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

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.

Disulfide-Driven Charge and Hydrophobicity Rearrangement of Remodeled Membrane Proteins toward Amyloid-Type Aggregation

As a common pathological hallmark, protein aggregation into amyloids is a highly complicated phenomenon, attracting extensive research interest for elucidating its structural details and formation mechanisms. Membrane deposition and disulfide-driven protein misfolding play critical roles in amyloid-type aggregation, yet the underlying molecular process remains unclear. Here, we employed sum frequency generation (SFG) vibrational spectroscopy to comprehensively investigate the remodeling process of lysozyme, as the model protein, into amyloid-type aggregates at the cell membrane interface. It was discovered that disulfide reduction concurrently induced the transition of membrane-bound lysozyme from predominantly α-helical to antiparallel β-sheet structures, under a mode switch of membrane interaction from electrostatic to hydrophobic, and subsequent oligomeric aggregation. These findings shed light on the systematic understanding of dynamic molecular mechanisms underlying membrane-interactive amyloid oligomer formation.

Fermi Resonance of the N-D Stretching Mode Probing the Local Hydrogen-Bonding Environment in Proteins

Local H-bonding interactions are crucial for proteins to undergo various structural transitions and form different secondary structures. However, identifying slight distinctions in the local H-bonding of proteins is rather challenging. Here, we demonstrate that the Fermi resonance of the N-D stretching mode can provide an effective probe for the localized H-bonding environment of proteins both at the surface/interface and in the bulk. Using sum frequency generation vibrational spectroscopy and infrared spectroscopy, we established a correlation between the Fermi resonance of the N-D mode and protein secondary structures. The H-bond of N-D···C═O splits the N-D modes into two peaks (∼2410 and ∼2470 cm-1). The relative strength ratio (R) between the ∼2410 cm-1 peak and the ∼2470 cm-1 peak is very sensitive to H-bond strength and protein secondary structure. R is less than 1 for α-helical peptides, while R is greater than 1 for β-sheet peptides. For R < 2.5, both α-helical/loop structures and β-sheet structures exhibit almost identical Fermi coupling strengths (W = 28 cm-1). For R > 2.5, W decreases from 28 to 14 cm-1 and depends on the aggregation degree of the β-sheet oligomers or fibrils. The initial local H-bonding status impacts the misfolding dynamics of proteins at the lipid bilayer interface.

Probing the Molecular Structure and Dynamics of Membrane-Bound Proteins during Misfolding Processes by Sum-Frequency Generation Vibrational Spectroscopy

Protein misfolding and amyloid formation are implicated in the protein dysfunction, but the underlying mechanism remains to be clarified due to the lack of effective tools for detecting the transient intermediates. Sum frequency generation vibrational spectroscopy (SFG-VS) has emerged as a powerful tool for identifying the structure and dynamics of proteins at the interfaces. In this review, we summarize recent SFG-VS studies on the structure and dynamics of membrane-bound proteins during misfolding processes. This paper first introduces the methods for determining the secondary structure of interfacial proteins: combining chiral and achiral spectra of amide A and amide I bands and combining amide I, amide II, and amide III spectral features. To demonstrate the ability of SFG-VS in investigating the interfacial protein misfolding and amyloid formation, studies on the interactions between different peptides/proteins (islet amyloid polypeptide, amyloid β, prion protein, fused in sarcoma protein, hen egg-white lysozyme, fusing fusion peptide, class I hydrophobin SC3 and class II hydrophobin HFBI) and surfaces such as lipid membranes are discussed. These molecular-level studies revealed that SFG-VS can provide a unique understanding of the mechanism of interfacial protein misfolding and amyloid formation in real time, in situ and without any exogenous labeling.

Diagrammatic theory of magnetic and quadrupolar contributions to sum-frequency generation in composite systems

Second-order nonlinear processes like Sum-Frequency Generation (SFG) are essentially defined in the electric dipolar approximation. However, when dealing with the SFG responses of bulk, big nanoparticles, highly symmetric objects, or chiral species, magnetic and quadrupolar contributions play a significant role in the process too. We extend the diagrammatic theory for linear and nonlinear optics to include these terms for single objects as well as for multipartite systems in interaction. Magnetic and quadrupolar quantities are introduced in the formalism as incoming fields, interaction intermediates, and sources of optical nonlinearity. New response functions and complex nonlinear processes are defined, and their symmetry properties are analyzed. This leads to a focus on several kinds of applications involving nanoscale coupled objects, symmetric molecular systems, and chiral materials, both in line with the existing literature and opening new possibilities for original complex systems.

Probing the interactions between amyloidogenic proteins and bio-membranes

Protein misfolding diseases (PMDs) in humans are characterized by the deposition of protein aggregates in tissues, including Alzheimer's disease, Parkinson's disease, type 2 diabetes, and amyotrophic lateral sclerosis. Misfolding and aggregation of amyloidogenic proteins play a central role in the onset and progression of PMDs, and these processes are regulated by multiple factors, especially the interaction between proteins and bio-membranes. Bio-membranes induce conformational changes in amyloidogenic proteins and affect their aggregation; on the other hand, the aggregates of amyloidogenic proteins may cause membrane damage or dysfunction leading to cytotoxicity. In this review, we summarize the factors that affect the binding of amyloidogenic proteins and membranes, the effects of bio-membranes on the aggregation of amyloidogenic proteins, mechanisms of membrane disruption by amyloidogenic aggregates, technical approaches for detecting these interactions, and finally therapeutic strategies targeting membrane damage caused by amyloidogenic proteins.

