12.6 Protein centrifugation Proteins and other macromolecules can be separated b

Question

12.6 Protein centrifugation Proteins and other macromolecules can be separated by size using centrifugation. The idea is to spin a sample con- taining proteins of different size in solution. The spinning produces a centrifugal force per unit mass gc, which leads to diffusion with a drift velocity that depends on the pro- tein size. We assume that a protein in the sample can be approximated as a ball of radius R. (a) Following the discussion in the chapter fill in the steps leading up to the formula for the drift velocity of the protein as a function of its radius (eqn 12.45), Vdrift - 91 where protein and solvent are the densities of the protein and the solvent, and is the solvent viscosity. (b) Estimate the drift velocity for hemoglobin in water in an ultracentrifuge with gc 105g, where gx10 m/ is the acceleration of freely falling objects in Earth's gravitational field. Assume a typical protein density of 1.2 g/cm3 (c) We would like to separate two similar proteins, having the same density, , 1.35 g/cm3. They have diameters of 4 nm and 5 nm respectively. The two protein species start out mixed together in a thin layer at the top of a 1 cm long centrifuge tube. How large should the centrifuge acceler- ation gc be so that the two proteins are separated before they drift to the end of the tube?

Explanation / Answer

Answer:

Based on the given information:

Vdrift = 2*(density of protein - density of solvent) * gc * R2 / (9* Solvent viscosity)

Vdrift = 2*(density of protein - density of solvent) * gc * R2 / (9* Solvent viscosity)

Vdrift = 2*(1.2 - 1.0) * 108* (4 x 10-7)2 / (9* 8.90 × 103) = 7.99*10-5 cm/s

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