The spontaneous magnetization of nickel

The original data of Weiss and Forrer on nickel are reanalyzed with the aim to obtain a simple empirical relation between the spontaneous magnetization and the temperature both below and above the Curie temperature. A simple relation between the spontaneous magnetization and the temperature is needed to separate easily between the ferromagnetic and the paramagnetic region in the plane given by temperature and field strength, needed for a comprehensive treatment of the magnetocaloric effect (compare J. Biol. Phys. 9, 27 (1981). Various methods of extrapolating to zero magnetic field strength are discussed and used to obtain specified values for the spontaneous magnetization at various temperatures. Starting with a simple expression of cooperativity among elements it is shown that a sufficiently good fit of models to the given data is only obtained with the assumption of three types of cooperativity differing in the intensity of coupling. This intensity of coupling is signified by the magnitude of the exponent in the independent variable (temperature).     

Magnetic characterization of Daphnia resting eggs

This study characterized the magnetic materials found within Daphnia resting eggs by measuring static magnetization with a superconducting quantum interference device (SQUID) magnetometer, after forming two types of conditions, each of which consists of zero-field cooling (ZFC) and field cooling (FC). Magnetic ions, such as Fe(3+), contained in Daphnia resting eggs existed as (1) paramagnetic and superparamagnetic particles, demonstrated by a magnetization and temperature dependence of the magnetic moments under an applied magnetic field after ZFC and FC, and (2) ferromagnetic particles with definite magnetic moments, the content of which was estimated to be very low, demonstrated by the Moskowitz test. Conventionally, biomagnets have been directly detected by transmission electron microscopes (TEM). As demonstrated in this study, it is possible to nondestructively detect small biomagnets by magnetization measurement, especially after two types of ZFC and FC. This nondestructive method can be applied in detecting biomagnets in complex biological organisms.     

CIDEP observations in photosystem I of green plant and algal photosynthesis.

Flash photolysis experiments with electron paramagnetic resonance detection were carried out between 10 K and 300 K on samples of green plant and algal species. Chemically induced dynamic electron polarization was evident for the signals observed in the g = 2.0 region for 100 KHz modulated detection and also for a system with no magnetic field modulation. The light reversible signals decaying in about 1 ms at low temperatures are interpreted as arising from photosystem I of the green plant and algal samples. Evidence is presented which indicates that the origin of the electron spin polarization is the well established radical-pair mechanism.     

Low-temperature magnetic circular dichroism spectra and magnetisation curves of 4Fe clusters in iron-sulphur proteins from Chromatium and Clostridium pasteurianum

The magnetic circular dichroism (MCD) spectra of the 4Fe clusters in the iron-sulphur proteins high-potential iron protein from Chromatium and the 8Fe ferredoxin from Clostridium pasteurianum have been measured over the wavelength range 300-800 nm at temperatures between approx. 1.5 and 50 K and at magnetic fields up to 5 tesla. In both cases the proteins have been studied in the oxidized and reduced states. The reduced state of high-potential iron protein gives a temperature-independent MCD spectrum up to 20 K, confirming the diamagetism of this state at low temperature. The MCD spectrum of samples of oxidized ferredoxin invariably show the presence of a low concentration of a paramagnetic species, in agreement with the observation that the EPR spectrum always shows a signal at g = 2.01. The paramagnetic MCD spectrum runs across the whole of the wavelength range studied and therefore most probably originates from an iron-sulphur centre. The diamagnetic component of the MCD spectrum of oxidized ferredoxin is very similar to that of reduced high-potential iron protein. The low-temperature MCD spectra of oxidized high-potential iron protein and reduced ferredoxin reveal intense, temperature-dependent bands. The spectra are highly structured with that of high-potential iron protein showing a large number of electronic transitions across the visible region. The MCD spectra of the two different oxidation levels are quite distinctive and should provide a means of establishing the identity of these state of 4Fe clusters in more complex proteins. MCD magnetisation curves have been constructed from detailed studies of the field and temperature dependence of the MCD spectra of the two paramagnetic oxidation states. These plots can be satisfactorily fitted to the theoretically computed curves for an S = 1/2 ground state with the g factors experimentally determined by EPR spectroscopy. The low-temperature MCD spectra of the reduced 2Fe-2S ferredoxin from Spirulina maxima are also presented and MCD magnetisation curves plotted and fitted to the experimentally determined g factors.     

