Protein crystals as a template for in situ formation of magnetite nanoparticles

Crystallisation in confned spaces is a widespread phenomenon in nature. Many processes like frost heave, biomineralisation (bones and seashells), growth of salt and ice crystals in the pores of masonry happen not in a bulk, but in limited volumes. Reducing the volume of the system infuences not only the probability of nucleation, but also the kinetic. As the results, in reduced volumes become possible stabilisation of metastable polymorphs, formation of crystals with preferred orientations, modifcation of morphology, etc. One of the most fascinating cases of crystallisation in confned spaces – formation of magnetite nanoparticles inside magnetotactic bacteria. The particles have unusual morphologies, homogeneous size and superparamagnetic properties. The control of the magnetite nanoparticles formation could be explained by a combination of physical and chemicals factors in which nucleation and growth of magnetite crystals is performed in confned vesicles (magnetosomes) modulated by the interaction with different proteins.bIn this work, we imitate this type of control over the precipitation of magnetite using channels of cross-linked protein crystals (CLPCs). CLPCs with different pore size and amino acid decoration were used to study their infuence on the magnetite precipitation. We obtained gradient distribution of nanoparticles with a narrow size distribution of around 2nm independently of the channel diameter size of the CLPCs. But the pores size infuenced the stabilisation of the Fe-rich phase. In case of lysozyme (the smallest pore size), the amorphous metastable intermediates of magnetite were stabilised, while in glucose isomerase crystals, the amorphous phase were recrystallised into the crystalline state of magnetite. Our results demonstrate control in the size and stability of those Fe-rich nanoparticles and the potential use of the CLPCs as excellent scaffolds to study the crystallisation in confnement.