Hybrid colloid analysis combining analytical ultracentrifugation and flow-field flow fractionation.

Ferritin, the iron storage protein, is an organic-inorganic hybrid colloid consisting of a hollow protein capsule, which is filled with ferrihydride with up to 4500 iron atoms. Owing to the varying iron content and the resulting density differences, as well as the protein oligomerization, a particle size distribution is superimposed with a density distribution, making a precise analysis of ferritin by analytical ultracentrifugation difficult. This study describes how the information of the sedimentation coefficient distribution can be combined with the diffusion coefficient distribution obtained from flow-field flow fractionation to yield the buoyant molar mass of the oligomers in the mixture, extending the information content of each individual analytical method. In addition, the sedimentation and diffusion coefficients are compatible with a simple hard-sphere aggregation model, suggesting that the ferritin oligomers up to the pentamer have a globular solution structure.