The sedimentation properties of ferritins. New insights and analysis of methods of nanoparticle preparation.

BACKGROUND

Ferritin exhibits complex behavior in the ultracentrifuge due to variability in iron core size among molecules. A comprehensive study was undertaken to develop procedures for obtaining more uniform cores and assessing their homogeneity.

METHODS

Analytical ultracentrifugation was used to measure the mineral core size distributions obtained by adding iron under high- and low-flux conditions to horse spleen (apoHoSF) and human H-chain (apoHuHF) apoferritins.

RESULTS

More uniform core sizes are obtained with the homopolymer human H-chain ferritin than with the heteropolymer horse spleen HoSF protein in which subpopulations of HoSF molecules with varying iron content are observed. A binomial probability distribution of H- and L-subunits among protein shells qualitatively accounts for the observed subpopulations. The addition of Fe(2+) to apoHuHF produces iron core particle size diameters from 3.8 + or - 0.3 to 6.2 + or - 0.3 nm. Diameters from 3.4 + or - 0.6 to 6.5 + or - 0.6 nm are obtained with natural HoSF after sucrose gradient fractionation. The change in the sedimentation coefficient as iron accumulates in ferritin suggests that the protein shell contracts approximately 10% to a more compact structure, a finding consistent with published electron micrographs. The physicochemical parameters for apoHoSF (15%/85% H/L subunits) are M=484,120 g/mol, nu=0.735 mL/g, s(20,w)=17.0 S and D(20,w)=3.21 x 10(-)(7) cm(2)/s; and for apoHuHF M=506,266 g/mol, nu=0.724 mL/g, s(20,w)=18.3S and D(20,w)=3.18 x 10(-)(7) cm(2)/s.

SIGNIFICANCE

The methods presented here should prove useful in the synthesis of size controlled nanoparticles of other minerals.