Properties of Protein and Polymer Systems

Interactions of proteins with other proteins, with polymers and with surfaces are of great practical importance within for instance protein purification, drug delivery, and food technology. The aim of this work is to increase the understanding of protein-protein, protein-polyelectrolyte, and protein-surface interactions, particularly concerning the role of electrostatic interactions in these systems. Properties that have been studied include the role of discrete charges of the protein in protein self-association, complexation with a polyelectrolyte, and adsorption to a surface.

The main method used in the present investigation has been Monte Carlo simulation. Lysozyme was chosen as a model protein because of its high degree of structural stability under a variety of conditions and because of the large amount of experimental data available for comparison with the simulation results. The protein model consists of a hard sphere with positive and negative charges beneath the surface. The positions of the charges have been taken from the lysozyme crystal structure and projected on a sphere.

In the first study the interaction between one protein and one polymer was investigated. It was found that the polymer was distributed unevenly over the surface of the protein because of the discrete charges. Furthermore, it was found that the discrete-charge protein model gave a higher level of adsorbed polyelectrolyte than a model with uniformly distributed charge. In the second study, a solution with many proteins was investigated, and the short-range interaction between the proteins was adjusted according to experimental light scattering data. Simulation results on protein self-association agreed with previously reported results in the literature. In the third study, different ways to represent the charges of the protein and different ways to include the small ions were compared. There was a small difference between the different charge representations. The difference between the ion models was minute at low ionic strength and slightly larger at higher ionic strength. In a fourth study, the system was extended to many proteins and many polymers. We were able to simulate the precipitation and redissolution, which is observed experimentally when a charged polymer is added to a protein solution. In a final study, the protein model was investigated at a surface. Simulated adsorption isotherms as well as lateral radial distribution functions for adsorbed proteins agreed well with experimental data.