Magnetic susceptibility measurements of deoxygenated hemoglobins and isolated chains

de la Fuente J, Fratila RM, Grazú V, Gutiérrez L. Iron Speciation in Animal Tissues Using AC Magnetic Susceptibility Measurements: Quantification of Magnetic Nanoparticles, Ferritin, and Other Iron-Containing Species

The simultaneous detection and quantification of several iron-containing species in biological matrices is a challenging issue. Especially in the frame of studies using magnetic nanoparticles for biomedical applications, no gold-standard technique has been described yet and combinations of different techniques are generally used. In this work, AC magnetic susceptibility measurements are used to analyze different organs from an animal model that received a single intratumor administration of magnetic nanoparticles. The protocol used for the quantification of iron associated with the magnetic nanoparticles is carefully described, including the description of the preparation of several calibration standard samples of nanoparticle suspensions with different degrees of dipolar interactions. The details for the quantitative analysis of other endogenous iron-containing species such as ferritin or hemoglobin are also described. Among the advantages of this technique are that tissue sample preparation is minimal and that large amounts of tissue can be characterized each time (up to hundreds of milligrams). In addition, the very high specificity of the magnetic measurements allows for tracking of the nanoparticle transformations. Furthermore, the high sensitivity of the instrumentation results in very low limits of detection for some of the iron-containing species. Therefore, the presented technique is an extremely valuable tool to track iron oxide magnetic nanoparticles in samples of biological origin.

MR venography of the fetal brain using susceptibility weighted imaging

PURPOSE

To evaluate the feasibility of performing fetal brain magnetic resonance venography using susceptibility weighted imaging (SWI).

MATERIALS AND METHODS

After obtaining informed consent, pregnant women in the second and third trimester were imaged using a modified SWI sequence. Fetal SWI acquisition was repeated when fetal or maternal motion was encountered. The median and maximum number of times an SWI sequence was repeated was four and six respectively. All SWI image data were systematically evaluated by a pediatric neuroradiologist for image quality using an ordinal scoring scheme: 1. diagnostic; 2. diagnostic with artifacts; and 3. nondiagnostic. The best score in an individual fetus was used for further statistical analysis. Visibility of venous vasculature was also scored using a dichotomous variable. A subset of SWI data was re-evaluated by the first and independently by a second pediatric neuroradiologist. Kappa coefficients were computed to assess intra-rater and inter-rater reliability.

RESULTS

SWI image data from a total of 22 fetuses were analyzed. Median gestational age and interquartile range of the fetuses imaged were 32 (29.9-34.9) weeks. In 68.2% of the cases (n = 15), there was no artifact; 22.7% (n = 5) had minor artifacts and 9.1% (n = 2) of the data was of nondiagnostic quality. Cerebral venous vasculature was visible in 86.4% (n = 19) of the cases. Substantial agreement (Kappa = 0.73; 95% confidence interval 0.44-1.00)) was observed for intra-rater reliability and moderate agreement (Kappa = 0.48; 95% confidence interval 0.19-0.77) was observed for inter-rater reliability.

CONCLUSION

It is feasible to perform fetal brain venography in humans using SWI.

Investigating the magnetic susceptibility properties of fresh human blood for noninvasive oxygen saturation quantification

Quantification of blood oxygen saturation on the basis of a measurement of its magnetic susceptibility demands knowledge of the difference in volume susceptibility between fully oxygenated and fully deoxygenated blood (Δχ(do) ). However, two very different values of Δχ(do) are currently in use. In this work we measured Δχ(do) as well as the susceptibility of oxygenated blood relative to water, Δχ(oxy) , by MR susceptometry in samples of freshly drawn human blood oxygenated to various levels, from 6 to 98% as determined by blood gas analysis. Regression analysis yielded 0.273 ± 0.006 and -0.008 ± 0.003 ppm (cgs) respectively, for Δχ(do) and Δχ(oxy) , in excellent agreement with previous work by Spees et al. (Magn Reson Med 2001;45:533-542).

