Magnetic anisotropy and the orientation of retinal rods in a homogeneous magnetic field

Signal compartments in ultra-high field multi-echo gradient echo MRI reflect underlying tissue microstructure in the brain

Gradient recalled echo magnetic resonance imaging (GRE-MRI) at ultra-high field holds great promise for new contrast mechanisms and delineation of putative tissue compartments that contribute to the multi-echo GRE-MRI signal may aid structural characterization. Several studies have adopted the three water-pool compartment model to study white matter brain regions, associating individual compartments with myelin, axonal and extracellular water. However, the number and identifiability of GRE-MRI signal compartments has not been fully explored. We undertook this task for human brain imaging data. Multiple echo time GRE-MRI data were acquired in five healthy participants, specific anatomical structures were segmented in each dataset (substantia nigra, caudate, insula, putamen, thalamus, fornix, internal capsule, corpus callosum and cerebrospinal fluid), and the signal fitted with models comprising one to six signal compartments using a complex-valued plane wave formulation. Information criteria and cluster analysis methods were used to ascertain the number of distinct compartments within the signal from each structure and to determine their respective frequency shifts. We identified five principal signal compartments with different relative contributions to each structure's signal. Voxel-based maps of the volume fraction of each of these compartments were generated and demonstrated spatial correlation with brain anatomy.

Three-dimensional analysis, modeling, and simulation of the effect of static magnetic fields on neurons

The effect of static magnetic fields on neuron function has been studied. None of the possible explanations are decisive or fully consistent with evidence in the literature. Therefore, the purpose of this paper is to examine the different possibilities, for the first time, through a three-dimensional modeling strategy in an effort to find out which possibility or combination is effective on cell function. A full-wave analysis was employed to simulate various effects of magnetic fields. The possibilities included force exerted on mobile ions, magnetophoretic force exerted on ions with permeability different from intracellular or extracellular fluids, magnetophoretic force exerted on sensor proteins in ion channels, and magnetophoretic pressure exerted on the membrane and spatial rotation of anisotropic diamagnetic particles. According to the simulations, the last two possibilities are more likely to be effective; therefore, their corresponding equations in this article were formulated to verify the results of the literature experiments. Bioelectromagnetics. 38:128-136, 2017. © 2016 Wiley Periodicals, Inc.

A torque balance measurement of anisotropy of the magnetic susceptibility in white matter

PURPOSE

Recent MRI studies have suggested that the magnetic susceptibility of white matter (WM) in the human brain is anisotropic, providing a new contrast mechanism for the visualization of fiber bundles and allowing the extraction of cellular compartment-specific information. This study provides an independent confirmation and quantification of this anisotropy.

METHODS

Anisotropic magnetic susceptibility results in a torque exerted on WM when placed in a uniform magnetic field, tending to align the WM fibers with the field. To quantify the effect, excised spinal cord samples were placed in a torque balance inside the magnet of a 7 T MRI system and the magnetic torque was measured as function of orientation.

RESULTS

All tissue samples (n = 5) showed orienting effects, confirming the presence of anisotropic susceptibility. Analysis of the magnetic torque resulted in reproducible values for the WM volume anisotropy that ranged from 13.6 to 19.2 ppb.

CONCLUSION

The independently determined anisotropy values confirm estimates inferred from MRI experiments and validate the use of anisotropy to extract novel information about brain fiber structure and myelination.

MR susceptibility imaging

This work reviews recent developments in the use of magnetic susceptibility contrast for human MRI, with a focus on the study of brain anatomy. The increase in susceptibility contrast with modern high field scanners has led to novel applications and insights into the sources and mechanism contributing to this contrast in brain tissues. Dedicated experiments have demonstrated that in most of healthy brain, iron and myelin dominate tissue susceptibility variations, although their relative contribution varies substantially. Local variations in these compounds can affect both amplitude and frequency of the MRI signal. In white matter, the myelin sheath introduces an anisotropic susceptibility that has distinct effects on the water compartments inside the axons, between the myelin sheath, and the axonal space, and renders their signals dependent on the angle between the axon and the magnetic field. This offers opportunities to derive tissue properties specific to these cellular compartments.

