Orientation determination of interfacial beta-sheet structures in situ

Light on the interactions between nanoparticles and lipid membranes by interface-sensitive vibrational spectroscopy

Nanoparticles are produced in natural phenomena or synthesized artificially for technological applications. Their frequent contact with humans has been judged potentially harmful for health, and numerous studies are ongoing to understand the mechanisms of the toxicity of nanoparticles. At the macroscopic level, the toxicity can be established in vitro or in vivo by measuring the survival of cells. At the sub-microscopic level, scientists want to unveil the molecular mechanisms of the first interactions of nanoparticles with cells via the cell membrane, before the toxicity cascades within the whole cell. Unveiling a molecular understanding of the nanoparticle-membrane interface is a tricky challenge, because of the chemical complexity of this system and its nanosized dimensions buried within bulk macroscopic environments. In this review, we highlight how, in the last 10 years, second-order nonlinear optical (NLO) spectroscopy, and specifically vibrational sum frequency generation (SFG), has provided a new understanding of the structural, physicochemical, and dynamic properties of these biological interfaces, with molecular sensitivity. We will show how the intrinsic interfacial sensitivity of second-order NLO and the chemical information of vibrational SFG spectroscopy have revealed new knowledge of the molecular mechanisms that drive nanoparticles to interact with cell membranes, from both sides, the nanoparticles and the membrane properties.

Role of Water during the Early Stages of Iron Oxyhydroxide Formation by a Bacterial Iron Nucleator

Understanding the nucleation of iron oxides and the underlying hydrolysis of aqueous iron species is still challenging, and molecular-level insights into the orchestrated response of water, especially at the hydrolysis interface, are lacking. We follow iron(III) hydrolysis in the presence of a synthetic bacterial iron nucleator, which is a magnetosome membrane specific peptide, by using a constant pH titration technique. Three distinct hydrolysis regimes were identified. Interface-selective sum frequency generation (SFG) spectroscopy was used to probe the interfacial reaction and water in direct contact with the peptide. SFG data reveal that iron(III) species react quickly with interfacial peptides while continuously enhancing water alignment into the later stages of hydrolysis. The gradually aligning water molecules are associated with initially promoted (regimes I and II) and later suppressed (regime III) hydrolysis after the saturation of water alignment has occurred until regime II. These interfacial insights are crucial for understanding the early stage of iron oxide biomineralization.

The Dysferlin C2A Domain Binds PI(4,5)P2 and Penetrates Membranes

Dysferlin is a large membrane protein found most prominently in striated muscle. Loss of dysferlin activity is associated with reduced exocytosis, abnormal intracellular Ca2+ and the muscle diseases limb-girdle muscular dystrophy and Miyoshi myopathy. The cytosolic region of dysferlin consists of seven C2 domains with mutations in the C2A domain at the N-terminus resulting in pathology. Despite the importance of Ca2+ and membrane binding activities of the C2A domain for dysferlin function, the mechanism of the domain remains poorly characterized. In this study we find that the C2A domain preferentially binds membranes containing PI(4,5)P2 through an interaction mediated by residues Y23, K32, K33, and R77 on the concave face of the domain. We also found that subsequent to membrane binding, the C2A domain inserts residues on the Ca2+ binding loops into the membrane. Analysis of solution NMR measurements indicate that the domain inhabits two distinct structural states, with Ca2+ shifting the population between states towards a more rigid structure with greater affinity for PI(4,5)P2. Based on our results, we propose a mechanism where Ca2+ converts C2A from a structurally dynamic, low PI(4,5)P2 affinity state to a high affinity state that targets dysferlin to PI(4,5)P2 enriched membranes through interaction with Tyr23, K32, K33, and R77. Binding also involves changes in lipid packing and insertion by the third Ca2+ binding loop of the C2 domain into the membrane, which would contribute to dysferlin function in exocytosis and Ca2+ regulation.

Conformation and Orientation of Antimicrobial Peptides MSI-594 and MSI-594A in a Lipid Membrane

There is significant interest in the development of antimicrobial compounds to overcome the increasing bacterial resistance to conventional antibiotics. Studies have shown that naturally occurring and de novo-designed antimicrobial peptides could be promising candidates. MSI-594 is a synthetic linear, cationic peptide that has been reported to exhibit a broad spectrum of antimicrobial activities. Investigation into how MSI-594 disrupts the cell membrane is important for better understanding the details of this antimicrobial peptide (AMP)'s action against bacterial cells. In this study, we used two different synthetic lipid bilayers: zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and anionic 7:3 POPC/1-palmitoyl-2-oleoyl-sn-glycero-3-phospho(1'-rac-glycerol) (POPG). Sum frequency generation (SFG) vibrational spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) were used to determine the orientations of MSI-594 and its analogue MSI-594A associated with zwitterionic POPC and anionic 7:3 POPC/POPG lipid bilayers. The simulated ATR-FTIR and SFG spectra using nuclear magnetic resonance (NMR)-determined structures were compared with experimental spectra to optimize the bent angle between the N- (1-11) and C- (12-24) termini helices and the membrane orientations of the helices; since the NMR structure of the peptide was determined from lipopolysaccharide (LPS) micelles, the optimization was needed to find the most suitable conformation and orientation in lipid bilayers. The reported experimental results indicate that the optimized MSI-594 helical hairpin structure adopts a complete lipid bilayer surface-bound orientation (denoted "face-on") in both POPC and 7:3 POPC/POPG lipid bilayers. The analogue peptide, MSI-584A, on the other hand, exhibited a larger bent angle between the N- (1-11) and C- (12-24) termini helices with the hydrophobic C-terminal helix inserted into the hydrophobic region of the bilayer (denoted "membrane-inserted") when interacting with both POPC and 7:3 POPC/POPG lipid bilayers. These experimental findings on the membrane orientations suggest that both peptides are likely to disrupt the cell membrane through the carpet mechanism.

