R A. Brooks, J Vymazal, Ronald B. Goldfarb, J W. Bulte, P Aisen
By combining nuclear magnetic ralaxometry on 39 ferritin samples with different iron loading with magnetometry, results were obtained that suggest a new interpretation of the core structure and magnetic properties of ferritin. These studies provide evidence that, countrary to most earlier reports, the ferritin ocre is antiferromagnetic (AFM) even at body temperature and possesses a superparamagnetic (SPM) moment due to incomplete cancellation of antiparellel sublattices, as predicted by Neel's theory. This moment also provides a likely explanation for the anomalous T2 shortening in ferritin solution. However, the number of SPM moments derived form this model is less than the number of ferritin molecules determined chemically, and a similar discrepancy was found by retrospectively fitting previously published magnetometry data. In other words, only a fraction of the ferritin moleclues seem to be SPM. The studies also provide evidence for paramagnetic (PM) Curie-Weiss iron ions at the core surface, where the local Neel temperature is lower; these ions are apparently responsible for the weaker T1 shortening. In fact, the conversion of uncompensated AFM lattice ions to PM ions could explain the small number of SPM particies. The apparent Curie Law behavior of ferritin thus appears to be a coincidental result of different temperature dependences of the PM and SPM components.