Molecular composition and mechanical properties of biopolymer interfaces studied by sum frequency generation vibrational spectroscopy and atomic force microscopy

Investigating buried polymer interfaces using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies of buried polymer interfaces using sum frequency generation (SFG) vibrational spectroscopy. Both buried solid/liquid and solid/solid interfaces involving polymeric materials are discussed. SFG studies of polymer/water interfaces show that different polymers exhibit varied surface restructuring behavior in water, indicating the importance of probing polymer/water interfaces in situ. SFG has also been applied to the investigation of interfaces between polymers and other liquids. It has been found that molecular interactions at such polymer/liquid interfaces dictate interfacial polymer structures. The molecular structures of silane molecules, which are widely used as adhesion promoters, have been investigated using SFG at buried polymer/silane and polymer/polymer interfaces, providing molecular-level understanding of polymer adhesion promotion. The molecular structures of polymer/solid interfaces have been examined using SFG with several different experimental geometries. These results have provided molecular-level information about polymer friction, adhesion, interfacial chemical reactions, interfacial electronic properties, and the structure of layer-by-layer deposited polymers. Such research has demonstrated that SFG is a powerful tool to probe buried interfaces involving polymeric materials, which are difficult to study by conventional surface sensitive analytical techniques.

Impact of surface chemistry

The applications of molecular surface chemistry in heterogeneous catalyst technology, semiconductor-based technology, medical technology, anticorrosion and lubricant technology, and nanotechnology are highlighted in this perspective. The evolution of surface chemistry at the molecular level is reviewed, and the key roles of surface instrumentation developments for in situ studies of the gas-solid, liquid-solid, and solid-solid interfaces under reaction conditions are emphasized.

In situ molecular level studies on membrane related peptides and proteins in real time using sum frequency generation vibrational spectroscopy

Sum frequency generation (SFG) vibrational spectroscopy has been demonstrated to be a powerful technique to study the molecular structures of surfaces and interfaces in different chemical environments. This review summarizes recent SFG studies on hybrid bilayer membranes and substrate-supported lipid monolayers and bilayers, the interaction between peptides/proteins and lipid monolayers/bilayers, and bilayer perturbation induced by peptides/proteins. To demonstrate the ability of SFG to determine the orientations of various secondary structures, studies on the interactions between different peptides/proteins (melittin, G proteins, alamethicin, and tachyplesin I) and lipid bilayers are discussed. Molecular level details revealed by SFG in these studies show that SFG can provide a unique understanding on the interactions between a lipid monolayer/bilayer and peptides/proteins in real time, in situ and without any exogenous labeling.

An investigation of the influence of chain length on the interfacial ordering of L-lysine and L-proline and their homopeptides at hydrophobic and hydrophilic interfaces studied by sum frequency generation and quartz crystal microbalance

Sum frequency generation vibrational spectroscopy (SFG) and quartz crystal microbalance with dissipation monitoring (QCM-D) are employed to study the interfacial structure and adsorbed amount of the amino acids L-lysine and L-proline and their corresponding homopeptides, poly-L-lysine and poly-L-proline, at two liquid-solid interfaces. SFG and QCM-D experiments of these molecules are carried out at the interface between phosphate buffered saline at pH 7.4 (PBS) and the hydrophobic deuterated polystyrene (d8-PS) surface as well as the interface between PBS and hydrophilic fused silica (SiO2). The SFG spectra of the amino acids studied here are qualitatively similar to their corresponding homopeptides; however, the SFG signal from amino acids at the solid/PBS interface is smaller in magnitude relative to their more massive homopeptides at the concentrations studied here. Substantial differences are observed in SFG spectra for each species between the hydrophobic d8-PS and the hydrophilic SiO2 liquid-solid interfaces, suggesting surface-dependent interfacial ordering of the biomolecules. Over the range of concentrations used in this study, QCM-D measurements also indicate that on both surfaces poly-L-lysine adsorbs to a greater extent than its constituent amino acid L-lysine. The opposite trend is demonstrated by poly-L-proline which sticks to both surfaces less extensively than its corresponding amino acid, L-proline. Lastly, we find that the adsorption of the molecules studied here can have a strong influence on interfacial water structure as detected in the SFG spectra.

Changes in adsorbed fibrinogen upon conversion to fibrin

The conversion of adsorbed fibrinogen to fibrin in the presence of the enzyme thrombin was studied using surface plasmon resonance (SPR), a quartz crystal microbalance (QCM), sum frequency generation (SFG), atomic force microscopy (AFM), and an elutability assay. Exposure of adsorbed fibrinogen to thrombin resulted in a mass loss at the surface consistent with fibrinopeptide release and conversion to fibrin. Changes in hydration upon conversion of adsorbed fibrinogen to fibrin were determined from comparisons of acoustic (QCM) and optical (SPR) mass adsorption data. Conversion to fibrin also resulted in the adsorbed layer becoming more strongly bound to the surface and more compact. The elutability of adsorbed fibrinogen by Triton X-100, studied with SPR, decreased from 90 +/- 5 to 6 +/- 2% after conversion to fibrin. The height of the adsorbed monolayer, as determined by AFM, decreased from 5.5 +/- 2.2 to 1.7 +/- 0.8 nm. We conclude that thrombin-catalyzed fibrinopeptide release triggers significant changes in fibrinogen conformation beyond peptide cleavage.