Protein-Polymer Interaction and Association in Aqueous Solution

Abstract: The aim of this thesis has been to study protein-polymer interaction and association. Two different techniques were used; Monte Carlo simulations were combined with partitioning in polymeric aqueous two-phase systems. Monte Carlo simulations have been used to evaluate the hydrophobic interaction in electrolyte solution. The results showed that monovalent salt only slightly increases the hydrophobic interaction whereas divalent salt drastically increases the hydrophobic interaction. In two subsequent studies the effect of protein surface hydrophobicity on protein-polymer association was investigated. The surface hydrophobicity was kept constant, while the distribution was altered from a heterogeneous to homogenous. In these studies it was established that proteins with a more heterogeneous surface hydrophobicity adsorb weakly hydrophobic polymer better than proteins with homogenous surface distributions. It was also shown that this also applies independently of polymer concentration, chain length, and chain flexibility. Protein adsorption to polymer-grafted surfaces was investigated with lattice Monte Carlo simulations. The free energy difference between protein adsorption to surfaces with and without end-grafted polymers was determined under different conditions. The effect of several different properties was examined, such as grafting density, chain length, protein size, and hydrophobicity of the protein, polymer and surface. The results showed that longer polymer chains and higher grafting densities reduce protein adsorption. Furthermore, the study also showed that hydrophilic polymers are better in preventing adsorption of large proteins, whereas hydrophobic polymers are better in preventing adsorption of small proteins. Protein-polymer association was also studied in polymeric aqueous two-phase systems containing a cationic hydrophobically modified polymer and sodium dodecyl sulfate (SDS). It was shown that it is possible to direct the proteins either to top or bottom phase depending on their net charge. Negatively charged BSA was shown to partition to the cationic polymer phase, whereas positively charged lysozyme was shown to partition to the polymer-depleted phase. In systems of both cationic polymer and SDS a net negatively charged polymer surfactant complex was formed. Here BSA was partitioned to the polymer/SDS depleted phase, while lysozyme was partitioned to the polymer/SDS phase.