Orientation of the Dysferlin C2A Domain is Responsive to the Composition of Lipid Membranes

Dysferlin is a 230 kD protein that plays a critical function in the active resealing of micron-sized injuries to the muscle sarcolemma by recruiting vesicles to patch the injured site via vesicle fusion. Muscular dystrophy is observed in humans when mutations disrupt this repair process or dysferlin is absent. While lipid binding by dysferlin's C2A domain (dysC2A) is considered fundamental to the membrane resealing process, the molecular mechanism of this interaction is not fully understood. By applying nonlinear surface-specific vibrational spectroscopy, we have successfully demonstrated that dysferlin's N-terminal C2A domain (dysC2A) alters its binding orientation in response to a membrane's lipid composition. These experiments reveal that dysC2A utilizes a generic electrostatic binding interaction to bind to most anionic lipid surfaces, inserting its calcium binding loops into the lipid surface while orienting its β-sheets 30-40° from surface normal. However, at lipid surfaces, where PI(4,5)P2 is present, dysC2A tilts its β-sheets more than 60° from surface normal to expose a polybasic face, while it binds to the PI(4,5)P2 surface. Both lipid binding mechanisms are shown to occur alongside dysC2A-induced lipid clustering. These different binding mechanisms suggest that dysC2A could provide a molecular cue to the larger dysferlin protein as to signal whether it is bound to the sarcolemma or another lipid surface.

Selective IR super-resolution imaging of β-keratins at the bulk or interface in feather detected by using a nonlinear optical process

We developed the new IR super-resolution microscope by using a 4-wave mixing (4-wave), which is a third-order nonlinear optical process, and carried out the IR super-resolution imaging of the cross section of the rachis of an avian feather. We clearly observed strong signals in the entire region of the rachis at the amide I vibration of β-keratin in both of the XXYY and YYXX polarization combination. These results are different from images detected by using the vibrational sum-frequency generation (VSFG) method. While the VSFG imaging detects molecules only from the interface, the 4-wave method enables us to observe the signal from the bulk area. We concluded that the four repeating units of β-keratins in the bulk area which are suggested by X-ray diffraction studies are visualized in the 4-wave detected method. We also applied two IR super-resolution microscopies for the barb and discuss the site dependence of the orientation, distribution and concentration of β-keratin.

Peptide Orientation at Emulsion Nanointerfaces Dramatically Different from Flat Surfaces

The adsorption of protein to nanoparticles plays an important role in toxicity, food science, pharmaceutics, and biomaterial science. Understanding how proteins bind to nanophase surfaces is instrumental for understanding and, ultimately, controlling nanoparticle (NP) biochemistry. Techniques probing the adsorption of proteins at NP interfaces exist; however, these methods have been unable to determine the orientation and folding of proteins at these interfaces. For the first time, we probe in situ with sum frequency scattering vibrational spectroscopy the orientation of model leucine-lysine (LK) peptides adsorbed to NPs. The results show that both α-helical and β-strand LK peptides bind the particles in an upright orientation, in contrast to the flat orientation of LKs binding to planar surfaces. The different binding geometry is explained by Coulombic forces between peptides across the particle volume.

Acidic pH Promotes Refolding and Macroscopic Assembly of Amyloid β (16-22) Peptides at the Air-Water Interface

Assembly by amyloid-beta (Aβ) peptides is vital for various neurodegenerative diseases. The process can be accelerated by hydrophobic interfaces such as the cell membrane interface and the air-water interface. Elucidating the assembly mechanism for Aβ peptides at hydrophobic interface requires knowledge of the microscopic structure of interfacial peptides. Here we combine scanning force microscopy, sum-frequency generation spectroscopy, and metadynamics simulations to probe the structure of the central fragment of Aβ peptides at the air-water interface. We find that the structure of interfacial peptides depends on pH: at neutral pH, the peptides adopt a less folded, bending motif by forming intra-hydrogen bonds; at acidic pH, the peptides refold into extended β-strand fibril conformation, which further promotes their macroscopic assembly. The conformational transition of interfacial peptides is driven by the reduced hydrogen bonds, both with water and within peptides, resulting from the protonation of acidic glutamic acid side chains.

Polarization-Dependent Heterodyne-Detected Sum-Frequency Generation Spectroscopy as a Tool to Explore Surface Molecular Orientation and Ångström-Scale Depth Profiling

Sum-frequency generation (SFG) spectroscopy provides a unique optical probe for interfacial molecules with interface-specificity and molecular specificity. SFG measurements can be further carried out at different polarization combinations, but the target of the polarization-dependent SFG is conventionally limited to investigating the molecular orientation. Here, we explore the possibility of polarization-dependent SFG (PD-SFG) measurements with heterodyne detection (HD-PD-SFG). We stress that HD-PD-SFG enables accurate determination of the peak amplitude, a key factor of the PD-SFG data. Subsequently, we outline that HD-PD-SFG can be used not only for estimating the molecular orientation but also for investigating the interfacial dielectric profile and studying the depth profile of molecules. We further illustrate the variety of combined simulation and PD-SFG studies.

Peptide Mimic of the Marine Sponge Protein Silicatein Fabricates Ultrathin Nanosheets of Silicon Dioxide and Titanium Dioxide

Two-dimensional (2D) materials have attracted attention for potential applications in light harvesting, catalysis, and molecular electronics. Mineral proteins involved in hard tissue biogenesis can produce 2D structures with high fidelity by using sustainable production routes. This study shows that a peptide mimic based on the catalytic triad of the marine sponge protein silicatein catalyzes the formation of nanometer thin and stable sheets of silicon dioxide and titanium dioxide.

