Visualization of the impurity depletion zone surrounding apoferritin crystals growing in gel with holoferritin dimer impurity

Colorless apoferritin (M congruent to 450 kDa) crystals were grown in the presence of red colored holoferritin dimers (M congruent to 900 kDa) within agarose gel by counterdiffusion with Cd2+ as precipitant. In agreement with our previous quantitative measurements showing preferential trapping of dimers (segregation coefficient K = 4), the apoferritin crystals became darkly colored while the gel solution around them was nearly colorless. This segregation coefficient and the absence of convective mixing resulted in a large colorless area surrounding the crystals which allowed us to demonstrate the existence of the impurity depletion zone by direct visual observation. (C) 2001 Elsevier Science B.V. All rights reserved.