Light Scattering by Liquids at 6937 Angstroms

Protein solutions close to liquid-liquid phase separation exhibit a universal osmotic equation of state and dynamical behavior

Liquid-liquid phase separation (LLPS) of protein solutions is governed by highly complex protein-protein interactions. Nevertheless, it has been suggested that based on the extended law of corresponding states (ELCS), as proposed for colloids with short-range attractions, one can rationalize not only the thermodynamics, but also the structure and dynamics of such systems. This claim is systematically and comprehensively tested here by static and dynamic light scattering experiments. Spinodal lines, the isothermal osmotic compressibility κ_T and the relaxation rate of concentration fluctuations Γ are determined for protein solutions in the vicinity of LLPS. All these quantities are found to exhibit a corresponding-states behavior. This means that, for different solution conditions, these quantities are essentially the same if considered at similar reduced temperature or second virial coefficient. For moderately concentrated solutions, the volume fraction ϕ dependence of κ_T and Γ can be consistently described by Baxter's model of adhesive hard spheres. The off-critical, asymptotic T behavior of κ_T and Γ close to LLPS is consistent with the scaling laws predicted by mean-field theory. Thus, the present work aims at a comprehensive experimental test of the applicability of the ELCS to structural and dynamical properties of concentrated protein solutions.

Low angle laser light scattering-absolute calibration

A new method for the calculation of the Rayleigh factor from low angle light scattering measurements is developed. This method does not require a uniform intensity illuminating beam, hence efficiently utilizes all the beam from a focused laser source. Scattering volume is then very small, reducing sample volume and interference from contaminant particles. All the parameters necessary for the calculation of the Rayleigh factor (including the exact dependence on sample refractive index) are measurable, hence absolute calibration is possible. Over-all error is estimated to be less than 2.3% under specified conditions. Measurements are possible at scattering angles as small as 2 degrees obviating the need for angular extrapolations in the determination of molecular weight of most dissolved samples. Rayleigh factors at 22 degrees C, 633 nm, and a scattering angle of 4 degrees for water, methanol, benzene, and toluene, were found to be, respectively, 0.907, 2.56, 12.15, and 13.45 x 10(-6) cm(-1).