Early sum frequency generation vibrational spectroscopic studies on peptides and proteins at interfaces

This paper summarizes the early research results on studying proteins and peptides at interfaces using sum frequency generation (SFG) vibrational spectroscopy. SFG studies in the C-H stretching frequency region to examine the protein side-chain behavior and in the amide I frequency region to investigate the orientation and conformation of interfacial peptides/proteins are presented. The early chiral SFG research and SFG isotope labeling studies on interfacial peptides/proteins are also discussed. These early SFG studies demonstrate the feasibility of using SFG to elucidate interfacial molecular structures of peptides and proteins in situ, which built a foundation for later SFG investigations on peptides and proteins at interfaces.

Choose your own adventure: Picosecond or broadband vibrational sum-frequency generation spectroscopy

Vibrational sum-frequency generation (VSFG) spectroscopy is a method capable of measuring chemical structure and dynamics within the interfacial region between two bulk phases. At the core of every experimental system is a laser source that influences the experimental capabilities of the VSFG spectrometer. In this article, we discuss the differences between VSFG spectrometers built with picosecond and broadband laser sources as it will impact everything from material costs, experimental build time, experimental capabilities, and more. A focus is placed on the accessibility of the two different SFG systems to newcomers in the SFG field and provides a resource for laboratories considering incorporating VSFG spectroscopy into their research programs. This Tutorial provides a model decision tree to aid newcomers when determining whether the picosecond or femtosecond laser system is sufficient for their research program and navigates through it for a few specific scenarios.

Accurate molecular orientation at interfaces determined by multimode polarization-dependent heterodyne-detected sum-frequency generation spectroscopy via multidimensional orientational distribution function

Many essential processes occur at soft interfaces, from chemical reactions on aqueous aerosols in the atmosphere to biochemical recognition and binding at the surface of cell membranes. The spatial arrangement of molecules specifically at these interfaces is crucial for many of such processes. The accurate determination of the interfacial molecular orientation has been challenging due to the low number of molecules at interfaces and the ambiguity of their orientational distribution. Here, we combine phase- and polarization-resolved sum-frequency generation (SFG) spectroscopy to obtain the molecular orientation at the interface. We extend an exponentially decaying orientational distribution to multiple dimensions, which, in conjunction with multiple SFG datasets obtained from the different vibrational modes, allows us to determine the molecular orientation. We apply this new approach to formic acid molecules at the air–water interface. The inferred orientation of formic acid agrees very well with ab initio molecular dynamics data. The phase-resolved SFG multimode analysis scheme using the multidimensional orientational distribution thus provides a universal approach for obtaining the interfacial molecular orientation.

Electrostatics Trigger Interfacial Self-Assembly of Bacterial Ice Nucleators

Ice active bacteria can catalyze water freezing at high subzero temperatures using ice nucleating proteins (INPs) located at their outer cell walls. INPs are the most effective ice nucleators known and are of significant interest for agriculture, climate research, and freeze/antifreeze technologies. The aggregation of INPs into large ice nucleation sites is a key step for effective ice nucleation. It has been proposed that ice active bacteria can drive the aggregation of INPs and thereby trigger ice nucleation. However, the mechanism of INP aggregate assembly and the molecular processes behind the activation are still unclear. Both biochemical pathways and activation through electrostatics have been proposed based on experiments with lysed ice active bacteria. For a more direct view on the assembly of INPs, we follow the structure and water interactions of a synthetic model INP of the well-studied ice bacterium Pseudomonas syringae at the air-water interface as a function of the subphase pH. By combining sum frequency generation spectroscopy with two-dimensional infrared spectra, we conclude that self-assembly and electrostatic interactions drive the formation of ordered INP structures capable of aligning interfacial water.

Direct Evidence for Aligned Binding of Cellulase Enzymes to Cellulose Surfaces

The conversion of biomass into green fuels and chemicals is of great societal interest. Engineers have been designing new cellulase enzymes for the breakdown of otherwise insoluble cellulose materials. A barrier to the rational design of new enzymes has been our lack of a molecular picture of how cellulase binding occurs. A critical factor is the attachment via the enzyme's carbohydrate binding module (CBM). To elucidate the structural and mechanistic details of cellulase adsorption, we have combined experimental data from sum frequency generation spectroscopy with molecular dynamics simulations to probe the equilibrium structure and surface alignment of a 14-residue peptide mimicking the CBM. The data show that binding is driven by hydrogen bonding and that tyrosine side chains within the CBM align the cellulase with the registry of the cellulose surface. Such an alignment is favorable for the translocation and effective cellulose breakdown and is therefore likely an important parameter for the design of novel enzymes.

Probing Orientations and Conformations of Peptides and Proteins at Buried Interfaces

Molecular structures of peptides/proteins at interfaces determine their interfacial properties, which play important roles in many applications. It is difficult to probe interfacial peptide/protein structures because of the lack of appropriate tools. Sum frequency generation (SFG) vibrational spectroscopy has been developed into a powerful technique to elucidate molecular structures of peptides/proteins at buried solid/liquid and liquid/liquid interfaces. SFG has been successfully applied to study molecular interactions between model cell membranes and antimicrobial peptides/membrane proteins, surface-immobilized peptides/enzymes, and physically adsorbed peptides/proteins on polymers and 2D materials. A variety of other analytical techniques and computational simulations provide supporting information to SFG studies, leading to more complete understanding of structure-function relationships of interfacial peptides/proteins. With the advance of SFG techniques and data analysis methods, along with newly developed supplemental tools and simulation methodology, SFG research on interfacial peptides/proteins will further impact research in fields like chemistry, biology, biophysics, engineering, and beyond.