Quantitative imaging of intrinsic magnetic tissue properties using MRI signal phase: an approach to in vivo brain iron metabolism?

Quantitative susceptibility mapping (QSM) based on gradient echo (GRE) magnetic resonance phase data is a novel technique for non-invasive assessment of magnetic tissue susceptibility differences. The method is expected to be an important means to determine iron distributions in vivo and may, thus, be instrumental for elucidating the physiological role of iron and disease-related iron concentration changes associated with various neurological and psychiatric disorders. This study introduces a framework for QSM and demonstrates calculation of reproducible and orientation-independent susceptibility maps from GRE data acquired at 3T. The potential of these susceptibility maps to perform anatomical imaging is investigated, as well as the ability to measure the venous blood oxygen saturation level in large vessels, and to assess the local tissue iron concentration. In order to take into account diamagnetic susceptibility contributions induced by myelin, a correction scheme for susceptibility based iron estimation is demonstrated. The findings suggest that susceptibility contrast, and therewith also phase contrast, are not only linked to the storage iron concentration but are also significantly influenced by other sources such as myelin. After myelin correction the linear dependence between magnetic susceptibilities and previously published iron concentrations from post mortem studies was significantly improved. Finally, a comparison between susceptibility maps and processed phase images indicated that caution should be exercised when drawing conclusions about iron concentrations when directly assessing processed phase information.

Sensitivity of MRI resonance frequency to the orientation of brain tissue microstructure

Recent advances in high-field (>or=7 T) MRI have made it possible to study the fine structure of the human brain at the level of fiber bundles and cortical layers. In particular, techniques aimed at detecting MRI resonance frequency shifts originating from local variation in magnetic susceptibility and other sources have greatly improved the visualization of these structures. A recent theoretical study [He X, Yablonskiy DA (2009) Proc Natl Acad Sci USA 106:13558-13563] suggests that MRI resonance frequency may report not only on tissue composition, but also on microscopic compartmentalization of susceptibility inclusions and their orientation relative to the magnetic field. The proposed sensitivity to tissue structure may greatly expand the information available with conventional MRI techniques. To investigate this possibility, we studied postmortem tissue samples from human corpus callosum with an experimental design that allowed separation of microstructural effects from confounding macrostructural effects. The results show that MRI resonance frequency does depend on microstructural orientation. Furthermore, the spatial distribution of the resonance frequency shift suggests an origin related to anisotropic susceptibility effects rather than microscopic compartmentalization. This anisotropy, which has been shown to depend on molecular ordering, may provide valuable information about tissue molecular structure.

The orientation effect of gramicidin A on bicelles and Eu3+-doped bicelles as studied by solid-state NMR and FT-IR spectroscopy

We have explored the effect of gramicidin A (gA) on bicelle (Bic) orientation in the absence and presence of Eu(3+) by (31)P and (2)H NMR at different DMPC/gA ratios. FT-IR spectroscopy was used to assess the lipid chain ordering and verify the transmembrane peptide conformation. Our results show a time-dependent flipping of the bilayer normal alignment at high temperatures and high proportion of gA. The results are explained by both the diamagnetic susceptibility anisotropy of the beta(6.3) helical peptides and viscosity of the lipid mixture. The concentration effect of gramicidin on Bic/Eu(3+) is compared to that on Eu(3+)-doped DMPC liposomes. The Bic/Eu(3+) system is no longer oriented in the presence of gA and adopts a vesicular morphology while the peptide incorporation induces the formation of ellipsoidal DMPC/Eu(3+) assemblies aligned with their normal parallel to the magnetic field. The difference is explained in terms of lipid chain disorder and size of the bilayers.

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.

