Comparative Studies of the Superparamagnetic Properties of Engineered H-rich and L-rich Human Ferritins Reconstituted with 500 ⁵⁷Fe-atoms / Protein

Ferritin, the iron storage protein, consists of an inorganic ferrihydrite core surrounded by an organic protein shell, and in humans, it is found in the liver, spleen, brain, and heart. The shell consists of 24 amino acid subunits of two types: heavy (H) and light (L). In the liver and spleen, it is L-rich, while in the brain and heart it is H-rich. Since iron deposits in the brain have been linked to neurodegenerative diseases, like Parkinson’s and Alzheimer's, it is important to study the structure/function relations in these types of ferritin. Thus, we used Mӧssbauer Spectroscopy (MS) to analyze the physical properties of the cores within L-rich and an H-rich human ferritin. The MS signatures obtained in the temperature range of 4.2 < T < 300K show that magnetic spitting of the L-rich core collapses to a quadrupole doublet at a higher temperature (T_B=21K) than the H-rich core (T_B=11K). We used the Néel uniaxial magnetic anisotropy model for spin-relaxation-time processes in magnetic nanoparticles to extract information on core size. We found that H-rich proteins tend to form smaller cores with a larger surface-to-volume ratio, favoring high iron trafficking; while L-rich proteins form larger cores with a smaller surface-to-volume ratio, favoring long-term storage.