Protein interactions in undersaturated and supersaturated solutions: a study using light and x-ray scattering

Area of Science:

• Biophysics
• Crystallography
• Physical Chemistry

Background:

• Understanding protein-protein interactions is crucial for controlling crystallization.
• Protein solutions exhibit varying interaction potentials based on concentration and ionic strength.

Purpose of the Study:

• To investigate protein interactions in undersaturated and supersaturated solutions.
• To determine the influence of ionic strength on protein-protein interactions and crystallization.
• To evaluate the applicability of a statistical mechanical model in describing these interactions.

Main Methods:

• Static and dynamic light scattering to determine the osmotic second virial coefficient (B(22)).
• Small-angle X-ray scattering (SAXS) to probe molecular structure and interactions.
• Fitting SAXS data with a statistical mechanical model based on Derjaguin-Landau-Verwey-Overbeek (DLVO) potential using Random Phase Approximation (RPA).

Main Results:

• Positive B(22) at low ionic strengths indicated repulsive protein-protein interactions, preventing crystallization.
• Negative B(22) at higher ionic strengths signified attractive interactions, facilitating protein crystallization.
• The DLVO-RPA model accurately described SAXS data in undersaturated solutions.
• The model showed limitations in supersaturated solutions, suggesting the involvement of hydration repulsion.

Conclusions:

• Ionic strength significantly modulates protein-protein interactions, driving the transition from repulsion to attraction and enabling crystallization.
• The DLVO-RPA model provides a useful framework for understanding interactions in dilute protein solutions.
• Further investigation is needed to elucidate the role of hydration forces in supersaturated protein solutions.