Klinische Hoch- und Ultrahochfeld-MR und ihre Wechselwirkung mit biologischen Systemen

The field strength of the static field in MRI has increased from 0.015 to 12 Tesla (T) during the last 25 years, which is about an 800 fold increase. In addition to low- and high field systems (1.5-4 T), ultra-high field systems with field strengths above 4 T are now available for human MRI. The extension of non-significant risk status for clinical fields up to 8 T by the FDA in July 2003 facilitates the further growth of this technology. The increase in field strength creates the need for a better understanding of the safety challenges to ensure safety for human imaging applications. This encompasses understanding the effects of the strong magnetic field at the atomic and molecular level and from biological tissue to organ systems. Moreover, in addition to the effects of a static magnetic field, the effects of radio-frequency- and gradient-fields have to be considered. This paper reviews the safety relevant issues for high- and ultrahigh field MR.

Effect of a 125 mT static magnetic field on the kinetics of voltage activated Na+ channels in GH3 cells

Voltage activated Na(+) channels were examined in GH3 cells, using the whole cell patch clamp method. Channel currents were recorded before, during, and after a 150 s exposure to a 125 mT static magnetic field. There was a slight shift in the current-voltage relationship and a less than 5% reduction in peak current during magnetic field exposure. More pronounced, however, was an increase in the activation time constant, tau(m), during and for at least 100 s following exposure to the field. This change in tau(m) was seen primarily at lower activation voltages. No change was noted in the inactivation time constant, tau(h). Changes were clearly temperature dependent, being evident only at and above 35 degrees C. These findings are consistent with the hypothesis that reorientation of diamagnetic anisotropic molecules in the cell membrane are capable of distorting imbedded ion channels sufficiently to alter their function. The temperature dependence of this phenomenon is probably due to the greater ease with which a liquid crystal membrane can be deformed.

Static magnetic field therapy for symptomatic diabetic neuropathy: a randomized, double-blind, placebo-controlled trial

OBJECTIVE

To determine if constant wearing of multipolar, static magnetic (450G) shoe insoles can reduce neuropathic pain and quality of life (QOL) scores in symptomatic diabetic peripheral neuropathy (DPN).

DESIGN

Randomized, placebo-control, parallel study.

SETTING

Forty-eight centers in 27 states.

PARTICIPANTS

Three hundred seventy-five subjects with DPN stage II or III were randomly assigned to wear constantly magnetized insoles for 4 months; the placebo group wore similar, unmagnetized device.

INTERVENTION

Nerve conduction and/or quantified sensory testing were performed serially.

MAIN OUTCOME MEASURES

Daily visual analog scale scores for numbness or tingling and burning and QOL issues were tabulated over 4 months. Secondary measures included nerve conduction changes, role of placebo, and safety issues. Analysis of variance (ANOVA), analysis of covariance (ANCOVA), and chi-square analysis were performed.

RESULTS

There were statistically significant reductions during the third and fourth months in burning (mean change for magnet treatment, -12%; for sham, -3%; P<.05, ANCOVA), numbness and tingling (magnet, -10%; sham, +1%; P<.05, ANCOVA), and exercise-induced foot pain (magnet, -12%; sham, -4%; P<.05, ANCOVA). For a subset of patients with baseline severe pain, statistically significant reductions occurred from baseline through the fourth month in numbness and tingling (magnet, -32%; sham, -14%; P<.01, ANOVA) and foot pain (magnet, -41%; sham, -21%; P<.01, ANOVA).

CONCLUSIONS

Static magnetic fields can penetrate up to 20mm and appear to target the ectopic firing nociceptors in the epidermis and dermis. Analgesic benefits were achieved over time.

