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Translational and Rotational Diffusion of Globular Protein in Concentrated Polymer Solutions

Lin Fu1, Brian Pethica1, Carlos R. N. Pacheco2, and Robert K. Prud'homme1. (1) Chemical engineering, Princeton University, Princeton, NJ 08540, (2) Chemistry, Princeton University, Princeton, NJ 08540

The transport of proteins (ovalbumin) in concentrated polymer solutions (hydroxylpropyl guar), including both translational diffusion and rotational diffusion, has been studied by NMR. Fluorine tagging of protein is introduced to differentiate the protein from polymer matrix. 19F Pulsed field gradient NMR is used to measure the translational diffusion coefficient, while T1 and T2 relaxation of 19F signal is measured to obtain rotational diffusion coefficient. Experimental results were compared with Stokes Einstein (SE) equation and effective medium theory. Good agreement between experimental data and effective medium theory was found. Both translational and rotational diffusion coefficient deviate from the Stokes Einstein (SE) equation significantly. For translational diffusion, this deviation indicates that on the length scale of protein size the protein molecule feels inhomogeneity of the matrix polymer. This results in a 67% decrease in protein mobility when the matrix polymer concentration is only 5wt%. Rotational diffusion coefficient is found to stay relatively constant when the tracer size is much smaller than the mesh size of polymer matrix, which is consistent with SE equation. But when the protein size is comparable to the mesh size of polymer solution, the SE model fails. In this range, proteins only feel a fraction of the hydrodynamic interaction of the polymer matrix and the relationship between the rotational diffusion coefficient and macroscopic viscosity obeys a power law. Further, translational and rotational diffusion correlation times are discovered to be coupled in a stretched exponential form.