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The structure and dynamics of the spherical protein Apoferritin in aqueous solution are studied over a wide range of protein concentrations and ionic strengths. At high ionic strength and low protein concentration, the intermolecular forces are screened and, therefore, the proteins behave like uncharged molecules. Under these conditions, the form factor of Apoferritin was measured by means of small angle x-ray scattering (SAXS) and the hydrodynamic radius was determined by means of dynamic light scattering (DLS). The sample was found to be highly monodisperse. By decreasing the content of salt added, interactions between the Apoferritin particles were initiated. These intermolecular forces lead to a pronounced maximum in the SAXS intensity. At the same time, a slow mode appears in the relaxation time distribution, additionally to the diffusive mode. The relative amplitudes and correlation times of the diffusive and the slow mode were investigated and compared with predictions of the coupled mode theory. By assuming the slow mode to be related to the correlated motion of ordered domains, the size of these domains was derived from the slow relaxation time. From the x-ray data and the Apoferritin form factor, structure factors of ordered solutions were calculated. The shape of the structure factor peaks was studied as a function of Apoferritin and salt concentration. Finally, by using the DLS information regarding the size of correlated domains, we analyzed the degree of polyelectrolyte ordering within the paracrystalline domains in Apoferritin solutions. (C) 2002 American Institute of Physics.
