Viscoelastic and small angle neutron scattering studies of concentrated protein solutions

Small angle neutron scattering (SANS) and rheological measurements have been used to study horse heart cytochrome C, a globular protein characterized by approximately spherical shape (a × b × b = 15 × 17 × 17 Å3) with a molecular weight of 12 384 Da and a pI = 10.2. Two series of very concentrated protein solutions have been investigated at pD 5.4 and 11.0, respectively, the volume fraction of the protein spanning from 0.1 to 0.5. The Krieger–Dougherty model was applied to describe the relation between relative high shear viscosity of the solution and volume fraction of the protein at both pD in order to elucidate the charge effect on the interaction potential. The SANS intensity distributions at pD 5.4 were fitted using the GOCM model with an excellent agreement between the theory and experiments up to the volume fraction ϕ of 0.4. At pD 11.0 the intensity distribution at ϕ = 0.1 can be fitted with a pure form factor (oblate ellipsoid), suggesting that under this condition the cytochrome C molecules are almost uncharged and preserve the native molecular size. Addition of salt induces the transformation from liquid to a gel. This is a result of formation of ordered fractal clusters internally as evident from appearance of a second interaction peak at very low Q (magnitude of the scattering vector). The appearance of the low Q peak is also accompanied by a strong increase in the relative viscosity. These phenomena taken together can be considered as the signature of the gelation process.