Visualizing the Impurity Depletion Zone Around Holoferritin Crystals Growing in Gel with Ferritin Dimers

Colorless transparent apoferritin (Mr = 450KDa) crystals have been grown from gel with Cd(2+) as precipitant in the presence of reddish brown-colored ferritin dimers (Mr = 900KDa). In agreement with our previous measurements, showing preferential trapping of dimers (distribution coefficient K = 4), the apoferritin crystals become strongly colored while the gel solution around them became nearly colorless. The depth of the depletion with respect to the colored dimer impurity allowed us to visualize the impurity depletion zone. Depletion with respect to impurity as compared to the crystallizing protein is discussed.