Room-temperature magnetic properties of oxy- and carbonmonoxyhemoglobin

The magnetic susceptibility and the density of human oxy-(HbO(2)) and carbonmonoxyhemoglobin (HbCO) solutions of various concentrations have been measured at room temperature, with pure water used as a calibrant. Solutions of unstripped and stripped HbO(2) at pH 7.2 in unbuffered water solvent were always found to be less diamagnetic than pure water, whereas solutions of HbCO in identical conditions were always found to be more diamagnetic than pure water. After correcting for concentration-dependent density changes and assuming the HbCO samples to be fully diamagnetic, the paramagnetic reduction of the diamagnetic susceptibility of HbO(2) corresponds to a molar susceptibility per heme (chi(M) (heme)) of 2460 +/- 600 x 10(-6) cgs/mol.

Bioinorganic transformations of liver iron deposits observed by tissue magnetic characterisation in a rat model

The magnetic properties and the ultrastructure, with special emphasis on the nanometric range, of liver tissues in an iron overload rat model have been investigated. The tissues of the animals, sacrificed at different times after a single iron dextran injection, have been characterised by magnetic AC susceptibility measurements together with transmission electron microscopy (TEM) and selected area electron diffraction (SAED) as helping techniques. It has been observed that few days after the iron administration the liver contains at least two iron species: (i) akaganéite nanoparticles, coming from iron dextran and (ii) ferrihydrite nanoparticles corresponding to ferritin. The magnetic susceptibility of the tissues depends not only on the elemental iron content but also on its distribution among chemical species, and varies in a remarkable regular manner as a function of the elapsed time since the iron administration. The results are of relevance with respect to non-invasive techniques for liver iron determination, directly or indirectly based on the magnetic susceptibility of the tissues, as biomagnetic liver susceptometry (BLS) and magnetic resonance (MRI) image treatment.

Electronic configuration assignment and the importance of low-lying excited states in high-spin imidazole-ligated iron(II) porphyrinates

The synthesis and characterization of six new high-spin deoxymyoglobin models (imidazole(tetraarylporphyrinato)iron(II)) are described. These have been intensively studied by temperature-dependent Mossbauer spectroscopy from 295 to 4.2 K. All complexes show a strong temperature dependence for the quadrupole splitting consistent with low-lying excited states of the same or lower multiplicity. An analysis of the data obtained in applied magnetic fields leads to the assignment of the sign of the quadrupole splitting. All model compounds as well as those of deoxymyoglobin and deoxyhemoglobin, previously studied, have a negative sign for the quadrupole splitting. Although not previously predicted, this experimental observation leads to the assignment of the ground-state electronic configuration for all high-spin imidazole-ligated iron(II) porphyrinates as (d(xz)())(2)(d(yz)())(1)(d(xy)())(1)(d(z)()()2)(1)(d(x)()()2(-)(y)()()2)(1). This is a distinctly different ground-state electronic configuration from other high-spin iron(II) porphyrinates; differences in structural details for the two classes of high-spin complexes are also discussed. The apparent anomaly of differing signs for the zero-field splitting constant between previously studied model complexes and the heme proteins is addressed; the difference appears to result from the fact that the assumptions used in the spin Hamiltonian approach that has been applied to these complexes are not adequately satisfied. Structures of four of the new five-coordinate species have been determined. Core conformations in these derivatives show variation, but these and previously studied compounds reveal a limited number of conformational patterns. The bond lengths and other geometrical parameters such as porphyrin core size and iron out-of-plane displacement support a high-spin state assignment for the iron(II).

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.

