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Sum frequency generation (SFG) vibrational spectroscopy was used to investigate the interfacial properties of several amino acids, peptides, and proteins adsorbed at the hydrophilic polystyrene solid-liquid and the hydrophobic silica solid-liquid interfaces. The influence of experimental geometry on the sensitivity and resolution of the SFG vibrational spectroscopy technique was investigated both theoretically and experimentally. SFG was implemented to investigate the adsorption and organization of eight individual amino acids at model hydrophilic and hydrophobic surfaces under physiological conditions. Biointerface studies were conducted using a combination of SFG and quartz crystal microbalance (QCM) comparing the interfacial structure and concentration of two amino acids and their corresponding homopeptides at two model liquid-solid interfaces as a function of their concentration in aqueous solutions. The influence of temperature, concentration, equilibration time, and electrical bias on the extent of adsorption and interfacial structure of biomolecules were explored at the liquid-solid interface via QCM and SFG. QCM was utilized to quantify the biological activity of heparin functionalized surfaces. A novel optical parametric amplifier was developed and utilized in SFG experiments to investigate the secondary structure of an adsorbed model peptide at the solid-liquid interface.
Sum frequency generation (SFG) vibrational spectroscopy was used to investigate the interfacial properties of several amino acids, peptides, and proteins adsorbed at the hydrophilic polystyrene solid-liquid and the hydrophobic silica solid-liquid interfaces. The influence of experimental geometry on the sensitivity and resolution of the SFG vibrational spectroscopy technique was investigated both theoretically and experimentally. SFG was implemented to investigate the adsorption and organization of eight individual amino acids at model hydrophilic and hydrophobic surfaces under physiological conditions. Biointerface studies were conducted using a combination of SFG and quartz crystal microbalance (QCM) comparing the interfacial structure and concentration of two amino acids and their corresponding homopeptides at two model liquid-solid interfaces as a function of their concentration in aqueous solutions. The influence of temperature, concentration, equilibration time, and electrical bias on the extent of adsorption and interfacial structure of biomolecules were explored at the liquid-solid interface via QCM and SFG. QCM was utilized to quantify the biological activity of heparin functionalized surfaces. A novel optical parametric amplifier was developed and utilized in SFG experiments to investigate the secondary structure of an adsorbed model peptide at the solid-liquid interface.
Sum frequency generation (SFG) vibrational spectroscopy has been used to study the interfacial structure of several polypeptides and amino acids adsorbed to hydrophobic and hydrophilic surfaces under a variety of experimental conditions. Peptide sequence, peptide chain length, peptide hydrophobicity, peptide side-chain type, surface hydrophobicity, and solution ionic strength all affect an adsorbed peptide's interfacial structure. Herein, it is demonstrated that with the choice of simple, model peptides and amino acids, surface specific SFG vibrational spectroscopy can be a powerful tool to elucidate the interfacial structure of these adsorbates. Herein, four experiments are described. In one, a series of isosequential amphiphilic peptides are synthesized and studied when adsorbed to both hydrophobic and hydrophilic surfaces. On hydrophobic surfaces of deuterated polystyrene, it was determined that the hydrophobic part of the peptide is ordered at the solid-liquid interface, while the hydrophilic part of the peptide appears to have a random orientation at this interface. On a hydrophilic surface of silica, it was determined that an ordered peptide was only observed if a peptide had stable secondary structure in solution. In another experiment, the interfacial structure of a model amphiphilic peptide was studied as a function of the ionic strength of the solution, a parameter that could change the peptide's secondary structure in solution. It was determined that on a hydrophobic surface, the peptide's interfacial structure was independent of its structure in solution. This was in contrast to the adsorbed structure on a hydrophilic surface, where the peptide's interfacial structure showed a strong dependence on its solution secondary structure. In a third experiment, the SFG spectra of lysine and proline amino acids on both hydrophobic and hydrophilic surfaces were obtained by using a different experimental geometry that increases the SFG signal. Upon comparison of these spectra to the SFG spectra of interfacial polylysine and polyproline it was determined that the interfacial structure of a peptide is strongly dependent on its chain length. Lastly, SFG spectroscopy has been extended to the Amide I vibrational mode of a peptide (which is sensitive to peptide secondary structure) by building a new optical parametric amplifier based on lithium thioindate. Evidence is presented that suggests that the interfacial secondary structure of a peptide can be perturbed by a surface.
This book summarizes the main surface analysis techniques that are being used to study biological specimens/systems. The compilation of chapters in this book highlight the benefits that surface analysis provides. The outer layer of bulk solid or liquid samples is referred to as the surface of the sample/material. At the surface, the composition, microstructure, phase, chemical bonding, electronic states, and/or texture is often different than that of the bulk material. The outer surface is where many material interactions/reactions take place. This is especially true biomaterials which may be fabricated into bio-devices and in turn implanted into tissues and organs. Surfaces of biomaterials (synthetic or modified natural materials) are of critical importance since the surface is typically the only part of the biomaterial/bio-device that comes in contact with the biological system. Analytical techniques are required to characterize the surface of biomaterials and quantify their impact in real-world biological systems. Surface analysis of biological materials started in the 1960’s and the number of researchers working in this area have increased very rapidly since then, a number of advances have been made to standard surface analytical instrumentation, and a number of new instruments have been introduced.
