Ionic Strength Dependence of the Complex Coacervation between Lactoferrin and β-Lactoglobulin

Area of Science:

• Biophysical Chemistry
• Colloid and Surface Science
• Food Science and Technology

Background:

• Heteroprotein complex coacervation involves liquid-liquid phase separation of oppositely charged proteins in aqueous solutions.
• Previous studies established conditions for lactoferrin and β-lactoglobulin coacervate formation at pH 5.5.
• Understanding environmental factors influencing coacervation is crucial for controlling protein assembly.

Purpose of the Study:

• To investigate the effect of varying ionic strength on complex coacervation between lactoferrin and β-lactoglobulin.
• To elucidate the role of charge screening and Debye length in modulating protein-protein interactions during coacervation.

Main Methods:

• Direct mixing and desalting protocols were employed to induce and study complex coacervation.
• Varying concentrations of sodium chloride (NaCl) were used to control ionic strength.
• Isothermal titration calorimetry (ITC) was utilized to quantify binding energy changes.

Main Results:

• Complex coacervation was highly sensitive to ionic strength, with no phase separation observed above 20 mM NaCl.
• Coacervate yield decreased significantly with increasing NaCl concentrations (0–60 mM).
• A low NaCl concentration (around 2.5 mM) was found to promote binding energy between lactoferrin and β-lactoglobulin.

Conclusions:

• Increasing ionic strength screens electrostatic interactions, reducing the driving force for heteroprotein complex coacervation.
• The observed sensitivity to salt concentration highlights the critical role of electrostatics in this process.
• Specific low ionic strengths can enhance protein binding, offering a potential mechanism for fine-tuning coacervate formation.