Biomimetic synthesis and characterization of magnetic proteins (magnetoferritin)

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 degrees 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 similar to 140 atoms/protein molecule and the Fe(II): Me3NO ratio to 3:2 in each increment, ferrimagnetic nanocrystals of magnetite/maghemite (Fe3O4)/(gamma-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 le ss 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.