Solubility of Lysozyme in PEG-Electrolyte Mixtures:
the Depletion Interaction and Ion-Specific Effects
Matjaz Boncina 1, Jurij Rescic 1* and Vojko Vlachy 1
1 University of Ljubljana, Faculty of Chemistry and Chemical Technology
* To whom correspondence should be addressed. E-mail: jurij.rescic{at}fkkt.uni-lj.si.
Submitted on January 3, 2008
Revised on January 28, 2008
Accepted on 28 March 2008
 |
Abstract |
|---|
The solubility of aqueous solutions of lysozyme in the presence of polyethylene glycol and various alkaline salts was studied experimentally. The protein-electrolyte mixture was titrated with polyethylene glycol and when precipitation of the protein occurred, a strong increase of the absorbance at 340 nm was observed. The solubility data were obtained as a function of experimental variables such as protein and electrolyte concentrations, electrolyte type, degree of polymerization of polyethylene glycol, and pH of the solution; the latter defines the net charge of the lysozyme. The results indicate that the solubility of lysozyme decreases with addition of polyethylene glycol; the solubility is lower for a polyethylene glycol with a higher degree of polymerization. Further, the logarithm of the protein solubility is a linear function of the polyethylene glycol concentration. The process is reversible and the protein remains in its native form. An increase of the electrolyte (NaCl) concentration decreases the solubility of lysozyme in the presence and absence of polyethylene glycol. The effect can be explained by screening of the charged amino residues of the protein. The solubility experiments were performed at two different pH values (pH = 4.0 and 6.0), where the lysozyme net charge was +11 and +8, respectively. Ion-specific effects were systematically investigated. Anions such as Br-, Cl-, F- and H2PO4- (all in combination with Na+), when acting as counterions to a protein with positive net charge, exhibit a strong effect on the lysozyme solubility. The differences in protein solubility for chloride solutions with different cations Cs+, K+, and Na+ (co-ions) were much smaller. The results at pH = 4.0 show that anions decrease the lysozyme solubility in the order F- < H2PO4- < Cl- < Br- (the inverse Hofmeister series), while cations follow the direct Hofmeister series (Cs+ < K+ < Na+) in this situation.
Key Words:
Hofmeister effect, depletion interaction, electrolyte, lysozyme, polyethylene glycol, solubility curves
