Quantum Magnetism of the Iron Core in Ferritin Proteins: A Re-Evaluation of the Giant-Spin Model

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The electron–electron, or zero-field interaction (ZFI) in the electron paramagnetic resonance (EPR) of high-spin transition ions in metalloproteins and coordination complexes, is commonly described by a simple spin Hamiltonian that is second-order in the spin S: ℋ=𝐷[𝑆2𝑧−𝑆(𝑆+1)/3+𝐸(𝑆2𝑥−𝑆2𝑦). Symmetry considerations, however, allow for fourth-order terms when S ≥ 2. In metalloprotein EPR studies, these terms have rarely been explored. Metal ions can cluster via non-metal bridges, as, for example, in iron-sulfur clusters, in which exchange interaction can result in higher system spin, and this would allow for sixth- and higher-order ZFI terms. For metalloproteins, these have thus far been completely ignored. Single-molecule magnets (SMMs) are multi-metal ion high spin complexes, in which the ZFI usually has a negative sign, thus affording a ground state level pair with maximal spin quantum number mS = ±S, giving rise to unusual magnetic properties at low temperatures. The description of EPR from SMMs is commonly cast in terms of the ‘giant-spin model’, which assumes a magnetically isolated system spin, and in which fourth-order, and recently, even sixth-order ZFI terms have been found to be required. A special version of the giant-spin model, adopted for scaling-up to system spins of order S ≈ 103–104, has been applied to the ubiquitous iron-storage protein ferritin, which has an internal core containing Fe3+ ions whose individual high spins couple in a way to create a superparamagnet at ambient temperature with very high system spin reminiscent to that of ferromagnetic nanoparticles. This scaled giant-spin model is critically evaluated; limitations and future possibilities are explicitly formulated.
Original languageEnglish
Article number2254
Number of pages17
JournalMolecules (Basel, Switzerland)
Issue number10
Publication statusPublished - 2024


  • ferritin
  • core
  • EPR
  • spin Hamiltonian
  • zero-field interaction
  • higher-order terms
  • giant spin
  • metal-ion clusters
  • single-molecule magnets
  • superparamagnetism


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