Analysis of low temperature magnetic circular dichroism of high spin ferrihemoproteids in the near UV region

Magnetic circular dichroism spectra of fluoride complexes of metmyoglobin, methemoglobin, and horseradish peroxidase in the region of 300-450 nm at temperatures from 300 to 2.1 K were measured and analyzed. The temperature dependence of magnetic circular dichroism in the Soret region was found to be different from that of other paramagnetic forms and from the theoretically predicted dependence. The difference is explained by the superposition of the pi-->pi*-transition of porphyrin with one (peroxidase) or two charge transfer transitions and by substantially different temperature dependences of magnetic circular dichroism for the transitions of the two types. By minimization of differences between the expected and observed temperature dependences of magnetic circular dichroism, the parameters of its temperature dependence for charge transfer transitions and the parameter D of the zero-field splitting of the electronic ground state of the heme were found. The values of D for the fluoride complexes of metmyoglobin (5.8 cm-1) and methemoglobin (6.1 cm-1) agree well with those obtained by other methods. The D value for the fluoride complex of horseradish peroxidase (8.8 cm-1) was determined for the first time.     

New dinuclear cobalt(II) octaaza macrocyclic complexes with high oxidation redox potentials: Their crystal structure and unusual magnetic properties

Dinuclear cobalt(II) complexes were synthesised from 1,4,8,11-tetrakis-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tmpc), [Co₂(tmpc)Cl₂][CoCl₄] (1), [Co₂(tmpc)Cl₂][PF₆]₂ (1a) and [Co₂(tmpc)(NO₃)₂][NO₃]₂ · MeOH (2) and characterised by magnetic, spectroscopic and electrochemical techniques and by single-crystal X-ray diffraction. The X-ray structures of 1 and 2 demonstrate that in both complexes the metal ion is exo-coordinated with respect to the macrocyclic ligand. In 1, the Co(II) ions are fivefold coordinated with trigonal bipyramidal geometry, while in 2 they are sixfold coordinated with a distorted octahedral geometry. The high magnetic moments obtained for these complexes are explained in terms of a spin–orbit coupling. Magnetic measurements show a deviation from the Curie–Weiss law at low temperatures. Two magnetic orderings were observed, at high temperatures an antiferromagnetic coupling was found, below 20 K a change to a weak ferromagnetic coupling was observed. Isothermal magnetic measurements at low temperature show a weak hysteresis, which was confirmed by the small coercive field found at low temperature. In addition, for the first time an explanation for the observation that these cobalt(II) compounds are very stable towards oxidation is offered in terms of the high redox potential values obtained for 1, 1a, and 2.     

SQUID measurement of metalloprotein magnetization. New methods applied to the nitrogenase proteins.

New techniques have been developed to exploit the sensitivity of a commercial SQUID susceptometer in the study of the magnetization of metalloproteins. Previous studies have ignored both the slow relaxation (hours) of spin I = 1/2 nuclei and residual ferromagnetic impurities in sample holders. These potential sources of noise were at or below the sensitivity of previous instruments. With these noise sources under control, one can now decrease the protein concentration by a factor of ten. In addition careful characterization of the frozen magnetization sample, including the use of a multi-instrument holder for combined study of the magnetization sample with M?ssbauer spectroscopy, is required for reliable interpretation of the data in the face of paramagnetic impurities common to metalloprotein samples. Many previous magnetic studies of metalloproteins have been carried out in the Curie region. Saturation magnetization studies down to 1.8 K and up to 5 T can determine zero-field splitting parameters in addition to the spin and exchange coupling parameters measured in previous studies at lower fields and higher temperatures. Applications of these techniques to the study of the nitrogenase proteins of Azotobacter vinelandii are presented as examples.     

Electron paramagnetic resonance study of the migratory ant Pachycondyla marginata abdomens

Electron paramagnetic resonance was used to investigate the magnetic material present in abdomens of Pachycondyla marginata ants. A g congruent with 4.3 resonance of high-spin ferric ions and a very narrow g congruent with 2 line are observed. Two principal resonance broad lines, one with g > 4.5 (LF) and the other in the region of g congruent with 2 (HF), were associated with the biomineralization process. The resonance field shift between these two lines, HF and LF, associated with magnetic nanoparticles indicates the presence of cluster structures containing on average three single units of magnetite-based nanoparticles. Analysis of the temperature dependence of the HF resonance linewidths supports the model picture of isolated magnetite nanostructures of approximately 13 nm in diameter with a magnetic energy of 544 K. These particles are shown to present a superparamagnetic behavior at room temperature. The use of these superparamagnetic particle properties for the magnetoreception process of the ants is suggested.     

