Ferromagnetic resonance of horse spleen ferritin: core blocking and surface ordering temperatures.

In nature, ferritin, an iron-storage molecule, is found in species ranging from bacteria to man. In the past 50 years its chemical, physical, and magnetic properties have been studied, searching to relate function and structure. Horse spleen ferritin has been investigated by EPR at temperatures between 7 and 290 K. These spectra change from an isotropic line at 290 K to an anisotropic one at 19 K, with a behavior consistent with a system of particles that undergoes superparamagnetic relaxation. A blocking temperature of (116+/-9) K is obtained. A new temperature-dependent signal is observed in the low field region at temperatures higher than 80 K. At 7 K no EPR signal appears, suggesting (14+/-5) K as the Néel temperature of surface spins. Analysis of the temperature dependence of the distance between EPR lines extrema, under the view of two theoretical models, allowed the evaluation of magnetic parameters. These parameters are 2K/M=2.7 x 10(3) Oe and MV=1.9 x 10(-17) emu or K/M=1.3 x 10(3) Oe and MV=2.0 x 10(-17) emu, where K is the anisotropy energy per unit volume, M is the sample magnetization, and V is the superparamagnetic core volume. The results are also discussed, and some structural models in the literature are considered.

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.

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.

Nitrosyl hemoglobins: EPR above 80 K.

The EPR spectra of nitrosyl hemoglobin and myoglobin in different conditions (native, denatured and lyophilized), as well as of hematin-NO were obtained in the temperature range of 80-280 K. There is a substantial and reversible decrease of the areas of the EPR spectra of all the hemoglobin samples above 150 K. The interpretation of the results implies the existence of two conformational states in thermal equilibrium, only one of which is EPR detectable. Thermodynamical parameters are determined for the hexa- and penta-coordinated cases.

Fractal analysis of photolysis of nitrosyl haemoglobin at low temperatures.

Photolysis of nitrosyl haemoglobin (HbNO) has been studied from 5.9 K to 20 K for R, T and RT conformations. It was observed that the experimental curves have two different behaviours at a given temperature in a particular conformation. At shorter time scales the data are well reproduced by a model based on fractal concepts, where the relevant parameter is the difference between the fractal dimension and the fraction. For simplicity at longer time scales a simple exponential was used to fit the curves.

Nitrosyl hemoglobin: EPR components at low temperatures.

The EPR spectrum of nitrosyl hemoglobin has been studied from 7.5 K to 104 K. It is composed of at least three components (A, B and C) which have a different dependence on temperature and power level. The A component decreases with increasing temperature. The B component disappears at around 30 K and is replaced by C. Relaxation of A follows the Orbach mechanism with an energy of 28 cm-1. This behavior can be attributed to phonon induced changes in the orientation of NO with respect to the heme plane.

E.p.r. studies of photolysis of nitrosyl haemoglobin at low temperatures: effects of quaternary structure.

Photolysis of nitrosyl haemoglobin (HbNO) has been studied from 6.5 K to 20 K for different NO saturation conditions. The kinetic curves are fitted equally well by a biphasic exponential and a distribution of activation energies. The parameters are straightforwardly related to the quaternary structure of the protein. The biphasic model indicates that two germinate processes in the NO reassociation to Hb dominate at low temperatures independent of the protein conformation.

E.p.r. studies of photolysis of nitrosyl haemoglobin at low temperatures.

Photolysis of HbNO has been studied from 6.2 K to 15.5 K by electron spin resonance during and after continuous illumination. Non-exponential kinetics of both dissociation and reassociation of NO was observed. The prolonged illumination separates the fast and slow ligands. This picture is consistent with NO tunnelling from two sites at different distances from the bound position. This result is obtained using a model of a sum of two exponentials or of conformational substates.