Molecule Diffusion

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According to the Stokes-Einstein equation
the diffusion coefficient D for a particle in a free volume depends on the Boltzmann constant (k), the absolute temperature (T), the viscosity of the solution (h), and the hydrodynamic radius (R) of the particle.

The mobility of a molecule in the cellular environment is affected by the following parameters:

  • The size of the molecule:an eightfold increase of the size of a soluble sperical protein decreases D by factor 2.
  • the viscosity of the cellular environment: e.g. membranes have a much higher viscosity than cytoplasm
  • protein-protein-interactions and binding to macromolecules can also slow down the diffusion
  • if flow or active transport is involved in the movement of the probed molecule, the measured movement rate can become significantly higher than the theoretical diffusion rate

The diffusion coefficient D of the probed molecule can be measured via the halftime of the recovery (thalf). This coefficient is influenced by the above stated parameters. Therefore this value is often termed as the effective diffusion coefficient (or apparent diffusion coefficient) Deff (in µm2 s-1) and reflects the mean squared displacement explored by the proteins through a random walk over time.

Changes of the effective diffusion coefficient, e.g. due to binding to a larger molecule, can be exploited to study the function of the protein of interest. For instance the endonuclease ERCC1/XPF is binding to DNA when damages are induced by UV light resulting in a decreased Deff (Houtsmuller et al. 1999).

By performing the FRAP experiment at different temperatures (e.g. 27 and 37 °C) it is possible to determine if energy-dependent processes are involved in the mobility of the investigated molecule. While the difference in molecular diffusion due to a 10 K change in absolute temperature is too small to be resolved by FRAP (D decreases only ~3%, Phair & Misteli 2000), energy-dependent processes are more sensitive to temperature (Hoogstraten et al. 2002).

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contact: Stefan Terjung           last update: 02/06/04