a3+ which has undergone a low-spin to high-spin state transition in the course of the titration. In the presence of carbon monoxide the oxidation-reduction properties of cytochromes a and a3 are markedly altered. The a32+. CO complex is fully formed prior to reduction of either cytochrome a3+ or the EPR-detectable copper. The g = 3 EPR signal attributed to cytochrome a3+ decreases as the MCD intensity of cytochrome a2+ increases; no significant high-spin intensity is observed at any intermediate stage of reduction. We interpret these Nernstian titration results to indicate that in the presence of ligands the oxidation-reduction potential of cytochrome a relative to cytochrome a3 is determined by the oxidation-reduction state of the stabilized cytochrome a3 ligand complex; if ligand binding occurs to reduced cytochrome a3 then cytochrome a titrates with a lower potential; cytochrome a titrates with a higher potential if oxidized cytochrome a3 is stabilized by ligand binding.

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).

ChemInform Abstract: STUDIES OF THE MIXED AMINO ACID COMPLEXES OF COBALT(III). IV. MAGNETIC CIRCULAR DICHROISM SPECTRA OF THE (CO(OX)X(GLY)Y(EN)Z) COMPLEXES

Chemischer InformationsdienstVolume 7, Issue 48 Physical Organic Chemistry ChemInform Abstract: STUDIES OF THE MIXED AMINO ACID COMPLEXES OF COBALT(III). IV. MAGNETIC CIRCULAR DICHROISM SPECTRA OF THE (CO(OX)X(GLY)Y(EN)Z) COMPLEXES N. MATSUOKA, N. MATSUOKASearch for more papers by this authorY. SHIMURA, Y. SHIMURASearch for more papers by this author N. MATSUOKA, N. MATSUOKASearch for more papers by this authorY. SHIMURA, Y. SHIMURASearch for more papers by this author First published: November 30, 1976 https://doi.org/10.1002/chin.197648045Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume7, Issue48November 30, 1976 RelatedInformation

Magnetic circular dichroism studies on microsomal aryl hydrocarbon hydroxylase: Comparison with cytochrome b5 and cytochrome P-450 cam

Magnetic circular dichroism spectra are reported for the visible and near ultraviolet spectral regions of liver microsomes from dimethylbenzanthracene-treated rats. The sequential addition of NADH, dithionite, and carbon monoxide enables us to determine contributions to the magnetic circular dichroism by cytochromes b 5 and P-450, which dominate the spectra. The magnetic circular dichroism of the microsomal preparation is compared with that of purified oxidized and reduced cytochrome b 5 from pig liver and with the camphor-complexed and camphor-free oxidized, reduced, and reduced carbonmonoxy cytochrome P-450 cam from Pseudomonas putida. The magnetic circular dichroism spectra of the membrane bound cytochrome b 5 are similar to those of the purified protein, indicating that little or no alteration in the environment of the heme occurs during the isolation procedure. The soluble bacterial cytochrome P-450 cam also appears to be a suitable model for microsomal P-450, although differences in the magnetic circular dichroism intensity are observed for the two enzymes. No effect of dimethylbenzanthracene on the magnetic circular dichroism spectra of induced compared to control rat microsomes could be observed.

Detection and assignment of weak absorption bands by magnetic circular dichroism: Spin-forbidden transitions of CoCl 42−

The magnetic circular dichroism (MCD) of CoCl 4 2−/Cs 3ZnCl 5 at 9°K shows ten weak bands between 8000 Å and 3000 Å due to spin-forbidden transitions. The signs and magnitudes of the MCD intensities yield a clear assignment of nine bands.