Biomimetic Synthesis and Characterization of Magnetic Proteins (Magnetoferritin)

Abstract

A new synthesis of the magnetic protein magnetoferritin is reported. Addition of increments of Fe(II) to anaerobic solutions of the demetalated protein, apoferritin, at pH = 8.6 and 65 °C, followed by stoichiometric amounts of the oxidant trimethylamine-N-oxide (Me3NO), results in the formation of a dispersed magnetic bioinorganic nanocomposite. By limiting the Fe:protein ratio to not more than ∼140 atoms/protein molecule and the Fe(II):Me3NO ratio to 3:2 in each increment, ferrimagnetic nanocrystals of magnetite/maghemite (Fe3O4)/(γ-Fe2O3) are synthesized in the 8 nm diameter protein cage. Controlling the number of stepwise cycles of the Fe(II)/oxidant additions produces biomimetic proteins with different iron loadings (100, 260, 530, 1000, 2040, and 3150 Fe atoms/protein molecule) and concomitant changes in the size of the inorganic nanocores. Magnetoferritins prepared with less than 1000 Fe atoms/molecule were discrete nanocomposites with protein-encapsulated magnetic cores. Samples with higher iron loadings were aggregated on the TEM grid and showed a progressive increase in the number of cores with dimensions greater than the protein cavity. The temperature-dependent magnetic properties of magnetoferritins with different Fe loadings were studied by SQUID magnetometry. An approximately linear dependence of the superparamagnetic blocking temperature with iron loading was determined.