Interaction of Amyloid-β-(1-42) Peptide and Its Aggregates with Lipid/Water Interfaces Probed by Vibrational Sum-Frequency Generation Spectroscopy

In this study, we use surface-sensitive vibrational sum-frequency generation (VSFG) spectroscopy to investigate the interaction between model lipid monolayers and Aβ(1-42) in its monomeric and aggregated states. Combining VSFG with atomic force microscopy (AFM) and thioflavin T (ThT) fluorescence measurements, we found that only small aggregates with probably a β-hairpin-like structure adsorbed to the zwitterionic lipid monolayer (DOPC). In contrast, larger aggregates with an extended β-sheet structure adsorbed to a negatively charged lipid monolayer (DOPG). The adsorption of small, initially formed aggregates strongly destabilized both monolayers, but only the DOPC monolayer was completely disrupted. We showed that the intensity of the amide-II' band in achiral (SSP) and chiral (SPP) polarization combinations increased in time when Aβ(1-42) aggregates accumulated at the DOPG monolayer. Nevertheless, almost no adsorption of preformed mature fibrils to DOPG monolayers was detected. By performing spectral VSFG calculations, we revealed a clear correlation between the amide-II' signal and the degree of amyloid aggregates (e.g., oligomers or (proto)fibrils) of various Aβ(1-42) structures. The calculations showed that only structures with a significant amyloid β-sheet content have a strong amide-II' intensity, in line with previous Raman studies. The combination of the presented results substantiates the amide-II(') band as a legitimate amyloid marker.

Backbone Structure of Diatom Silaffin Peptide R5 in Biosilica Determined by Combining Solid-State NMR with Theoretical Sum-Frequency Generation Spectra

Silaffin peptide R5 is key for the biogenesis of silica cell walls of diatoms. Biosilification by the R5 peptide has potential in biotechnology, drug development, and materials science due to its ability to precipitate stable, high fidelity silica sheets and particles. A true barrier for the design of novel peptide-based architectures for wider applications has been the limited understanding of the interfacial structure of R5 when precipitating silica nanoparticles. While R5-silica interactions have been studied in detail at flat surfaces, the structure within nanophase particles is still being debated. We herein elucidate the conformation of R5 in its active form within silica particles by combining interface-specific vibrational spectroscopy data with solid-state NMR torsion angles using theoretical spectra. Our calculations show that R5 is structured and undergoes a conformational transition from a strand-type motif in solution to a more curved, contracted structure when interacting with silica precursors.

Intrinsically Disordered Osteopontin Fragment Orders During Interfacial Calcium Oxalate Mineralization

Calcium oxalate (CaC₂O₄) is the major component of kidney stone. The acidic osteopontin (OPN) protein in human urine can effectively inhibit the growth of CaC₂O₄ crystals, thereby acting as a potent stone preventer. Previous studies in bulk solution all attest to the importance of binding and recognition of OPN at the CaC₂O₄ mineral surface, yet molecular level insights into the active interface during CaC₂O₄ mineralization are still lacking. Here, we probe the structure of the central OPN fragment and its interaction with Ca²⁺ and CaC₂O₄ at the water–air interface using surface‐specific non‐linear vibrational spectroscopy. While OPN peptides remain largely disordered in solution, our results reveal that the bidentate binding of Ca²⁺ ions refold the interfacial peptides into well‐ordered and assembled β‐turn motifs. One critical intermediate directs mineralization by releasing structural freedom of backbone and binding side chains. These insights into the mineral interface are crucial for understanding the pathological development of kidney stones and possibly relevant for calcium oxalate biomineralization in general.

Molecular Structure of the Surface-Immobilized Super Uranyl Binding Protein

Recently, a super uranyl binding protein (SUP) was developed, which exhibits excellent sensitivity/selectivity to bind uranyl ions. It can be immobilized onto a surface in sensing devices to detect uranyl ions. Here, sum frequency generation (SFG) vibrational spectroscopy was applied to probe the interfacial structures of surface-immobilized SUP. The collected SFG spectra were compared to the calculated orientation-dependent SUP SFG spectra using a one-excitonic Hamiltonian approach based on the SUP crystal structures to deduce the most likely surface-immobilized SUP orientation(s). Furthermore, discrete molecular dynamics (DMD) simulation was applied to refine the surface-immobilized SUP conformations and orientations. The immobilized SUP structures calculated from DMD simulations confirmed the SUP orientations obtained from SFG data analyzed based on the crystal structures and were then used for a new round of SFG orientation analysis to more accurately determine the interfacial orientations and conformations of immobilized SUP before and after uranyl ion binding, providing an in-depth understanding of molecular interactions between SUP and the surface and the effect of uranyl ion binding on the SUP interfacial structures. We believe that the developed method of combining SFG measurements, DMD simulation, and Hamiltonian data analysis approach is widely applicable to study biomolecules at solid/liquid interfaces.

Ice-nucleating proteins are activated by low temperatures to control the structure of interfacial water

Ice-nucleation active (INA) bacteria can promote the growth of ice more effectively than any other known material. Using specialized ice-nucleating proteins (INPs), they obtain nutrients from plants by inducing frost damage and, when airborne in the atmosphere, they drive ice nucleation within clouds, which may affect global precipitation patterns. Despite their evident environmental importance, the molecular mechanisms behind INP-induced freezing have remained largely elusive. We investigate the structural basis for the interactions between water and the ice-nucleating protein InaZ from the INA bacterium Pseudomonas syringae. Using vibrational sum-frequency generation (SFG) and two-dimensional infrared spectroscopy, we demonstrate that the ice-active repeats of InaZ adopt a β-helical structure in solution and at water surfaces. In this configuration, interaction between INPs and water molecules imposes structural ordering on the adjacent water network. The observed order of water increases as the interface is cooled to temperatures close to the melting point of water. Experimental SFG data combined with molecular-dynamics simulations and spectral calculations show that InaZ reorients at lower temperatures. This reorientation can enhance water interactions, and thereby the effectiveness of ice nucleation.

Ice-nucleating proteins promote ice formation at high sub-zero temperatures, but the mechanism is still unclear. The authors investigate a model ice-nucleating protein at the air-water interface using vibrational sum frequency generation spectroscopy and simulations, revealing its reorientation at low temperatures, which increases contact with water molecules and promotes their ordering.