Orientation of paramecium swimming in a DC magnetic field

We found that a ciliated protozoan, Paramecium, swam perpendicular to a static (DC) magnetic field (0.68 T). The swimming orientation was similar even when the ionic current through the cell membrane disappeared after saponin treatment. To determine the diamagnetic anisotropy of intracellular organs, macronuclei, cilia, and secretory vesicles, trichocysts, were selectively isolated. Both cilia and trichocysts tended to align their long axis parallel to the magnetic field (0.78 T). Paramecium mutants that lack trichocysts also swam perpendicular to the magnetic field, although the proportion fraction was smaller than the normal population. Since large numbers of cilia and trichocysts are arranged at right angles to the long axis of the cell, the diamagnetic anisotropies of cilia and trichocysts cause the long axis of the cell to align perpendicular to the magnetic field. In contrast to the DC magnetic field, an alternative (AC) magnetic field (60 Hz, 0.65 T) had almost no effect on the swimming orientation of Paramecium.

Structure and orientation of a G protein fragment in the receptor bound state from residual dipolar couplings

Residual dipolar couplings for a ligand that is in fast exchange between a free state and a state where it is bound to a macroscopically ordered membrane protein carry precise information on the structure and orientation of the bound ligand. The couplings originate in the bound state but can be detected on the free ligand using standard high resolution NMR. This approach is used to study an analog of the C-terminal undecapeptide of the alpha-subunit of the heterotrimeric G protein transducin when bound to photo-activated rhodopsin. Rhodopsin is the major constituent of disk-shaped membrane vesicles from rod outer segments of bovine retinas, which align spontaneously in the NMR magnet. Photo-activation of rhodopsin triggers transient binding of the peptide, resulting in measurable dipolar contributions to 1J(NH) and 1J(CH) splittings. These dipolar couplings report on the time-averaged orientation of bond vectors in the bound peptide relative to the magnetic field, i.e. relative to the membrane normal. Approximate distance restraints of the bound conformation were derived from transferred NOEs, as measured from the difference of NOESY spectra recorded prior to and after photo-activation. The N-terminal eight residues of the bound undecapeptide adopt a near-ideal alpha-helical conformation. The helix is terminated by an alpha(L) type C-cap, with Gly9 at the C' position in the center of the reverse turn. The angle between the helix axis and the membrane normal is 40 degrees (+/-4) degrees. Peptide protons that make close contact with the receptor are identified by analysis of the NOESY cross-relaxation pattern and include the hydrophobic C terminus of the peptide.

Safety of strong, static magnetic fields

Issues associated with the exposure of patients to strong, static magnetic fields during magnetic resonance imaging (MRI) are reviewed and discussed. The history of human exposure to magnetic fields is reviewed, and the contradictory nature of the literature regarding effects on human health is described. In the absence of ferromagnetic foreign bodies, there is no replicated scientific study showing a health hazard associated with magnetic field exposure and no evidence for hazards associated with cumulative exposure to these fields. The very high degree of patient safety in strong magnetic fields is attributed to the small value of the magnetic susceptibility of human tissues and to the lack of ferromagnetic components in these tissues. The wide range of susceptibility values between magnetic materials and human tissues is shown to lead to qualitatively differing behaviors of these materials when they are exposed to magnetic fields. Mathematical expressions are provided for the calculation of forces and torques.

Cleavage planes in frog eggs are altered by strong magnetic fields

Early cleavages of Xenopus embryos were oriented in strong, static magnetic fields. Third-cleavage planes, normally horizontal, were seen to orient to a vertical plane parallel with a vertical magnetic field. Second cleavages, normally vertical, could also be oriented by applying a horizontal magnetic field. We argue that these changes in cleavage-furrow geometries result from changes in the orientation of the mitotic apparatus. We hypothesize that the magnetic field acts directly on the microtubules of the mitotic apparatus. Considerations of the length of the astral microtubules, their diamagnetic anisotropy, and flexural rigidity predict the required field strength for an effect that agrees with the data. This observation provides a clear example of a static magnetic-field effect on a fundamental cellular process, cell division.