Dynamics of the First‐Order Metamagnetic Transition in Magnetocaloric La(Fe,Si)13: Reducing Hysteresis

The influence of dynamics and sample shape on the magnetic hysteresis in first‐order magnetocaloric metamagnetic LaFe_13–xSi_x with x = 1.4 is studied. In solid‐state magnetic cooling, reducing magnetic and thermal hysteresis is critical for refrigeration cycle efficiency. From magnetization measurements, it is found that the fast field‐rate dependence of the hysteresis can be attributed to extrinsic heating directly related to the thickness of the sample and the thermal contact with the bath. If the field is paused partway through the transition, the subsequent relaxation is strongly dependent on shape due to both demagnetizing fields and thermal equilibration; magnetic coupling between adjacent sample fragments can also be significant. Judicious shaping of the sample can both increase the onset field of the ferromagnetic–paramagnetic (FM–PM) transition but have little effect on the PM–FM onset, suggesting a route to engineer the hysteresis width by appropriate design. In the field‐paused state, the relaxation from one phase to the other slows with increasing temperature, implying that the process is neither thermally activated or athermal; comparison with the temperature dependence of the latent heat strongly suggests that the dynamics reflect the intrinsic free energy difference between the two phases.     

Magnetically aligned phospholipid bilayers with positive ordering: a new model membrane system.

A stable smectic phospholipid bilayer phase aligned with the director parallel to the magnetic field can be generated by the addition of certain trivalent paramagnetic lanthanide ions to a bicellar solution of dimyristoylphosphatidylcholine (DMPC) and dihexanoylphosphatidylcholine (DHPC) in water. Suitable lanthanide ions are those with positive anisotropy of their magnetic susceptibility, namely Eu3+, Er3+, Tm3+, and Yb3+. For samples doped with Tm3+, this phase extends over a wide range of Tm3+ concentrations (6-40 mM) and temperatures (35-90 degrees C) and appears to undergo a transition from a fluid nematic discotic to a fluid, but highly ordered, smectic phase at a temperature that depends on the thulium concentration. As a membrane mimetic, these new, positively ordered phospholipid phases have high potential for structural studies using a variety of techniques such as magnetic resonance (EMR and NMR), small-angle x-ray and neutron diffraction, as well as optical and infrared spectroscopy.     

Electron paramagnetic resonance study of honeybee Apis mellifera abdomens

Although ferromagnetic material has been detected in Apis mellfera abdomens and identified as suitable for magnetic reception, physical and magnetic properties of these particles are still lacking. Electron paramagnetic resonance is used to study different magnetic materials in these abdomens. At least four iron structures are identified: isolated Fe3+ ions, amorphous FeOOH, isolated magnetite nanoparticles of about 3 x 10(2) nm3 and 10(3) nm3 volumes, depending on the hydration degree of the sample, and aggregates of these particles. A low-temperature transition (52-91 K) was observed and the temperature dependence of the magnetic anisotropy constant of those particles was determined. These results imply that biomineralized magnetites are distinct from inorganic particles and the parameters presented are relevant for the refinement of magnetoreception models in honeybees.     

Magnetic studies of Chromatium flavocytochrome C552. A mechanism for heme-flavin interaction.

Electron paramagnetic resonance and magnetic susceptibility studies of Chromatium flavocytochrome C552 and its diheme flavin-free subunit at temperatures below 45 degrees K are reported. The results show that in the intact protein and the subunit the two low-spin (S = 1/2) heme irons are distinguishable, giving rise to separate EPR signals. In the intact protein only, one of the heme irons exists in two different low spin environments in the pH range 5.5 to 10.5, while the other remains in a constant environment. Factors influencing the variable heme iron environment also influence flavin reactivity, indicating the existence of a mechanism for heme-flavin interaction.     

Study of the mechanism for fast ion heating in the GOL-3 multimirror magnetic confinement system

Results are presented from experimental studies of ion heating in the GOL-3 device. The experiments were carried out in a multimirror configuration with a local magnetic well. It was found that, during the injection of a relativistic electron beam, a decrease in the local density of the beam in a magnetic well, which is proportional to the decrease in the strength of the longitudinal magnetic field, results in the formation of a short plasma region with a low electron temperature. The measured longitudinal gradient of the plasma pressure corresponds to an electron temperature gradient of ～2–3 keV/m. Axially nonuniform heating of the plasma electrons gives rise to the macroscopic motion of the plasma along the magnetic field in each cell of the multimirror confinement system. The mixing of the counterpropagating plasma flows inside each cell leads to fast ion heating. Under the given experimental conditions, the efficiency of this heating mechanism is higher than that due to binary electron-ion collisions. The collision and mixing of the counterpropagating plasma flows is accompanied by a neutron and γ-ray burst. The measured ratio of the plasma pressure to the vacuum magnetic field pressure in these experiments reaches 0.2.     