Role of Water in CaCO3 Biomineralization

Biomineralization occurs in aqueous environments. Despite the ubiquity and relevance of CaCO₃ biomineralization, the role of water in the biomineralization process has remained elusive. Here, we demonstrate that water reorganization accompanies CaCO₃ biomineralization for sea urchin spine generation in a model system. Using surface-specific vibrational spectroscopy, we probe the water at the interface of the spine-associated protein during CaCO₃ mineralization. Our results show that, while the protein structure remains unchanged, the structure of interfacial water is perturbed differently in the presence of both Ca²⁺ and CO₃^2– compared to the addition of only Ca²⁺. This difference is attributed to the condensation of prenucleation mineral species. Our findings are consistent with a nonclassical mineralization pathway for sea urchin spine generation and highlight the importance of protein hydration in biomineralization.

Glutathione Self-Assembles into a Shell of Hydrogen-Bonded Intermolecular Aggregates on "Naked" Silver Nanoparticles

A detailed understanding of the molecular structure in nanoparticle ligand capping layers is crucial for their efficient incorporation into modern scientific and technological applications. Peptide ligands render the nanoparticles as biocompatible materials. Glutathione, a γ-ECG tripeptide, self-assembles into aggregates on the surface of ligand-free silver nanoparticles through intermolecular hydrogen bonding and forms a few nanometer-thick shells. Two-dimensional nonlinear infrared (2DIR) spectroscopy suggests that aggregates adopt a conformation resembling the β-sheet secondary structure. The shell thickness was evaluated with localized surface plasmon resonance spectroscopy and X-ray photoelectron spectroscopy. The amount of glutathione on the surface was obtained with spectrophotometry of a thiol-reactive probe. Our results suggest that the shell consists of ∼15 stacked molecular layers. These values correspond to the inter-sheet distances, which are significantly shorter than those in amyloid fibrils with relatively bulky side chains, but are comparable to glycine-rich silk fibrils, where the side chains are compact. The tight packing of the glutathione layers can be facilitated by hydrogen-bonded carboxylic acid dimers of glycine and the intermolecular salt bridges between the zwitterionic γ-glutamyl groups. The structure of the glutathione aggregates was studied by 2DIR spectroscopy of the amide-I vibrational modes using ¹³C isotope labeling of the cysteine carbonyl. Isotope dilution experiments revealed the coupling of modes forming vibrational excitons along the cysteine chain. The coupling along the γ-glutamyl exciton chain was estimated from these values. The obtained coupling strengths are slightly lower than those of native β-sheets, yet they appear large enough to point onto an ordered conformation of the peptides within the aggregate. Analysis of the excitons' anharmonicities and the strength of the transition dipole moments generally is in agreement with these observations.

Direct Evidence That Mutations within Dysferlin's C2A Domain Inhibit Lipid Clustering

Mechanical stress on sarcolemma can create small tears in the muscle cell membrane. Within the sarcolemma resides the multidomain dysferlin protein. Mutations in this protein render it unable to repair the sarcolemma and have been linked to muscular dystrophy. A key step in dysferlin-regulated repair is the binding of the C2A domain to the lipid membrane upon increased intracellular calcium. Mutations mapped to this domain cause loss of binding ability of the C2A domain. There is a crucial need to understand the geometry of dysferlin C2A at a membrane interface as well as cell membrane lipid reorientation when compared to that of a mutant. Here, we describe a comparison between the wild-type dysferlin C2A and a mutation to the conserved aspartic acids in the domain binding loops. To identify both the geometry and the cell membrane lipid reorientation, we applied sum frequency generation (SFG) vibrational spectroscopy and coupled it with simulated SFG spectra to observe and quantify the interaction with a model cell membrane composed of phosphotidylserine and phosphotidylcholine. Observed changes in surface pressure demonstrate that calcium-bridged electrostatic interactions govern the initial interaction of the C2A domains docking with a lipid membrane. SFG spectra taken from the amide-I region for the wild type and variant contain features near 1642, 1663, and 1675 cm^-1 related to the C2A domain β-sandwich secondary structure, indicating that the domain binds in a specific orientation. Mapping simulated SFG spectra to the experimentally collected spectra indicated that both wild-type and variant domains have nearly the same orientation to the membrane surface. However, examining the ordering of the lipids that make up a model membrane using SFG, we find that the wild type clusters the lipids as seen by the increase in the ratio of the CD₃ and CD₂ symmetric intensities by 170% for the wild type and by 120% for the variant. This study highlights the capabilities of SFG to probe with great detail biological mutations in proteins at cell membrane interfaces.

Orientation and Conformation of Proteins at the Air-Water Interface Determined from Integrative Molecular Dynamics Simulations and Sum Frequency Generation Spectroscopy

Understanding the assembly of proteins at the air-water interface (AWI) informs the formation of protein films, emulsion properties, and protein aggregation. Determination of protein conformation and orientation at an interface is difficult to resolve with a single experimental or simulation technique alone. To date, the interfacial structure of even one of the most widely studied proteins, lysozyme, at the AWI remains unresolved. In this study, molecular dynamics (MD) simulations are used to determine if the protein adopts a side-on, head-on, or axial orientation at the AWI with two different forcefields, GROMOS-53a6 + SPC/E and a99SB-disp + TIP4P-D. Vibrational sum frequency generation (SFG) spectroscopy experiments and spectral SFG calculations validate consistency between the structure determined from MD and experiments. Overall, we show with strong agreement that lysozyme adopts an axial conformation at pH 7. Further, we provide molecular-level insight as to how pH influences the binding domains of lysozyme resulting in side-on adsorption near the isoelectric point of the lysozyme.

IR super-resolution imaging of avian feather keratins detected by using vibrational sum-frequency generation

IR super-resolution imaging of the cross section of the rachis of an avian feather was carried out by using a vibrational sum-frequency generation (VSFG) detected IR microscope with a sub-micrometer spatial resolution. In the YYX polarization combination, we clearly observed strong signals in the entire region of the rachis at the amide I vibration of β-keratin. On the other hand, the signal disappears from most of the cross section in the XXY polarization combination. Because the VSFG imaging detects the signal only from the interface, we conclude that the interfacial deflection inside of a rachis was detected.

