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With many polymers being used as biomaterials, it is important to probe the interaction of proteins adsorbed onto these surfaces once the biomaterial is implanted inside of the body. SFG and AFM have been applied to examine the structural and conformational information of various proteins after adsorption onto different polymer surfaces. Results suggest that adsorbed proteins adopt certain conformations and/or orientations. These conformations and/or orientations are characteristic of certain secondary structures of the protein and may change as a function of time.
The interactions between polymers and proteins have a significant impact on whether a material is biocompatible. The study of these interactions must focus on the behavior of these molecules at the interface. A nonlinear vibrational spectroscopic technique, sum frequency generation (SFG) vibrational spectroscopy, has been used to investigate these surfaces and interfaces. Research focusing on the surface structures of plasticized polyurethanes, commonly used for biosensors, indicates that plasticizers can segregate to the polyurethane surface not only in air but also in water. In addition, plasticizer content can affect the protein adsorption behavior of these surfaces.
Phospholipids are one of the major structural components of biological membranes. The hydrophilic headgroups of lipid molecules interact with the adjacent interfacial water molecules. The hydrogen bonding structure of interfacial water is determined by the chemical structure of the lipid headgroups themselves. Moreover, the first few molecular layers of interfacial water mediate all the chemical interactions at the membrane/water interface. Herein, the changes in interfacial water structure associated with lipid-lipid and ion-lipid interactions next to various zwitterionic and charged lipid membranes are investigated. In the first part of the dissertation, the interfacial water structure and phosphate group hydration of 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) monolayers at air/water interfaces are described. Both vibrational sum frequency spectroscopy (VSFS) and Langmuir monolayer compression measurements were made. The PC lipids oriented water molecules predominantly through their phosphate-choline (P-N) dipoles and carbonyl moieties. Upon the introduction of low concentrations of 1,2 dioleoyl-3-trimethylammonium propane (DOTAP), a positively charged double chain surfactant, the TAP headgroups were attracted to the phosphate moieties on adjacent PC lipids. This attraction caused the monolayers to contract, expelling water molecules that were hydrogen bonded to the phosphate groups. Moreover, amplitude of the OH stretch signal decreased. At higher DOTAP concentrations, the positive charge on the monolayer caused an increase in the area per headgroup and water molecules in the near-surface bulk region became increasingly aligned. Under these latter conditions, the OH stretch amplitude was linearly proportional to the surface potential. By contrast, introducing 1,2-dioleoyl-sn-glycero-3-phosphatidylglycerol (DOPG), a negatively charged lipid, did not change the area per lipid or the phosphate-water hydrogen bonding network. As the interfacial potential grew more negative, the OH stretch amplitude increased continuously. Significantly, changes in the interfacial water spectrum were independent of the chemistry employed to create the positive or negative interfacial potential. For example, Ca2+ and tetracaine (both positively charged) disrupted the water structure similarly to low DOTAP concentrations, while SCN- and ibuprofen (both negatively charged) enhanced the water structure. The changes in interfacial water peaks as a function of increasing surface potential were deconvoluted into changes in directly bound water molecules and water molecules ordered by the electric field. At high surface potential values, the water molecules ordered by the electric field were found to dominate the VSFS signal changes. In a second set of experiments, the effect of inverting the P-N dipole on the interfacial water ordering was studied using VSFS. The inverted headgroup dipole was found to order less water molecules compared to PC headgroups. The second part of this dissertation describes various ion-lipid interactions and the associated changes in interfacial water structure. Using a combination of fluorescence imaging, surface pressure-area isotherms and VSFS, the differences between the interaction of Cu2+, Ca2+, Mg2+ and Zn2+ ions with the PS headgroup were characterized. Cu2+ ions bound to two PS lipids without attenuating the surface charge, whereas Ca2+, Mg2+ and Zn2+ ions neutralized the negative charge on the headgroups. Zn2+ and Ca2+ ions induced three-dimensional bleb formation in PS lipid bilayers because of their ability to form contact ion pairs with the PS headgroup moieties and to reduce the headgroup area dramatically. The molecular details of phase changes within a PS lipid monolayer induced by Zn2+ were further characterized in detail using VSFS. Zn2+ ions were found to dehydrate the PS lipid headgroups and rigidify the lipid alkyl tails. The changes in interfacial water structure next to PS lipid monolayers were utilized to determine the dissociation constant for Zn2+-PS interactions. The approach developed in this dissertation to interpret interfacial water signal changes during ion-lipid and lipid-lipid interactions can potentially be useful for future nonlinear optical experiments.
Describes the supramolecular properties of molecular assemblies that contain a solid phase, offering an integrated approach to measurement and addressibility. * Offers an integrated approach to measurement and addressibility. * Features case studies describing the major devices developed using this technology. * The prospects for the future of interfacial supramolecular assemblies are considered.
Work from our laboratory on vibrational sum frequency spectroscopic investigations of molecular ordering at the carbon tetrachloride-water interface is reviewed. Simple charged surfactants adsorbed at the liquid-liquid interface are seen to induce alignment of interfacial water molecules to a degree which is dependent on the induced surface potential. Saturation of water molecule alignment occurs at a surfactant surface concentration corresponding to a calculated surface potential of approximately 160 mV. In complementary studies, the relative degree of hydrocarbon chain ordering within monolayers of symmetric phosphatidylcholines of different chain lengths is inferred by the relative signal contributions of the methyl and methylene symmetric stretch modes. The degree of hydrocarbon chain disorder observed depends strongly on the method of monolayer preparation. By one method, a decrease in hydrocarbon chain order is seen with increasing chain length. Another method of monolayer formation yielded very well ordered hydrocarbon chains for the longest chain phosphatidylcholine studied, and showed much greater disorder in shorter chain species which was comparable to the other preparation method. These studies are a foundation for further work with this technique geared towards understanding molecular-level structural features in membrane-like assemblies and surface biochemical interactions of relevance to biomedical research.