Overview

Protein crystallization is an ongoing area of research. Much theoretical work exists in this field, however currently there is no universal theory that predicts the conditions at which proteins crystallize. A better understanding of protein behavior in different solutions well help create high quality crystals suitable for crystallography and will also allow us to prevent and understand the onset of certain diseases. In this thesis, we examine many interesting models of protein self-assembly in the presence of aqueous electrolyte solutions to extend the understanding of phase transitions taking place in such systems. It is well known experimentally that salts play an important role in phase transitions of protein solutions, among other things. We analyze a salt specific model of lysozyme based on ion-dispersion interactions in order to determine where the liquid-liquid and liquid-solid phase transitions occur. Using Monte Carlo simulations, we show that the system has a metastable fluid-fluid transition, which is consistent with experimental observations of this system at similar salt concentrations. We also examine the role of salts on the metastable fluid-fluid curve in a model of aluminum oxide nanoparticles. The methods we employ include finite-size scaling, multicanonical histogram reweighting and Gibbs ensemble Monte Carlo. We show that, as expected, this interaction potential belongs to the Ising universality class. The scaling fields and critical point parameters are obtained in the thermodynamic limit of infinite system size by extrapolation of our finite size scaling results. We also quantitatively demonstrate how different salts have an effect on the critical point of the metastable fluid-fluid curve. Ion-specific interactions may manifest themselves in many ways. In our most recent study, we examine another model of lysozyme at different salt concentrations which includes interactions due to ion-specific and hydrophobic surface effects. We use a potential of mean force along with Monte Carlo simulations to determine the liquid-liquid and liquid-solid phase diagrams for different salt concentrations of NaCl. In one case, we observe a stable liquid-solid phase transition while for the higher salt concentration, we observe that the system is just slightly metastable. We attribute this to the fact that the potential for the higher salt concentration has a repulsive maximum, which effectively shortens the range of the attraction. Lastly, we present preliminary results on gelation for a simple model of globular proteins in which the liquid-liquid and liquid-solid phase diagram is already known. In this model, we use canonical ensemble Monte Carlo simulations to investigate the gel region below the critical point where we observe that the arrested gel state is preceded by spinodal decomposition. In some cases, we also observe local crystallization within the gel state.

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