Two globular proteins diffuse in water at ( )

PHYSICS · Multi-part · Fermi University Physics · Biophysics & Medical Physics · Physical foundations of the living cell · Physical foundations of the living cell

Life at Thermal Energy, Ch Random walks, diffusion and the crowded cell

Problem

Two globular proteins diffuse in water at

25^\circ\mathrm{C}

(

\eta = 0.89\,\mathrm{mPa\,s}

). Protein A has molar mass

14\,\mathrm{kDa}

and radius

a_A = 1.9\,\mathrm{nm}

. Protein B has molar mass

66\,\mathrm{kDa}

(about

4.7

times heavier than A) and radius

a_B = 3.0\,\mathrm{nm}

. A student argues: "B is

4.7

times heavier than A, so B must diffuse

4.7

times more slowly." ((a)) Use the Stokes-Einstein relation to predict

D_A

and

D_B

. ((b)) Find the true ratio

D_A/D_B

and explain what is wrong with this reasoning.

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