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

We have carried out analysis of the electronic level scheme of the high-spin ferrous hemoproteins by simultaneous fit of the adjustable parameters of a 4-term theoretical model to low-temperature magnetic circular dichroism (MCD), room temperature absorption spectra and available magnetic susceptibility and or Mössbauer data of myoglobin, horseradish peroxidase and cytochrome P450. The high reliability of the ligand field parameter values obtained for deoxymyoglobin is confirmed by good agreement between the predicted and observed magnetic field dependences of MCD and magnetization not used in the fit procedure. In addition, an energy gap between the ground and first excited singlets, estimated to be 4.2 cm-1, agrees well with the value of approximately 4 cm-1 derived from the far-infrared magnetic resonance. Our computer and explicit theoretical analyses give strong evidence that large distinctions in the shape, intensity and temperature behaviour of the MCD of Mb and HRP from those of cytochrome P450 can be described only if the ground manifold in these proteins is 5E eta and 5B2, respectively. The changes in relative energies of the one-electron 3d-orbitals on substitution of an imidazole of histidine for a sulphur anion of cysteine as a protein-derived heme iron ligand are rationalized by the lower ionization potential of the negatively charged sulphur ligand and the higher pi-orbital overlap of its lone pair orbitals with the iron d pi-orbitals compared to the imidazole ligand.

Comparison of T2 relaxation in blood, brain, and ferritin

T2 was measured in samples of human blood and monkey brain over a field range of 0.02-1.5 Tesla, with variable interecho times, and was compared with previous data on ferritin solutions (taken with the same apparatus). 1/T2 in deoxygenated blood increased quadratically with field strength, as noted previously, but in brain gray matter the increase was linear, as also was the case in ferritin solution. In both deoxygenated blood and gray matter, 1/T2 increased with interecho time, but appeared to level off at times around 50 msec, as expected from the theory of diffusion through magnetic gradients. Diffusion times estimated by using the chemical exchange approximation were 3.4 msec for deoxygenated blood and 5.7 msec for the globus pallidus. The quadratic field dependence in blood is consistent with this same theory, but the linear dependence in brain tissue and in ferritin solutions remains unexplained.

Proton transverse nuclear magnetic relaxation in oxidized blood: a numerical approach

When red blood cells are deoxygenated, hemoglobin, which is then transformed into deoxyhemoglobin or methemoglobin, becomes paramagnetic. The transverse nuclear magnetic relaxation rate of water protons is considerably enhanced by this chemical transformation. A general agreement exists about the origin of the phenomenon--local field inhomogeneities induced by paramagnetic centers randomly distributed within the cell--but the localization of the region that dominates the relaxation is unclear. We addressed this problem with a computer simulation devoted to the determination of transverse magnetic relaxation of water protons in the presence of superparamagnetic MRI contrast agent candidates. The simulation confirms an earlier experimental result that shares equitably the responsibility for the observed relaxation between intracellular and extracellular water.

On the origin of paramagnetic inhomogeneity effects in blood

Hydrogen, sodium, and fluorine (added F-) NMR spectra of venous and oxygenated blood were measured. The fluorine resonance was seen as a single peak in both samples, and all three resonances exhibited the same deoxy-oxy shift. Because F- exchanges slowly across the red cell membrane, and because sodium is 95% extracellular, these results suggest that the intra-extracellular field difference delta B is less than 0.1 ppm. A small value of delta B tends to rule out transmembrane exchange as an important contributor to relaxation in MRI of blood and hematomas. However, the broadening of the resonances with deoxygenation, by 0.3-0.4 ppm, indicates that both intra- and extracellular gradients are of comparable and sufficient magnitude to produce the T2-weighted hypointensity seen in clinical magnetic resonance images of hematomas at high fields.

Integer-spin electron paramagnetic resonance of iron proteins

A quantitative interpretation is presented for EPR spectra from integer-spin metal centers having large zero-field splittings. Integer-spin, or non-Kramers, centers are common in metalloproteins and many give EPR signals, but a quantitative understanding has been lacking until now. Heterogeneity of the metal's local environment will result in a significant spread in zero-field splittings and in broadened EPR signals. Using the spin Hamiltonian Hs = S.D.S + beta S.g.B and some simple assumptions about the nature of the zero-field parameter distributions, a lineshape model was devised which allows accurate simulation of single crystal and frozen solution spectra. The model was tested on single crystals of magnetically dilute ferrous fluosilicate. Data and analyses from proteins and active-site models are presented with the microwave field B1 either parallel or perpendicular to B. Quantitative agreement of observed and predicted signal intensities is found for the two B1 orientations. Methods of spin quantitation are given and are shown to predict an unknown concentration relative to a standard with known concentration. The fact that the standard may be either a non-Kramers or a Kramers center is further proof of the model's validity. The magnitude of the splitting in zero magnetic field is of critical importance; it affects not only the chance of signal observation, but also the quantitation accuracy. Experiments taken at microwave frequencies of 9 and 35 GHz demonstrate the need for high-frequency data as only a fraction of the molecules give signals at 9 GHz.

