Effect of electrostatic interactions on biomolecular self-assembly processes

Title of dissertation: EFFECT OF ELECTROSTATIC INTERACTIONS ON BIOMOLECULAR SELF-ASSEMBLY PROCESSES Hongcheng Xu Doctor of Philosophy, 2018 Dissertation directed by: Dr. Silvina Matysiak, Associate Professor Fischell Department of Bioengineering Biophysics Program Molecular level self-assembly processes are not only ubiquitous in living cells, but also widely applied in industry to synthesize and fabricate a variety of nanoscale biomaterials. The emergence of ordered aggregates from disordered components typically requires driving forces from electrostatic interactions to hydrophobic-hydrophilic effects. This thesis aims to elucidate the effect of electrostatic interactions, and the intricate balance between electrostatic and hydrophobic interactions in dictating spontaneous self-assembly processes with three case studies covering various types of biomolecules. For the first case study, we have examined the pH-induced polysaccharide hydrogel network formation. The polysaccharide molecule chitosan forms hydrogels composed of water-filled cross-linking polymer chains. The pH-responsive selfassembly behavior of chitosan hydrogel has been utilized in fabricating nanomaterials with a wide range of applications. To investigate the role of electrostatic interactions in the chitosan hydrogel network formation, we have developed a novel coarse-grained (CG) chitosan polymer model that captures the pH-dependent selfassembly behavior. The structural, mechanical, and thermodynamical properties of chitosan polymer hydrogel have been characterized well in the simulations and agree very well with experimental observations. For the second case study, the anticancer peptide folding induced by phospholipid membrane was investigated. Peptide folding in an aqueous environment is a self-assembly process that has been well studied over the years. However, the folding in a membranous environment is complicated by the heterogeneity in phospholipid distributions and membrane-peptide interactions. To provide information about the driving forces behind membrane peptide folding and the effect of lipid composition on folding behavior, my work has combined our recently developed Water-Explicit Polarizable Protein (WEPPRO) and Membrane (WEPMEM) model to explore the driving forces behind model anticancer peptide SVS-1 folding and how they can be affected by changing the membrane composition. For the third case study, we have studied the formation of nanodomains in mixed lipid bilayers. Phospholipid membranes are essential components in animal cells. The heterogeneous distribution of phospholipids on the membrane bilayer plays an important role in cellular structure and function such as signal transduction and membrane fusion. Interactions between a mixture of lipids and different ligands give rise to interesting patterns that are yet to be understood. Model lipid bilayers with a content of anionic lipids have been shown experimentally to be sensitive to the presence of certain ions. Monovalent cation Li induces membrane phase transition similarly as Ca and Mg, while distinctive from other monovalent cations like Na and K. We have evaluated the role of electrostatics interactions in the sizedependent cation-induced lipid nanodomain formation with binary mixed bilayers composed of zwitterionic and anionic lipids. EFFECT OF ELECTROSTATIC INTERACTIONS ON BIOMOLECULAR SELF-ASSEMBLY PROCESSES