Action of melittin on the DPPC-cholesterol liquid-ordered phase: a solid state 2H-and 31P-NMR study

Solid-state deuterium and phosphorus-31 nuclear magnetic resonance studies of deuterium-labeled beta--[2,2',3,4,4',6-2H6]-cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine have been undertaken to monitor the action of melittin on model membranes containing 30 mol% cholesterol, both at the molecular and macroscopic level. Cholesterol totally inhibits the toxin-triggered formation of large unilamellar vesicles and strongly restricts the appearance of small discs. The latter remain stable over a wide temperature range (20-60 degrees C) because of an increase in their cholesterol content as the temperature increases. This process is related to a constant disc hydrophobic thickness of approximately 29 A. The system, when not in the form of discs, appears to be composed of very large vesicles on which melittin promotes magnetically induced ellipsoidal deformation. This deformation is the greatest when the maximum of discs is observed. A model to describe both the disc formation and stability is proposed.

Effects of MR exposure on cell proliferation and migration of chick motoneurons

The effect of magnetic resonance (MR) exposure on the proliferation and migration of motoneurons was examined in chick embryos. Embryos were exposed in ovo to a static magnetic field of 1.5 T for 6 hours and to 64-MHz radio-frequency field pulses and a switched magnetic field gradient with an amplitude of 0.6 G/cm for 4 hours. For cell proliferation studies, embryos were exposed to MR fields during the developmental stage at which motoneuron proliferation is most active. For cell migration studies, embryos were exposed to MR fields at the developmental stage just before lateral motoneuron migration. The results show that the birth dates, migration, and proliferation of lateral motoneurons were unaffected by the MR exposure conditions in this study.

A proposed mechanism for the action of strong static magnetic fields on biomembranes

Experimental studies have demonstrated a temperature dependent effect by strong static magnetic fields on synaptic function. It is proposed that these findings may be explained by the diamagnetic properties of membrane phospholipids. The change in diamagnetic anisotropy coincidental with membrane thermotropic phase transition is responsible for the temperature dependence of this phenomenon and provides insight into the mechanism of action of these fields. At the prephase transition temperature highly diamagnetic anisotropic gel phase domains exist within a more fluid liquid-crystal phase. The partial magnetic reorientation of these domains results in membrane distortion and, thereby, functional impairment of contiguous ion specific channels. This mechanism adequately explains observations of the effects of static magnetic fields both on the central nervous system and at the neuromuscular junction. It is suggested that the same mechanism may be operative in other biosystems.

Magnetic field influence on acetylcholine release at the neuromuscular junction

The effects of a static magnetic field on the frequency of miniature end-plate potentials (MEPPs), recorded from the murine phrenic nerve-diaphragm preparation, were studied. In the presence of a 1,200-G field, statistically significant changes in MEPP frequency were observed. There was a modest increase in frequency at temperatures at and below 34 degrees C and a prominent decrease in frequency at temperatures above 35 degrees C. This temperature-dependent phenomenon was not seen in the absence of calcium in the perfusate. These results suggest that, at its phase transition temperature, the diamagnetic anisotropy of the presynaptic membrane is sufficient to influence neurotransmitter release by altering the function of the transmembrane calcium transfer mechanism.

Optimizing and characterizing alignment of oriented lipid bilayers containing gramicidin D

31P NMR spectroscopy and optical microscopy have been used to characterize samples of gramicidin D in oriented lipid bilayers. Correlations have been made between the defect structures observed under crossed polarizers by optical microscopy and characteristic features of 31P NMR spectra. The sample preparation protocol has been improved using these techniques to achieve minimal dispersion of the bilayer normal and minimal amounts of unoriented sample. The molar ratio of gramicidin to dimyristoyl-phosphatidylcholine, the extent of hydration, and the cosolubilizing solvent system were used as the protocol variables. While hydration level and solvent system had profound effects on the sample orientation the molar ratio did not. However, the 31P chemical shift anisotropy is very sensitive to the molar ratio and can be used as an in situ method for determining the molar ratio.