Effects of an inhomogeneous magnetic field on flowing erythrocytes

Effects of an inhomogeneous magnetic field on narrow erythrocyte streams in a wide and transparent laminar buffer flow were studied. The stream line of erythrocytes containing paramagnetic hemoglobin showed distinct displacement toward the stronger magnetic field. The displacement increased in the order, oxygenated erythrocytes (no displacement), erythrocytes containing cyanomethemoglobin, deoxygenated erythrocytes, erythrocytes containing methemoglobin in the high spin state; more precisely the displacement was proportional to the square of the paramagnetic moment of hemoglobin contained in the erythrocytes. In addition, the displacement was proportional to the product of the magnetic flux density and its gradient, and approximately proportional to the hematocrit of the flowing-erythrocyte suspension, and was much larger than that calculated for a single erythrocyte. These phenomena could be successfully interpreted by the interaction of paramagnetic erythrocytes with the inhomogeneous magnetic field, the resistance force (Stokes Law) from the bulk water, and the hydrodynamic interaction between erythrocytes.

Drift of an erythrocyte flow line due to the magnetic field

Drifts of erythrocyte-flow lines due to inhomogeneous magnetic field in a laminar flow in a buffer solution are shown for the first time, and are interpreted as being due to the paramagnetism of hemoglobins included in the erythrocytes. The drifts were dependent on the hematocrit of the flowing erythrocyte suspension.

Magnetic and spectral properties of carp carbonmonoxyhemoglobin. Competitive effects of chloride ions and inositol hexakisphosphate

We have extended our studies on the magnetic properties of carp carbonmonoxyhemoglobin and the dependence of these properties upon solution variables. Using an improved version of the superconducting magnetometer, we have found that the magnetic susceptibility of carp carbonmonoxyhemoglobin is sensitive to both inositol hexakisphosphate and chloride ion. The dependence upon chloride ion concentration is complex. At relatively low concentrations this anion reverses the effect of inositol hexakisphosphate, restoring paramagnetism. At higher chloride concentrations the protein is converted to a roughly diamagnetic state in the absence of inositol hexakisphosphate. Along with these susceptibility studies, we have examined the effects of these anions on other properties of carp carbonmonoxyhemoglobin. The positions of the Soret bands of human and carp methemoglobin derivatives are correlated with spin state; changes in the magnetic susceptibility of carbonmonoxyhemoglobin are similarly associated with alterations in this spectral band. We have also examined the effects of these anions on the proton nuclear magnetic resonance spectrum of carp carbonmonoxyhemoglobin. Both chloride and inositol hexakisphosphate alter the position of the proton resonances in the ring-current-shifted region of the spectrum.

Oxygenation dependence of the transverse relaxation time of water protons in whole blood at high field

At high and medium magnetic field, the transverse NMR relaxation rate (T-1(2)) of water proteins in blood is determined predominantly by the oxygenation state of haemoglobin. T-1(2) depends quadratically on the field strength and on the proportion of haemoglobin that is deoxygenated. Deoxygenation increases the volume magnetic susceptibility within the erythrocytes and thus creates local field gradients around these cells. From volume susceptibility measurements and the dependence of T-1(2) on the pulse rate in the Carr-Purcell-Meiboom-Gill experiment, we show that the increase in T-1(2) with increasing blood deoxygenation arises from diffusion of water through these field gradients.

