Ferrous Iron Binding Key to Mms6 Magnetite Biomineralisation: A Mechanistic Study to Understand Magnetite Formation Using pH Titration and NMR Spectroscopy

Formation of magnetite nanocrystals by magn eto-tactic bacteria is controlled by specificproteins whichregu-late the particles’ nucleation and growth.One such proteinis Mms6. This small, amphiph ilic protein can self-assembleand bind ferric ions to aid in magnetite formation. To under-stand the role of Mms6 duringinvitro iron oxide precipita-tion we have performed in situ pH titrations. We find Mms6has little effect duringferric salt precipitation, but exertsgreatest influence during the incorporation of ferrous ionsand conversion of this salt to mixed-valence iron mineral s,suggesting Mms6 has ahitherto unrecorded ferrous iron in-teracting property which promotes the formation of mag-netiteinferrous-rich solutions.Weshow ferrous binding tothe DEEVE motif within the C-term inal region of Mms6 byNMR spectroscopy,and model these binding events usingmolecular simulations. We conclude that Mms6 functions asamagnetite nucleating protein under cond itions where fer-rous ions predominate.