Effect of precipitation temperature and number of iron atoms per molecule on the structure of hydrated iron(III) oxyhydroxide ferritin cores synthesised in vitro

Horse spleen apoferritin was reconstituted with varying numbers of iron atoms per protein shell at 25 and 50°C. Samples with mean particle sizes ranging from approximately 20 to 1100 iron atoms per particle were synthesised. Transmission electron microscopy combined with elemental analysis and protein assays indicated a characteristic iron density for the iron(III) oxyhydroxide of 6.4 ± 0.6 iron atoms per nm3 thus suggesting a maximum possible loading per ferritin shell of 1700 ± 200 iron atoms. Room temperature Mössbauer spectra of the iron(III) oxyhydroxide particles formed within the ferritin protein shells were doublets with slightly larger quadrupole splittings when precipitated at 25°C compared with those precipitated at 50°C. The quadrupole splittings were not significantly correlated with the number of iron atoms per protein shell although the general trend was for smaller particles to have larger quadrupole splittings. A very small but statistically significant increase in the centre shift of the Mössbauer spectra with increasing particle size was observed. These results, taken together with results from other studies, indicate that reconstituted ferritin cores are highly disordered and low density when compared with those found in native horse spleen ferritin. The results also indicate that raising the temperature of reconstitution increases the structural order within the particles slightly but that increasing the number of iron atoms per ferritin molecule above 300 has little influence on the apparent overall structural order