Distribution coefficients of protein impurities in ferritin and lysozyme crystals - Self-purification in microgravity

Ribonuclease, insulin, cytochrome C, myoglobin and ovalbumin were introduce d into solutions from which ferritin and lysozyme crystals were grown. Thes e measurements were also performed for the ferritin dimers trapped by growi ng ferritin crystals. The crystals were later dissolved in a pure solvent, the impurity concentrations were measured by high performance liquid chroma tography and the effective impurity distribution coefficient, K, was evalua ted relative to the initial concentrations of ferritin or lysozyme. The den sity of impurity species in crystal relative to its density in mother solut ion were used to calculate volumetric distribution coefficient, k. These di stribution coefficients were found to exceed unity (k > 1) in terrestrial c ondition for all impurity species, except for insulin and cytochrome C lyso zyme. For ferritin dimers, K = 4, k = 1.8 x 10(3). Crystals grown in space under the otherwise identical conditions incorporated lower amounts of all of these impurities, majority of them below the detection limit. The lower impurity incorporation obtained in stagnant solution may be partially due t o more difficult impurity supply through the impurity depletion zone arisin g around the growing crystals at k > 1 in the absence of buoyancy driven co nvection or stirring. Analytical estimates of the depletion zone show reaso nable agreement with measurements for ferritin dimers. Step bunching and ot her flow-dependent surface processes may also contribute to lower distribut ion coefficient. (C) 2000 Elsevier Science B.V. All rights reserved.