Structure Determination of Hen Egg-White Lysozyme Aggregates Adsorbed to Lipid/Water and Air/Water Interfaces

We use vibrational sum-frequency generation (VSFG) spectroscopy to study the structure of hen egg-white lysozyme (HEWL) aggregates adsorbed to DOPG/D₂O and air/D₂O interfaces. We find that aggregates with a parallel and antiparallel β-sheet structure together with smaller unordered aggregates and a denaturated protein are adsorbed to both interfaces. We demonstrate that to retrieve this information, fitting of the VSFG spectra is essential. The number of bands contributing to the VSFG spectrum might be misinterpreted, due to interference between peaks with opposite orientation and a nonresonant background. Our study identified hydrophobicity as the main driving force for adsorption to the air/D₂O interface. Adsorption to the DOPG/D₂O interface is also influenced by hydrophobic interaction; however, electrostatic interaction between the charged protein's groups and the lipid's headgroups has the most significant effect on the adsorption. We find that the intensity of the VSFG spectrum at the DOPG/D₂O interface is strongly enhanced by varying the pH of the solution. We show that this change is not due to a change of lysozyme's and its aggregates' charge but due to dipole reorientation at the DOPG/D₂O interface. This finding suggests that extra care must be taken when interpreting the VSFG spectrum of proteins adsorbed at the lipid/water interface.

Lasalocid Acid Antibiotic at a Membrane Surface Probed by Sum Frequency Generation Spectroscopy

Carboxyl polyether ionophores (CPIs) are widely used as veterinary antibiotics and to increase food utilization in ruminating animals. Furthermore, CPIs can target drug-resistant bacteria, but detailed knowledge about their mode-of-action is needed to develop agents with a reasonable therapeutic index. It has been suggested that ionophores bind to membranes and incur large structural changes to shield a bound ion from the hydrophobic environment of the lipid bilayer for transport. One crucial piece of information is missing, however: Is it necessary for the free ionophore to adsorb on the membrane surface before interacting with a cation to facilitate cross-membrane ion transport? To answer this question, we applied sum-frequency generation (SFG) vibrational spectroscopy and surface tensiometry to identify the interaction between the prototypical CPI lasalocid acid (LA) and a model membrane. Observed changes in the surface pressure demonstrate that the free LA undergoes a self-assembly process with the lipid monolayer. Spectra taken from the lipid monolayer show that the free acid inserts partially into the lipid monolayer and then after complexation with sodium chloride disrupts the lipid monolayer. Overall, this study strongly suggests that this must be the crucial step of LA and metal ion complexation that allows the ionophore to traverse a lipid membrane.

Otoferlin C2F Domain-Induced Changes in Membrane Structure Observed by Sum Frequency Generation

Proteins that contain C2 domains are involved in a variety of biological processes, including encoding of sound, cell signaling, and cell membrane repair. Of particular importance is the interface activity of the C-terminal C2F domain of otoferlin due to the pathological mutations known to significantly disrupt the protein's lipid membrane interface binding activity, resulting in hearing loss. Therefore, there is a critical need to define the geometry and positions of functionally important sites and structures at the otoferlin-lipid membrane interface. Here, we describe the first in situ probe of the protein orientation of otoferlin's C2F domain interacting with a cell membrane surface. To identify this protein's orientation at the lipid interface, we applied sum frequency generation (SFG) vibrational spectroscopy and coupled it with simulated SFG spectra to observe and quantify the otoferlin C2F domain interacting with model lipid membranes. A model cell membrane was built with equal amounts of phosphatidylserine and phosphatidylcholine. SFG measurements of the lipids that make up the model membrane indicate a 62% increase in amplitude from the SFG signal near 2075 cm^-1 upon protein interaction, suggesting domain-induced changes in the orientation of the lipids and possible membrane curvature. This increase is related to lipid ordering caused by the docking interaction of the otoferlin C2F domain. SFG spectra taken from the amide-I region contain features near 1630 and 1670 cm^-1 related to the C2F domains beta-sandwich secondary structure, thus indicating that the domain binds in a specific orientation. By mapping the simulated SFG spectra to the experimentally collected SFG spectra, we found the C2F domain of otoferlin orients 22° normal to the lipid surface. This information allows us to map what portion of the domain directly interacts with the lipid membrane.

Peptide-Controlled Assembly of Macroscopic Calcium Oxalate Nanosheets

The fabrication of two-dimensional (2D) biomineral nanosheets is of high interest owing to their promise for applications in electronics, filtration, catalysis, and chemical sensing. Using a facile approach inspired by biomineralization in nature, we fabricate laterally macroscopic calcium oxalate nanosheets using β-folded peptides. The template peptides are composed of repetitive glutamic acid and leucine amino acids, self-organized at the air-water interface. Surface-specific sum frequency generation spectroscopy and molecular dynamics simulations reveal that the formation of oxalate nanosheets relies on the peptide-Ca²⁺ ion interaction at the interface, which not only restructures the peptides but also templates Ca²⁺ ions into a calcium oxalate dihydrate lattice. Combined, this enables the formation of a critical structural intermediate in the assembly pathway toward the oxalate sheet formation. These insights into peptide-ion interfacial interaction are important for designing novel inorganic 2D materials.