A comparative study of the effect of cholesterol on bicelle model membranes using X-band and Q-band EPR spectroscopy

X-band and Q-band electron paramagnetic resonance (EPR) spectroscopic techniques were used to investigate the structure and dynamics of cholesterol containing phospholipid bicelles based upon molecular order parameters (S_mol), orientational dependent hyperfine splittings and line shape analysis of the corresponding EPR spectra. The nitroxide spin-label 3-β-doxyl-5-α-cholestane (cholestane) was incorporated into DMPC/DHPC bicelles to report the alignment of bicelles in the static magnetic field. The influence of cholesterol on aligned phospholipid bicelles in terms of ordering, the ease of alignment, phase transition temperature have been studied comparatively at X-band and Q-band. At a magnetic field of 1.25 T (Q-band), bicelles with 20 mol% cholesterol aligned at a much lower temperature (313 K), when compared to 318 K at a 0.35 T field strength for X-band, showed better hyperfine splitting values (18.29 G at X-band vs. 18.55 G at Q-band for perpendicular alignment and 8.25 G at X-band vs. 7.83 G at Q-band for the parallel alignment at 318 K) and have greater molecular order parameters (0.76 at X-band vs. 0.86 at Q-band at 318 K). Increasing cholesterol content increased the bicelle ordering, the bicelle-alignment temperature and the gel to liquid crystalline phase transition temperature. We observed that Q-band is more effective than X-band for studying aligned bicelles, because it yielded a higher ordered bicelle system for EPR spectroscopic studies.     

Analysis of the distribution of flowing erythrocytes in a model vessel under an inhomogeneous magnetic field

Changes in the distribution of flowing erythrocytes in a straight cylinder were studied under an inhomogeneous magnetic field. The magnetic field was applied perpendicular to a cylinder, which had a 90° side vessel at the end (oriented towards the magnetic field) to detect changes in the erythrocyte distribution within the cylinder. (1) The attraction of paramagnetic erythrocytes by the magnetic field was demonstrated by an increase in the concentration (or number) of erythrocytes drawn into the side vessel. The flow of diamagnetic, oxygenated erythrocytes was unaffected. (2) The degree of attraction of the paramagnetic erythrocytes was proportional to ``(magnetic susceptibility)' and to ``(magnetic flux density) × (magnetic field gradient)' up to 10 T²/m, but it saturated at high magnetic field. The onset of the saturation depended on the magnetic susceptibility of the erythrocytes. (3) The degree of attraction depended on the hematocrit of the flowing erythrocyte suspension, with a maximum value at a low hematocrit. These phenomena are explained on the basis of the balance between the paramagnetic attractive force of the magnetic field and the collision rate between erythrocytes. Received: 2 May 1996 / Accepted: 1 July 1996     

Paramagnetic artifact and safety criteria for human brain mapping

Biological effects of magnetic field and their safety criteria, especially effects of gradient magnetic field on the cerebral and pulmonary circulation during functional brain mapping are still unclear. Here we estimated that magnetically induced artifacts for the blood oxygenation level- and flow- based functional magnetic resonance imaging are less than 0.1%, and disturbance in the pulmonary circulation is less than 1.3% even if the field strength of magnetic resonance system is risen up to 10 tesla. These paramagnetic effects are considered to be small and harmless during human brain mapping.     

Functional brain mapping by blood oxygenation level-dependent contrast magnetic resonance imaging. A comparison of signal characteristics with a biophysical model.

It recently has been demonstrated that magnetic resonance imaging can be used to map changes in brain hemodynamics produced by human mental operations. One method under development relies on blood oxygenation level-dependent (BOLD) contrast: a change in the signal strength of brain water protons produced by the paramagnetic effects of venous blood deoxyhemoglobin. Here we discuss the basic quantitative features of the observed BOLD-based signal changes, including the signal amplitude and its magnetic field dependence and dynamic effects such as a pronounced oscillatory pattern that is induced in the signal from primary visual cortex during photic stimulation experiments. The observed features are compared with the results of Monte Carlo simulations of water proton intravoxel phase dispersion produced by local field gradients generated by paramagnetic deoxyhemoglobin in nearby venous blood vessels. The simulations suggest that the effect of water molecule diffusion is strong for the case of blood capillaries, but, for larger venous blood vessels, water diffusion is not an important determinant of deoxyhemoglobin-induced signal dephasing. We provide an expression for the apparent in-plane relaxation rate constant (R2*) in terms of the main magnetic field strength, the degree of the oxygenation of the venous blood, the venous blood volume fraction in the tissue, and the size of the blood vessel.     