NMR investigation of the electric properties and magnetic states of hemes and hemoproteins : the S = 2 state in ferrous porphyrins

The proton NMR data of a synthetic model of deoxy-myoglobin and hemoglobin show one that:(i) the ground state of the high spin iron (II) complex must be described by at least two electronic levels separated by ca.200 cm-1 and resulting from the removal of the degeneracy of the 5E level; (ii) this in-plane rhombic distorsion is directed towards the methine bridges of the porphyrin ring. It is proposed that the rhombicity results from specific interactions of the lambda orbitals of the axial base with the iron d lambda orbitals.

Variability of the magnetic moment of carbon monoxide hemoglobin from carp

Deionized carp carbon monoxide hemoglobin in distilled water or in bis(2-hydroxyethyl)imino-tris(hydroxymethyl)methane or Tris buffer exhibits a slight but significant paramagnetism. This is most clearly demonstrated by the decrease in this paramagnetism that is caused by the addition of inositol hexaphosphate to this protein in the former buffer at pH 6.3-6.4. No such effect is seen when inositol hexaphosphate is added to carp cyanomethemoglobin, demonstrating that the change observed with carbon monoxide derivative is not due to a modification in the diamagnetic properties of the protein.

Jahn-Teller effect in biomolecules

The role played by the Jahn-Teller effect for a correct interpretation of the spectroscopic and structural data of some biological systems is stressed and a short review of the investigations so far performed is presented. Afterwards, on the basis of the angular overlap model, several expressions are given for the coupling constants of the transition metal ions, which mainly occur in biomolecules (Fe, Co, Cu, Mo) in order to have an approximate evaluation of the Jahn-Teller effect magnitude. The above expressions are then applied to obtain the vibronic coupling constants of the deoxygenated hemoglobin and myoglobin. The results obtained are compared with the predictions previously made by the author on these systems.

High magnetic field Mössbauer studies of deoxymyoglobin, deoxyhemoglobin, and synthetic analogues

Mössbauer spectra of deoxymyoglobin, deoxyhemoglobin, and the synthetic analogues, iron (II) 2-methylimidazole meso-tetraphenylporphyrin, and iron (II) 1,2-dimethylimidazole meso-tetraphenylporphyrin have been observed in high magnetic fields and over a wide range of temperature. At temperatures greater than 20 K all materials exhibit remarkably similar spectra, with anisotropic internal magnetic fields decreasing as 1/T. All have negative quadrupole interaction, and both this and the magnetic anisotropy imply that the orbital of the odd electron is prolate in the ground quintet, with little unquenched orbital angular momentum. At 4.2 K the spectra differ, suggesting different detailed structure within the quintet. In contrast to the proteins, the 2-methyl model exhibits spectra at 4.2 K which imply that the lowest spin state has high susceptibility in a single direction.

The spin-state transition of the hemochrome non-equilibrium conformation in partially reduced human methemoglobin. A pulse-radiolysis study of aqueous-methanol solutions of methemoglobin

The effect of external parameters on the relaxation process of the hemochrome-type non-equilibrium conformation in partially reduced methemoglobin has been investigated. The relaxation of the intermediate ferrous low-spin state to the high-spin equilibrium conformation of hemoglobin appears to be facilitated particularly by protons and phosphate ions. In addition to studying the spin-state transition in aquomethemoglobin we have also studied it in complexes of the heme group in methemoglobin with fluoride, azide and cyanide anions.

Investigation of the electronic term scheme of deoxygenated human haemoglobin by a least squares fit procedure using simultaneously magnetic susceptibility and Mössbauer data

The calculation of the magnetic susceptibility from a published term scheme for the ferrous iron in deoxygenated human haemoglobin is discussed and a procedure for the simultaneous least squares fit of susceptibility and Mössbauer data is presented. The application of this procedure to the appropriate measurements on human haemoglobin leads to a rearrangement of the low lying electronic levels of the iron. The term schemes received as results of two different sets of susceptibility data used in combination with one set of Mössbauer data overlap with their error bars. The obtained level scheme of the Fe is correlated with the distance of the iron atom from the haem plane and the distance Fe-HIS F8, and some biological implications of these correlations are discussed.