Controlled formation of magnetite crystal by partial oxidation of ferrous hydroxide in the presence of recombinant magnetotactic bacterial protein Mms6

Mms6 is a small acidic protein that is tightly associated with bacterial magnetite in Magnetospirillum magneticum AMB-1. This protein has previously shown iron binding activity, allowing it to generate uniform magnetic crystals by co-precipitation of ferrous and ferric ions. Here, magnetite crystals were formed by the partial oxidation of ferrous hydroxide in the presence and absence of Mms6. The crystals synthesised were systematically characterised according to their sizes and morphologies using high-resolution transmission electron microscopy. Mms6-mediated synthesis of magnetite by this methods produced crystals of a uniform size and narrow size distribution with a cubo-octahedral morphology, similar to bacterial magnetite observed in M. magneticum AMB-1. The crystals formed in the absence of Mms6 were octahedral, larger with an increased size distribution. Protein quantification analysis of Mms6 in the synthesised particles indicated tight association of this protein onto the crystal. Furthermore, high affinities to iron ions and a highly charged electrostatic quality suggest that the protein acts as a template for the nucleus formation and/or acts as a growth regulator by recognising crystal faces. The method introduced in this study presents an alternative route for controlling the size and shape of magnetite crystals without the use of organic solvent and high temperatures.