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

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=[2−(+1)/3+(2−2). Symmetry considerations, however, allow for fourth-order terms...