| Biomolecules at the interfaces are crucial for the functionalities of all biological organisms, including homeostasis maintenance, intercellular communication, and transportation. Understanding of the structures, behaviors, and functionalities of biomolecules at the interfaces is also essential for tackling challenges in biomedical sciences and engineering. One of the leading challenges in studying interfacial behaviors of biomolecules is the lack of surface-specific techniques that exclude contribution from the bulk. Sum frequency generation (SFG) spectroscopy is a second-order nonlinear optical technique that is intrinsically surface selective and orientation sensitive. The application of SFG spectroscopy in biological systems opens up huge potentials in tackling interface-related challenges such as molecular transport across cell membranes, transmembrane signal transduction, cell recognition and adhesion, drug delivery across cell membranes, immunological responses, biosensors, heterogeneous biocatalysts, and enzymes on the electrodes of biofuel cells. All of these topics in applied science and engineering require understanding of biomolecules at interfaces. In this dissertation, SFG spectroscopy is utilized in combination with conventional surface characterization techniques to gain comprehensive fundamental knowledge of biomolecular behaviors at the interfaces, followed by the insights to guide the design of protein-based biocompatible materials.;First, the supported lipid bilayer was investigated as the model system for cell membranes. Cell membranes are crucial for many biological processes. Due to the complexity of the cell membrane, lipid bilayers are often used as model systems in biophysical studies. Lipid structures influence the physical properties of bilayers, but their interplay, especially in multiple-component lipid bilayers, has not been fully explored. The Langmuir--Blodgett method was applied to make mono- and bilayers of 1,2-dihexadecanoyl-sn-glycero-3- phospho-( 1 '-rac-glycerol) (DPPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-(1'-racglycerol) (POPG), and 1-hexadecanoyl-2-(9Z-octadecenoyl)-sn-glycero-3-phospho-l-serine (POPS) as well as their 1:1 binary mixtures. The fluidity, stability, and rigidity of these structures were then investigated using sum frequency generation (SFG) spectroscopy combined with analyses of surface pressure---area isotherms, compression moduli, and stability. The results show that single-component bilayers, both saturated and unsaturated, may not be ideal membrane mimics because of their low fluidity and/or stability. However, the binary saturated and unsaturated DPPG/POPG and DPPG/POPS systems show not only high stability and fluidity, but also high resistance to changes in surface pressure, especially in the range of 25-35 mN/m, typical of cell membranes. Because the ratio of saturated to unsaturated lipids is highly regulated in cells, the results underline the possibility of modulating biological properties using lipid compositions. Also, the usage of flat optical windows as solid substrates in SFG experiments should make the SFG method more compatible with other techniques, enabling more comprehensive surface characterizations of bilayers in the future.;Second, a surface-active biofilm protein Bs1A obtained from Bacillus subtilis biofilm was investigated at the air-water interface using various surface characterization techniques and spectroscopies. Biofilm is an extracellular matrix of bacteria and serves as a protective shield of bacterial communities. It is crucial for microbial growth and one of the leading causes of human chronic infections as well. However, the structures and molecular mechanism of biofilm formation remain largely unknown. A surface-active protein Bs1A, expressed in the biofilms of Bacillus subtilis, was investigated at the air-water interface. Using techniques in surface chemistry and spectroscopy, Bs1A was found to form a stable and robust Langmuir monolayer at the air/water interface. The results show that the Bs1A Langmuir monolayer underwent two-stage elasticity in the solid state phase upon mechanical compression: one is possibly due to the intermolecular interaction and the other is likely due to both the intermolecular interaction and the intramolecular distortion. The Langmuir monolayer of Bs1A shows abrupt changes in rigidities and elasticities at ~25 mN/m. This surface pressure is close to the one at which B1sA saturates the air/water interface as a self-assembled film without mechanical compression, corresponding to a mean molecular area of ~700 A2 per molecule. Based on the results of surface UV-visible spectroscopic and infrared reflective-absorption spectroscopic studies, it is proposed that the Bs1A Langmuir monolayer carries intermolecular elasticity before ~25 mN/m and both intermolecular/intramolecular elasticity after ~25 mN/m. These results provide valuable insights into the understanding of biofilm-associated protein under high mechanical force, shedding light on the further investigation of biofilm structure and functionalities.;Finally, based on the knowledge obtained from interfacial characterization of the BslA protein, SpyTag was genetically introduced into the BslA protein to functionalize the protein for potential biomaterial design and engineering. Proteins are desirable building blocks to create self-assembled, spatially defined structures and interfaces on length-scales that are inaccessible by traditional methods. A novel approach was introduced to create functionalized monolayers using the proteins Bs1A and SpyCatcher/SpyTag. BslA is a bacterial hydrophobin whose amphiphilic character underlies its ability to assemble into a monolayer at both air/water and oiUwater interfaces. The work demonstrates that having the SpyTag peptide fused at the N- or C-terminus does not affect the formation of such monolayers. We also establish the creation of stable oil-in-water microcapsules using Bs1A, and also show the fabrication of capsules outwardly displaying the reactive SpyTag peptide by fusing it to the C-terminus of Bs1A. Such capsules can be covalently labeled by reacting the surface-displayed SpyTag with SpyCatcher fused to any desired protein. We demonstrate this principle by labeling microcapsules using green fluorescent protein (GFP). All components are genetically encodable, the reagents can be readily prepared in large quantities, and all reactions occur at ambient temperature in aqueous solution. Thus, this straightforward, modular, scalable strategy has myriad potential applications in the creation of novel, functional materials and interfaces. |
