Measurement of protein diffusion in Dextran solutions by holographic interferometry

Theories and experimental measurements related to the diffusion of globular macromolecules and small spheres in polymer solutions are discussed. It is shown that the Kirkwood‐Riseman point scatterer and Brinkman models, two theoretical approaches that lead to hydrodynamic screening, are equivalent. Holographic interferometry is presented as a new method for measuring gradient diffusion of proteins in transparent polymer solutions and gels. This method is used to examine the effect of ionic strength, polymer concentration and polymer molecular weight on the diffusion of bovine serum albumin (BSA) in dextran solutions. The data are interpreted in light of the hydrodynamic screening and Stokes‐Einstein models of diffusion. In particular, it is shown that while the Stokes‐Einstein equation may be appropriate for the diffusion of relatively large latex spheres in polymer solutions, it is inappropriate for predicting diffusion coefficients of BSA and comparable proteins in such solutions.