Ab initio Modeling of the Vibrational Sum-Frequency Generation Spectrum of Interfacial Water

Understanding the structural and dynamical features of interfacial water is of greatest interest in physics, chemistry, biology, and materials science. Vibrational sum-frequency generation (SFG) spectroscopy, which is sensitive to the molecular orientation and dynamics on the surfaces or at the interfaces, allows one to study a wide variety of interfacial systems. The structural and dynamical features of interfacial water at the air/water interface have been extensively investigated by SFG spectroscopy. However, the interpretations of the spectroscopic features have been under intense debate. Here, we report a simulated SFG spectrum of the air/water interface based on ab initio molecular dynamics simulations, which covers the OH stretching, bending, and libration modes of interfacial water. Quantitative agreement between our present simulations and the most recent experimental studies ensures that ab initio simulations predict unbiased structural features and electrical properties of interfacial systems. By utilizing the kinetic energy spectral density (KESD) analysis to decompose the simulated spectra, the spectroscopic features can then be assigned to specific hydrogen-bonding configurations of interfacial water molecules.

Structure of von Willebrand factor A1 on polystyrene determined from experimental and calculated sum frequency generation spectra

The blood-clotting protein von Willebrand factor (vWF) can be activated by small molecules, high shear stress, and interactions with interfaces. It subsequently binds platelet receptor glycoprotein Ibα (GPIbα) at the surface of platelets, thereby playing a crucial role in blood clotting due to platelet activation, which is an important process to consider in the design of cardiovascular implants and biomaterials used in blood-contacting applications. The influence of surfaces on the activation and the molecular-level structure of surface-bound vWF is largely unknown. Recent studies have indicated that when bound to hydrophobic polystyrene (PS), the A1 domain of vWF remains accessible for GPIbα binding. However, the detailed secondary structure and exact orientation of vWF A1 at the PS surface is still unresolved. Here, the authors resolve these features by studying the system with sum-frequency generation (SFG) spectroscopy. The data are consistent with a scenario where vWF A1 maintains a native secondary structure when bound to PS. Comparison of experimental and calculated SFG spectra combined with previously reported time-of-flight secondary ion mass spectrometry data suggests that A1 assumes an orientation with the GPIbα binding domain oriented away from the solid surface and exposed to the solution phase. This structural information will benefit future in vitro experiments with surface-adsorbed A1 domain and may have relevance for the design of novel blood-contacting biomaterials and wound-healing applications.

Engineering Proteins at Interfaces: From Complementary Characterization to Material Surfaces with Designed Functions

Once materials come into contact with a biological fluid containing proteins, proteins are generally—whether desired or not—attracted by the material's surface and adsorb onto it. The aim of this Review is to give an overview of the most commonly used characterization methods employed to gain a better understanding of the adsorption processes on either planar or curved surfaces. We continue to illustrate the benefit of combining different methods to different surface geometries of the material. The thus obtained insight ideally paves the way for engineering functional materials that interact with proteins in a predetermined manner.

Interchain Overlap Affects Formation of Silk Fibroin Secondary Structures on Hydrophobic Polystyrene Surface Detected via Achiral/Chiral Sum Frequency Generation

Silk fibroin (SF) has been engineered in the biomedical applications on account of its structural robustness, biocompatibility, and biodegradability. However, in situ study is still lacking with respect to the formation of SF secondary structures at the interface. In this paper, by using methanol as an inducing agent, the formation of SF secondary structures at the polystyrene (PS)/SF solution interfaces was detected with achiral and chiral sum frequency generation (SFG) vibrational spectroscopy. SF solutions with two concentrations above and below the critical overlapping concentration ( C*) of SF (∼1.8 mg/mL) were chosen, namely, 90 and 1 mg/mL. We found that above C*, before adding methanol to the protein solution, no ordered SF secondary structures could be detected at the PS/SF solution interface; oppositely, after adding methanol to the protein solution, ordered SF secondary structure, for example, antiparallel β-sheet, could be formed at the PS/protein solution interface. Below C*, both before and after adding methanol to the SF solution, ordered SF secondary structure such as antiparallel β-sheet could be formed. Besides, the addition of methanol could induce the formation of an extended helical structure, verified by the achiral and chiral characteristic bands. Because C* represents a critical solution concentration above which the SF chains can interact with each other and below which the SF chains are isolated in the solution, this achiral/chiral SFG study emphasizes the importance of the chain-chain interaction or spatial confinement on the formation of the protein secondary structures, which provides an additional dimension for the future study of interfacial protein folding.

Nonlinear Optical Methods for Characterization of Molecular Structure and Surface Chemistry

The principles, strengths and limitations of several nonlinear optical (NLO) methods for characterizing biological systems are reviewed. NLO methods encompass a wide range of approaches that can be used for real-time, in-situ characterization of biological systems, typically in a label-free mode. Multiphoton excitation fluorescence (MPEF) is widely used for high-quality imaging based on electronic transitions, but lacks interface specificity. Second harmonic generation (SHG) is a parametric process that has all the virtues of the two-photon version of MPEF, yielding a signal at twice the frequency of the excitation light, which provides interface specificity. Both SHG and MPEF can provide images with high structural contrast, but they typically lack molecular or chemical specificity. Other NLO methods such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) can provide high-sensitivity imaging with chemical information since Raman active vibrations are probed. However, CARS and SRS lack interface and surface specificity. A NLO method that provides both interface/surface specificity as well as molecular specificity is vibrational sum frequency generation (SFG) spectroscopy. Vibration modes that are both Raman and IR active are probed in the SFG process, providing the molecular specificity. SFG, like SHG, is a parametric process, which provides the interface and surface specificity. SFG is typically done in the reflection mode from planar samples. This has yielded rich and detailed information about the molecular structure of biomaterial interfaces and biomolecules interacting with their surfaces. However, 2-D systems have limitations for understanding the interactions of biomolecules and interfaces in the 3-D biological environment. The recent advances made in instrumentation and analysis methods for sum frequency scattering (SFS) now present the opportunity for SFS to be used to directly study biological solutions. By detecting the scattering at angles away from the phase-matched direction even centrosymmetric structures that are isotropic (e.g., spherical nanoparticles functionalized with self-assembled monolayers or biomolecules) can be probed. Often a combination of multiple NLO methods or a combination of a NLO method with other spectroscopic methods is required to obtain a full understanding of the molecular structure and surface chemistry of biomaterials and the biomolecules that interact with them. Using the right combination methods provides a powerful approach for characterizing biological materials.