Physical interactions of static magnetic fields with living tissues

Clinical magnetic resonance imaging (MRI) was introduced in the early 1980s and has become a widely accepted and heavily utilized medical technology. This technique requires that the patients being studied be exposed to an intense magnetic field of a strength not previously encountered on a wide scale by humans. Nonetheless, the technique has proved to be very safe and the vast majority of the scans have been performed without any evidence of injury to the patient. In this article the history of proposed interactions of magnetic fields with human tissues is briefly reviewed and the predictions of electromagnetic theory on the nature and strength of these interactions are described. The physical basis of the relative weakness of these interactions is attributed to the very low magnetic susceptibility of human tissues and the lack of any substantial amount of ferromagnetic material normally occurring in these tissues. The presence of ferromagnetic foreign bodies within patients, or in the vicinity of the scanner, represents a very great hazard that must be scrupulously avoided. As technology and experience advance, ever stronger magnetic field strengths are being brought into service to improve the capabilities of this imaging technology and the benefits to patients. It is imperative that vigilance be maintained as these higher field strengths are introduced into clinical practice to assure that the high degree of patient safety that has been associated with MRI is maintained.     

Dose-response relationship between body temperature and birth defects in radiofrequency-irradiated rats

Five groups of pregnant Sprague-Dawley rats were irradiated for 10-40 min on gestation day 9 in a 27.12-MHz radiofrequency field at a magnetic field strength of 55 A/m and an electric field strength of 300 V/m. The specific absorption rate was 10.8 +/- 0.3 W/kg. Exposures were terminated after the rat's colonic temperature reached 41.0 degrees C, 41.5 degrees C, 42.0 degrees C, 42.5 degrees C, or 43.0 degrees C. A control group was sham irradiated at 0 A/m and 0 V/m on gestation day 9, whereas a second control group was untreated. The incidence of both birth defects and prenatal death was directly related to maternal body temperature once a temperature threshold was exceeded. The temperature threshold for both types of effects was approximately 41.5 degrees C. A few pregnant rats died after exposure to 43.0 degrees C, and higher temperatures were nearly always lethal.     

Influence of quaternary structure of the globin on thermal spin equilibria in different methemoglobin derivatives.

We have measured the paramagnetic susceptibilities of sperm whale azide metmyoglobin and of carp azide, thiocyanate, and nitrite methemoglobin in the quaternary oxy (R) and deoxy (T) structures between about 300 and 90 K, using a new sensitive superconducting magnetometer. We have also measured the pressure dependence of the high- and low-spin optical absorption bands of azide metmyoglobin and of carp azide methemoglobin in the R and T structures between 1 and 2000-4000 atmospheres. At low temperatures all the derivatives show normal Curie behavior, but above 200-250 K this is reversed, so that a thermal spin equilibrium is set up and the paramagnetic susceptibilities rise steeply with rising temperature. At all temperatures the effective magnetic moments in the T structure are higher than in the R structure. The magnetic data for azide methemoglobin have been subjected to detailed analysis. Below 250 K the magnetic moment in the R structure is 1.98 microB, characteristic of pure low spin, but that in the T structure is 2.80 microB, suggestive of a random mixture of high- and low-spin centers which have become frozen in by the immobility of the surrounding protein. Comparison of the thermal spin equilibria above 250 K shows that in the T structure the equilibrium is biased toward higher spin by the equivalent of about 1 kcal/mol relative to the R structure. Hydrostatic pressure reduces the optical density of the high-spin band at 630 nm and increases that of the low-spin bands at 541 and 573 nm. We have calibrated the optical density of the band at 630 nm against the measured paramagnetic susceptibilities of sperm whale azide metmyoglobin and carp azide methemoglobin in the R and T structures and have used this calibration to determine the dependence of the spin equilibria on hydrostatic pressure; this has allowed us to calculate the volume contraction associated with the transition from the fully high to the fully low-spin state. This amounts to -6.7 and -13.3 mL/mol heme for carp azide methemoglobins in the R and T structures, respectively, and to -12.5 mL/mol heme for azide metmyoglobin. These volume contractions are larger than those of about -4 mL/mol Fe found in synthetic iron chelates. Apparently stereochemical changes of the globin surrounding the heme also contribute to the volume changes; these must be larger in the T than in the R structure. The significance of these observations for the mechanism of heme-heme interaction is discussed.