Chemical and Biomolecular Engineering - Theses
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ItemPhysical interaction forces in biological soft matter( 2012)Biological soft matter such as simple proteins to more complex systems such as membranes and cells are composed of a variety of self-assembling units that involve a myriad of dynamic interactions. Understanding the physical interaction forces that occur between simpler biological molecules can give valuable insights to the interactions in whole biological systems in an attempt to manipulate them efficiently. This thesis has examined the direct interaction forces in a series of systems with increasing complexity from well-defined colloidal systems with adsorbed biomolecular coatings, to the interactions in immensely complex living cancer cells. Highly sophisticated sensitive force measurement techniques of Total Internal Reflection Microscopy (TIRM) and Atomic Force Microscopy (AFM) were used to probe these interactions. TIRM was used to measure protein interactions in various electrolyte solutions to determine favorable interactions conducive to protein crystallization at very low concentrations previously unexplored in detail in literature. A fundamental understanding of weak protein interactions is useful for purification of protein mixtures, understanding protein diffusion in concentrated mixtures, and for stabilization of proteins in therapeutic formulations. Trends in protein interactions versus the type of electrolyte comparable to the Hofmeister series were apparent for certain proteins at much lower electrolyte concentrations than observations in literature. Variations in the trends were also seen for other proteins, providing evidence that pure protein interactions are complicated and affected by various factors such as the protein concentration and the surface density, in addition to the salt type and concentration. TIRM proved to be a very promising tool for quantifying pure protein interactions at very low salt concentrations. AFM was used to investigate interaction forces between smart new reversibly switchable biosurfactants to control foam stability. Previous work using AFM on drops and bubbles has demonstrated the importance of surface active molecules in these systems. The principles of such studies were applied to explore biomolecules, such as peptides at deformable interfaces. Direct force measurements showed that the adsorption of proteins was a complex process and the investigation of protein foam stability via air-water interfaces adds complexity to bubble-bubble interactions. The surface forces for very similar proteins were found to be vastly different. This study also highlighted that in general, bubble stability is governed by three key factors: the hydrodynamic drainage behavior of the thin film in between the bubbles, the bubble deformation, and the equilibrium surface forces in the thin film, regardless of the presence of various surfactants at the interface. Direct forces were also investigated in highly complex and poorly characterized biological systems such as single, living cancer cells and the milk fat globule in bovine milk. Direct forces measured using AFM helped elucidate morphological and mechanical changes that occur in cancer cells through its progression of cancer which became stiffer at the latter stages of the disease. In addition, AFM also showed changes to the milk fat globule membrane as it encountered mechanical and shear forces in various steps of the milk manufacturing process. AFM aided in characterizing and quantifying differences to give insight to such complex biological soft matter systems. This thesis has demonstrated the importance of investigating and understanding fundamental physical interaction behavior of biological soft matter to gain insight to interactions and other processes in complex biological systems. This thesis has further contributed to the understanding of interactions between proteins and other biomolecules under various solution conditions in determining how changes in the process properties affect whole soft matter systems.