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 magn etic properties have been studied, searching to relate function and structu re. Horse spleen ferritin has been investigated by EPR at temperatures betw een 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 parti cles 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 sig nal appears, suggesting (14 +/- 5) K as the Neel temperature of surface spi ns. Analysis of the temperature dependence of the distance between EPR line s 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, whe re K is the anisotropy energy per unit volume, M is the sample magnetizatio n, and V is the superparamagnetic core volume. The results are also discuss ed, and some structural models in the literature are considered. (C) 2001 A cademic Press.