Orientation of lipid tubules by a magnetic field

Lipid tubules, which are straight hollow cylinders consisting of lipid bilayers, are shown to orient in strong magnetic fields. Birefringence measurements were made of dilute samples of tubules of 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3-phosphocholine (DC23PC) in magnetic fields of up to 4 T. The tubules were found to orient with their long axes parallel to the field direction, with saturated orientation [P2 (cos theta] approximately greater than 0.95) found at approximately 2 T. From known distributions of lengths and the number of bilayers in the walls, a value delta chi = (-7 +/- 1) X 10(-9) erg cm-3 G-2 was calculated for the tubules, which compares well with some previously reported values for phosphatidylcholines. Magnetic alignment will permit more sophisticated structural studies of monomeric and polymeric tubules, and provide a method of orienting macromolecules in the tubule walls or interior.

Magnetic resonance imaging. Part I--physical principles

Magnetic resonance (MR) imaging is the most complex imaging technology available to clinicians. Whereas most imaging technologies depict differences in one, or occasionally two, tissue characteristics, MR imaging has five tissue variables-spin density, T(1) and T(2) relaxation times and flow and spectral shifts-from which to construct its images. These variables can be combined in various ways by selecting pulse sequences and pulse times to emphasize any desired combination of tissue characteristics in the image. This selection is determined by the user of the MR system before imaging data are collected. If the selection is not optimal, the imaging process must be repeated at a cost of time and resources. The optimal selection of MR imaging procedures and the proper interpretation of the resultant images require a thorough understanding of the basic principles of MR imaging. Included in this understanding should be at least the rudiments of how an MR imaging signal is produced and why it decays with time; the significance of relaxation constants; the principles of scanning methods such as saturation recovery, inversion recovery and spin echo; how data obtained by these methods are used to form an image, and how the imaging data are complied by multi-slice and volumetric processes. In selecting an MR imaging unit, information about different magnet designs (resistive, superconductive and permanent) is useful. Although no bioeffects are thought to be associated with an MR imaging examination, some knowledge of the attempts to identify bioeffects is helpful in alleviating concern in patients.

Magnetic anisotropy of lecithin membranes. A new anisotropy susceptometer

Cylindrical giant vesicles prepared from egg lecithin and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) are oriented in an external magnetic field and observed by phase contrast microscopy. The anisotropic part of the diamagnetic susceptibility of the lecithin membrane is determined from the distribution of angles between the magnetic field and the long cylinder axis due to thermal fluctuations. The anisotropy of DMPC is found to be larger by a factor of 2 than that of egg lecithin. This is attributed to the presence of unsaturated acyl chains in egg lecithin.

The effects of direct-current magnetic fields on turtle retinas in vitro

Direct-current magnetic fields of 10 to 100 gauss cause a significant short-term reduction of the in vitro electroretinographic b-wave response in turtle retina. This response compression is not accompanied by the usual reduction in retinal sensitivity that occurs with background illumination. Furthermore, this effect is obtained only briefly after the offset of ambient lighting in the diurnal light-dark cycle of nonhibernating animals.

Rotation of sickle cells in homogeneous magnetic fields

Deoxygenated sickle cells (HbS) have been shown to orient in homogeneous magnetic fields because of magnetic anisotropy of the hemes. The time of rotation is proportional to 1/H2 and is in agreement with theory. Structural information concerning the orientation of HbS molecules in the fibers is obtained from these experiments and is shown to agree substantially with existing models.

Effect of homogeneous magnetic fields on responses to toxic stimulation in Spirostomum ambiguum

Effects of DC magnetic fields, up to 125,000 G, on the survival of the ciliated protozoan Spirostomum ambiguum exposed to the toxic substance 2,2'-dipyridyldisulfide are reported. The magentic field diminishes the ability of the organism to survive the drug, and lengthens the extension phase of the contraction cycle. It is proposed that the magnetic field alters the regulation of intracellular (Ca2+) transients.