Interactions between Surface-Immobilized Antimicrobial Peptides and Model Bacterial Cell Membranes

Sum frequency generation (SFG) vibrational spectroscopy was used to study surface immobilization effects on the interactions between antimicrobial peptide cecropin P1 (CP1) and model cell membranes. While free CP1 in solution interacted with a model cell membrane composed of a phosphatidylglycerol (PG) bilayer, electrostatic interaction led to the attachment of CP1 molecules onto the PG surface and the hydrophobic domain in the lipid bilayer enabled the peptides to insert into the bilayer and form α-helices from random coil structures. While CP1 molecules immobilized on a self-assembled monolayer interacted with PG lipid vesicles, the intensity of the SFG peak for the peptide α-helix decreased as the PG vesicle concentration increased. It was believed that when surface-immobilized CP1 molecules interacted with lipid vesicles, they lay down on the surface or became random coils. When the immobilized CP1 interacted with a PG lipid monolayer on water, the strong interaction led to the lying-down orientation of all of the surface-immobilized peptides as well. Differently, no significant interactions between surface-immobilized CP1 with the mammalian cell membrane model 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer were observed. Our results suggest that, instead of membrane insertion, the electrostatic interactions between the surface cationic charges of CP1 and anionic bacterial membranes may play an important role in the antimicrobial activity of the surface-immobilized CP1 peptide.

Sum Frequency Generation Vibrational Spectroscopy for Characterization of Buried Polymer Interfaces

Sum frequency generation vibrational spectroscopy (SFG-VS) has become one of the most appealing technologies to characterize molecular structures at interfaces. In this focal point review, we focus on SFG-VS studies at buried polymer interfaces and review many of the recent publications in the field. We also cover the essential theoretical background of SFG-VS and discuss the experimental implementation of SFG-VS.

Determination of Absolute Orientation of Protein α-Helices at Interfaces Using Phase-Resolved Sum Frequency Generation Spectroscopy

Understanding the structure of proteins at surfaces is key in fields such as biomaterials research, biosensor design, membrane biophysics, and drug design. A particularly important factor is the orientation of proteins when bound to a particular surface. The orientation of the active site of enzymes or protein sensors and the availability of binding pockets within membrane proteins are important design parameters for engineers developing new sensors, surfaces, and drugs. Recently developed methods to probe protein orientation, including immunoessays and mass spectrometry, either lack structural resolution or require harsh experimental conditions. We here report a new method to track the absolute orientation of interfacial proteins using phase-resolved sum frequency generation spectroscopy in combination with molecular dynamics simulations and theoretical spectral calculations. As a model system we have determined the orientation of a helical lysine-leucine peptide at the air-water interface. The data show that the absolute orientation of the helix can be reliably determined even for orientations almost parallel to the surface.

Identifying the selectivity of antimicrobial peptides to cell membranes by sum frequency generation spectroscopy

Cationic amphiphilic peptides have been engineered to target both Gram-positive and Gram-negative bacteria while avoiding damage to other cell types. However, the exact mechanism of how these peptides target, bind, and disrupt bacterial cell membranes is not understood. One specific peptide that has been engineered to selectively capture bacteria is WLBU2 (sequence: RRWVRRVRRWVRRVVRVVRRWVRR). It has been suggested that WLBU2 activity stems from the fact that when interacting with bacterial cell membranes the peptide assumes an α-helical structure and inserts itself into the membrane. Alternatively, in the presence of mammalian cell membranes, the peptide assumes an inert β-sheet structure. To test this hypothesis, the authors applied sum frequency generation (SFG) spectroscopy and surface tensiometry to identify the structure of WLBU2 as it interacts with model lipid monolayers that mimic mammalian and bacterial cell membranes. Model mammalian cell membranes were built upon zwitterionic 1,2-dipalmitoyl-sn-glycero-3-phosphocholine lipids while bacterial cell membranes were constructed with negatively charged 1,2-dimyristoyl-sn-glycero-3-phospho-(1'-rac-glycerol) lipids. Observed changes in surface pressure at the peptide-lipid-air interface demonstrate that the peptide has a clear binding preference toward negatively charged bacteria-like lipids. The structure of both the lipids and peptides were characterized by SFG spectra collected at the monolayer interface. Changes in monolayer structure as the peptide binds were observed by tracking the intensities of SFG vibrational modes related to the acyl chains within the lipids. Peptide structures when bound to both types of lipids were determined by SFG spectra collected within the amide I vibrational band. The SFG spectra of WLBU2 interacting with the model mammalian lipid monolayer contain two peaks near 1642 and 1678 cm^-1 indicative of an inactive β-sheet structure. SFG spectra collected from the peptide bound to a bacteria-like lipid monolayer contains just a single peak near 1651 cm^-1 which corresponds to an active α-helix structure. Combined, the tensiometry and SFG results demonstrate that WLBU2 both possesses a higher binding affinity toward and is in an active α-helix structure when bound to bacterial cell membranes.

Observation of pH-Induced Protein Reorientation at the Water Surface

Hydrophobins are surface-active proteins that form a hydrophobic, water-repelling film around aerial fungal structures. They have a compact, particle-like structure, in which hydrophilic and hydrophobic regions are spatially separated. This surface property renders them amphiphilic and is reminiscent of synthetic Janus particles. Here we report surface-specific chiral and nonchiral vibrational sum-frequency generation spectroscopy (VSFG) measurements of hydrophobins adsorbed to their natural place of action, the air-water interface. We observe that hydrophobin molecules undergo a reversible change in orientation (tilt) at the interface when the pH is varied. We explain this local orientation toggle from the modification of the interprotein interactions and the interaction of hydrophobin with the water solvent, following the pH-induced change of the charge state of particular amino acids.