Computer simulations and theoretical aspects of the depletion interaction in protein-oligomer mixtures

Phase stability of aqueous mixtures of bovine serum albumin with low molecular mass salts in presence of polyethylene glycol

The stability of bovine serum albumin (BSA) solutions against phase separation caused by cooling the system is studied under the combined influence of added poly(ethylene glycol) (PEG) and alkali halide salts in water as solvent. The phase stability of the system depends on the concentration of the added PEG and its molecular mass, the concentration of the low molecular mass electrolyte and its nature, as also on the pH of the solution. More specifically, the addition of NaCl to the BSA-PEG mixture promotes phase separation at pH = 4.0, where BSA carries the net positive charge in aqueous solution, and it increases the stability of the solution at pH=4.6, i.e., near the isoionic point of the protein. Moreover, at pH = 4.6, the cloud-point temperature decreases in the order from NaF to NaI and from LiCl to CsCl. The order of the salts at pH = 4.0 is exactly reversed: LiCl and NaF show the weakest effect on the cloud-point temperature and the strongest decrease in stability is caused by RbCl and NaNO3. An attempt is made to correlate these observations with the free energies of hydration of the added salt ions and with the effect of adsorption of salt ions on the protein surface on the protein-protein interactions. Kosmotropic salt ions decrease the phase stability of BSA-PEG-salt solutions at pH < pI, while exactly the opposite is true at pH = pI.

Explicit-water theory for the salt-specific effects and Hofmeister series in protein solutions

Effects of addition of salts on stability of aqueous protein solutions are studied theoretically and the results are compared with experimental data. In our approach, all the interacting species, proteins, ions, and water molecules, are accounted for explicitly. Water molecules are modeled as hard spheres with four off-center attractive square-well sites. These sites serve to bind either another water or to solvate the ions or protein charges. The ions are represented as charged hard spheres, and decorated by attractive sites to allow solvation. Spherical proteins simultaneously possess positive and negative groups, represented by charged hard spheres, attached to the surface of the protein. The attractive square-well sites, mimicking the protein-protein van der Waals interaction, are located on the surface of the protein. To obtain numerical results, we utilized the energy route of Wertheim's associative mean spherical approximation. From measurable properties, we choose to calculate the second virial coefficient B2, which is closely related to the tendency of proteins to aggregate and eventually crystalize. Calculations are in agreement with experimental trends: (i) For low concentration of added salt, the alkali halide salts follow the inverse Hofmeister series. (ii) At higher concentration of added salt, the trend is reversed. (iii) When cations are varied, the salts follow the direct Hofmeister series. (iv) In contrast to the colloidal theories, our approach correctly predicts the non-monotonic behavior of B2 upon addition of salts. (v) With respect to anions, the theory predicts for the B2 values to follow different sequences below and above the iso-ionic point, as also confirmed experimentally. (vi) A semi-quantitative agreement between measured and calculated values for the second virial coefficient, as functions of pH of solution and added salt type and concentration, is obtained.

Solubility of lysozyme in polyethylene glycol-electrolyte mixtures: the depletion interaction and ion-specific effects

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 last defines the net charge of the lysozyme. The results indicate that the solubility of lysozyme decreases with the 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 the 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 H(2)PO(4)(-) (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(+) (coions) were much smaller. The results at pH = 4.0 show that anions decrease the lysozyme solubility in the order F(-) < H(2)PO(4)(-) < Cl(-) < Br(-) (the inverse Hofmeister series), whereas cations follow the direct Hofmeister series (Cs(+) < K(+) < Na(+)) in this situation.