Orientation and mobility of molecules in membranes studied by polarized light spectroscopy

Biological membranes are composed of mainly lipids and proteins. The physical properties of the lipids, forming a bilayer structure, are of crucial importance for the living cell, since the plasma membrane is the guardian barrier towards the environment. Thus, the functioning cell needs a highly stable lipid bilayer, which depends on molecular packing and orientation properties of the various membrane components (Wieslanderet al.1980). The spatial arrangement of the membrane proteins incorporated in the lipid matrix plays an essential role for the different chemical processes occurring at or within the membrane. Information about molecular orientation and mobility is therefore necessary for unravelling the functional mechanisms of a biological membrane.

Magnetic anisotropy of the visual pigment rhodopsin

A new estimate of diamagnetic anisotropy of the frog rhodopsin is reported. The estimate is obtained by combining the data of magnetic field induced orientation of isolated frog rod outer segments as measured by Chagneux and Chalazonitis (1972) and the data of diamagnetic anisotropy of lecithin membranes as recently reported by Boroske and Helfrich (1978). The anisotropy of the volume susceptibilities of frog rhodopsin is calculated to be 4.4 X 10(-8) cgs unit/cm3, which corresponds to 1.5 X 10(-27) cgs unit/molecule, or 9.0 X 10(-4) cgs unit/mol.

Theory of fluorescence polarization in magnetically oriented photosynthetic systems

Many cells and cell fragments are known to assume specific alignments with respect to an applied magnetic field. One indicator of this alignment is a difference between the intensities of fluorescence observed in polarizations parallel and perpendicular to the magnetic filed. We calculate these two intensities using a model that assumes axially symmetric membranes and that covers a wide variety of shapes from flat disk to right cylinder. The fluorescence is assumed to originate at chromophores randomly exicted but nonrandomly oriented in the membranes. The membrane alignment is assumed to be due to the net torque on a nonrandom distribution of diamagnetically anisotropic molecules. The predicted results are consistent with most magnetoorientation data from green cells, but we are able to show that Chlorella data are not consistent with the hypothesis that the membranes have, and maintain, a cuplike configuration.

Diamagnetic anisotropy and orientation of alpha helix in frog rhodopsin and meta II intermediate

The diamagnetic anisotropy of retinal rod outer segments, and its variation upon bleaching, have been measured with a rotating field device. A large molar diamagnetic asymmetry is found for rhodopsin. This cannot be explained by an anisotropy of the aromatic side chains of the protein, nor by the orientation of the retinal chromophore. However, it can be accounted for by an orientation perpendicular to the disc membrane of a major proportion of the alpha-helical segments of the protein. Upon bleaching a decrease of 9 +/- 2% of the diamagnetic asymmetry is observed when going to the meta II intermediate. This change is not mainly due to a reorientation of the retinal, since it is practically insensitive to detachment of the chromophore by addition of NH2OH. Comparison with recent UV linear dichroism results indicate that it may be due to the rotation of a trytophan residue in the bleaching sequence.

Structural origins of diamagnetic anisotropy in proteins

Magnetic anisotropy in proteins and polypeptides can be attributed to the diamagnetic anisotropy of the planar peptide bonds. The alpha helix in particular has large anisotropy due to the axial alignment of the peptide bonds. The regular arrangements of the peptide bonds in beta pleated sheet and collagen structures also produce substantial anisotropy, but less than for alpha helix. The anisotropy permits orientation of small structures of these types in magnetic fields of several kilogauss.

Decrease in magnetic anisotropy of external segments of the retinal rods after a total photolysis

The magnetic anisotropy of a polymembrane cell, such as the external segment of the frog retinal rod, is defined as the difference between the axial magnetic susceptibility (chiar), chia - chir = deltachi of the segment. After the total photolysis of the rhodopsin in situ, deltachi decreases a significantly by 20%. This decrease in magnetic anisotrophy should involve a subtle molecular disorder, mostly due to an alteration of the rhodopsin molecule.