Viscosity of concentrated protein solutions

Abstract

The viscosity of solutions of four proteins (Bovine Serum Albumin, Ovalbumin, α _{s -1} Casein, Lysozyme), brought to the random coil conformation, has been measured over a large concentration range extending into the entanglement region. A master curve is obtained in the dilute and semi-dilute regions with the reduced variables $\tilde \eta = {\eta _{sp}}/\left[ \eta \right]c$ and $\tilde c = c/y$ of Simha and Utracki.

By using Graessley’s expression for the polymer coil expansion at a given concentration in the semi-dilute region (c* ≤ c ≤ c**), a simple equation is established giving the relative viscosity η _r , as a function of concentration c: for ${c^ * } \leqslant c \leqslant {c^{ * * }},\;{\text{in}}\;{\eta _r} = 2a\left[ \eta \right]{c^ * }{\left( {c/{c^ * }} \right)^{1/2a}} - (2a - 1)\left[ \eta \right]{c^ * };$ where c* is the incipient overlap concentration, [η] the intrinsic viscosity, and a the Mark-Houwink exponent for the polymer-solvent considered.

This equation fits well the experimental results. The adjustment yields for the parameter a values which are comprised between 0.6 and 0.7, as expected, for Bovine Serum Albumin and Ovalbumin, but very close to 0.5 for α _{s -1} Casein and Lysozyme. This can be explained by the fact that the molecular weights of the two latter proteins are lower than, or very close, the critical molecular weight; the critical molecular weight is estimated to be about 20000.