Vibrational sum frequency spectroscopy (VSFS) is a nonlinear optical process. The sum frequency signal is proportional to the square of second order nonlinear susceptibility, which is proportional to the average of polarizabilities of molecules, which is related to molecular orientation. Since the polarizabilities of molecules in bulk phase will be canceled out, a sum frequency signal can only be generated from interfaces where the inversion symmetry is broken. Because of its interfacial specificity, VSFS has been applied to study many interfacial phenomena. In this dissertation we investigated various biological interfaces with VSFS. Fibrinogen adsorption was studied at the protein/solid interface in combination with atomic force microscopy (AFM), immunoassay, and VSFS. Astonishing changes in the interfacial water orientation accompanied by the pH changes provided fibrinogen0́9s adsorption mechanism up to the amino acid level. Enzymatic fragmentation of fibrinogen revealed that the adsorption property of fibrinogen was mainly from the alpha C fragments of the protein. Mimicking of the fibrinogen binding site with polypeptides was successfully performed and showed very similar properties of fibrinogen adsorption. Protein stability is sensitive to the salts in solutions. The ability of ions to stabilize protein was ordered by Hofmeister in 1888 and the order is SO42− =̃ HPO42−> F−> Cl−> Br−> NO3−> I− (=̃ ClO4−)> SCN−. Even though the phenomenon was observed in various biological systems, the origin of those ionic effects is still not well understood. We studied ion effects on alkyl chain ordering and interfacial water structure for octadecylamine, dimethyldidodecylammonium bromide, and dilauroylphosphotidyl choline monolayers. Because of its ability to probe a hydrophobic moiety and interfacial water at the same time, VSFS provided further information to understand the Hofmeister series. We found that the Hofmeister effect is a combinatorial effect of screening effects, ion binding, and dispersion forces.
Vibrational Spectroscopy in Protein Research offers a thorough discussion of vibrational spectroscopy in protein research, providing researchers with clear, practical guidance on methods employed, areas of application, and modes of analysis. With chapter contributions from international leaders in the field, the book addresses basic principles of vibrational spectroscopy in protein research, instrumentation and technologies available, sampling methods, quantitative analysis, origin of group frequencies, and qualitative interpretation. In addition to discussing vibrational spectroscopy for the analysis of purified proteins, chapter authors also examine its use in studying complex protein systems, including protein aggregates, fibrous proteins, membrane proteins and protein assemblies. Emphasis throughout the book is placed on applications in human tissue, cell development, and disease analysis, with chapters dedicated to studies of molecular changes that occur during disease progression, as well as identifying changes in tissues and cells in disease studies. Provides thorough guidance in implementing cutting-edge vibrational spectroscopic methods from international leaders in the field Emphasizes in vivo, in situ and non-invasive analysis of proteins in biomedical and life science research more broadly Contains chapters that address vibrational spectroscopy for the study of simple purified proteins and protein aggregates, fibrous proteins, membrane proteins and protein assemblies
Surfaces/interfaces are omnipresent in nature, ranging from physics and chemistry to biology and material sciences. To characterize the interfacial structures and understand the surface phenomena, many useful tools have been developed. Vibrational sum frequency generation (VSFG) spectroscopy has proven to be a powerful second-order non-linear optical technique for this purpose and is gaining more and more attention nowadays. With VSFG, two pulsed laser beams, one at infrared frequency and one at visible frequency, are incident on the surface and generate a new beam with a frequency equal to the sum of the IR and visible frequencies. When the IR frequency matches a surface vibrational mode frequency, this process would be resonantly enhanced. In this dissertation, this surface-sensitive technique was adopted to investigate several interfaces with special relevance to biology. The first topic of interest is the unusual orientation of a strong protein stabilizer, trimethylamine N-oxide (TMAO), at two aqueous/hydrophobic interfaces (air/water interface and OTS/water interface). By interpretation of the relative phase of VSFG spectra coupled with a numerical algorithm, the maximum entropy method (MEM) anaylysis of the molecular orientation, it is found that the methyl groups of TMAO prefer to point into the aqueous medium, while the oxide moieties (N+-O-) orient towards the hydrophobic air or OTS. This unusual orientation may be attributed to the more hydrophilic nature of methyl groups that is attached to a strong electron withdrawing atom such as a quaternary nitrogen. These results could help elucidate the stabilizing effect of TMAO on proteins the increased need to keep the methyl group hydrated would cause them to be excluded from protein interface and thereby lead to protein stabilization.The other major issue focused in this dissertation is based on the ion specific interactions at a charged interface, which plays a decisive role in various physico-chemical and biological processes. Binding affinity of different cations to monolayers of amphiphilic molecules (e.g. fatty acids, phospholipids) at the air/aqueous surfactant interfaces, may provide molecular level clues on various functions of cell membranes that are resembled by these amphiphilic molecules. Specifically, the binding events of several alkali cations to the hydrophilic carboxylate headgroups of long chain fatty acid, inferred from interfacial water structures, are thoroughly investigated by VSFG measurement. Results show that Li+ binds strongest to the negatively charged carboxylate groups, followed by Na+, then K+ although the difference is slight. The ranking of the alkali metal cations' binding abilities differs from the sequence predicted by the law of match water affinities (LMWA) and also varies with different headgroups in the model system, which may suggest the distinct solvation behaviors of these ions. Such findings should help to elucidate the molecular-level binding behavior to proteins in aqueous solutions.