Sum-frequency vibrational spectroscopy of centrosymmetric molecule at interfaces

The centrosymmetric benzene molecule has zero first-order electric dipole hyperpolarizability, which results in no sum-frequency vibrational spectroscopy (SFVS) signal at interfaces, but it shows very strong SFVS experimentally. We perform a theoretical study on its SFVS, which is in good agreement with the experimental results. Its strong SFVS mainly comes from the interfacial electric quadrupole hyperpolarizability rather than the symmetry-breaking electric dipole, bulk electric quadrupole, and interfacial and bulk magnetic dipole hyperpolarizabilities, which provides a novel and completely unconventional point of view.

Inhibitory effects of fluorinated benzenesulfonamides on insulin fibrillation

Amyloid fibrils are protein aggregates formed by protein assembly through cross β structures. Inhibition of amyloid fibril formation may contribute to therapy against amyloid-related disorders like Parkinson's, Alzheimer's, and type 2 diabetes. Here we report that several fluorinated sulfonamide compounds, previously shown to inhibit human carbonic anhydrase, also inhibit the fibrillation of different proteins. Using a range of spectroscopic, microscopic and chromatographic techniques, we found that the two fluorinated sulfonamide compounds completely inhibit insulin fibrillation over a period of 16 h and moderately suppress α-synuclein and Aβ fibrillation. In addition, these compounds decreased cell toxicity of insulin incubated under fibrillation-inducing conditions. We ascribe these effects to their ability to maintain insulin in the native monomeric state. Molecular dynamic simulations suggest that these compounds inhibit insulin self-association by interacting with residues at the dimer interface. This highlights the general anti-aggregative properties of aromatic sulfonamides and suggests that sulfonamide compounds which inhibit carbonic anhydrase activity may have potential as therapeutic agents against amyloid-related disorders.

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.

In Situ Identification of Secondary Structures in Unpurified Bombyx mori Silk Fibrils Using Polarized Two-Dimensional Infrared Spectroscopy

The mechanical properties of biomaterials are dictated by the interactions and conformations of their building blocks, typically proteins. Although the macroscopic behavior of biomaterials is widely studied, our understanding of the underlying molecular properties is generally limited. Among the noninvasive and label-free methods to investigate molecular structures, infrared spectroscopy is one of the most commonly used tools because the absorption bands of amide groups strongly depend on protein secondary structure. However, spectral congestion usually complicates the analysis of the amide spectrum. Here, we apply polarized two-dimensional (2D) infrared spectroscopy (IR) to directly identify the protein secondary structures in native silk films cast from Bombyx mori silk feedstock. Without any additional peak fitting, we find that the initial effect of hydration is an increase of the random coil content at the expense of the helical content, while the β-sheet content is unchanged and only increases at a later stage. This paper demonstrates that 2D-IR can be a valuable tool for characterizing biomaterials.

The Diatom Peptide R5 Fabricates Two-Dimensional Titanium Dioxide Nanosheets

Diatoms use peptides based on the protein silaffin to fabricate their silica cell walls. To the interest of material scientists, silaffin peptides can also produce titanium dioxide nanoparticles. Peptide-based synthesis could present an environmentally friendly route to the synthesis of titanium dioxide nanomaterials with potential applications in water splitting and for biocompatible materials design. Two-dimensional nanomaterials have exceptional surface-to-volume ratios and are particularly suited for these applications. We here demonstrate how the silaffin peptide R5 can precipitate free-standing and self-supported sheets of titanium dioxide at the air-water interface, which are stable over tens of micrometers.

Structure and Orientation of the SARS-Coronavirus-2 Spike Protein at Air-Water Interfaces

The SARS coronavirus 2 (SARS-CoV-2) spike protein is located at the outermost perimeter of the viral envelope and is the first component of the virus to make contact with surrounding interfaces. The stability of the spike protein when in contact with surfaces plays a deciding role for infection pathways and for the viability of the virus after surface contact. While cryo-EM structures of the spike protein have been solved with high resolution and structural studies in solution have provided information about the secondary and tertiary structures, only little is known about the folding when adsorbed to surfaces. We here report on the secondary structure and orientation of the S1 segment of the spike protein, which is often used as a model protein for in vitro studies of SARS-CoV-2, at the air-water interface using surface-sensitive vibrational sum-frequency generation (SFG) spectroscopy. The air-water interface plays an important role for SARS-CoV-2 when suspended in aerosol droplets, and it serves as a model system for hydrophobic surfaces in general. The SFG experiments show that the S1 segment of the spike protein remains folded at the air-water interface and predominantly binds in its monomeric state, while the combination of small-angle X-ray scattering and two-dimensional infrared spectroscopy measurements indicate that it forms hexamers with the same secondary structure in aqueous solution.

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.

Sum-frequency vibrational spectroscopy of methanol at interfaces due to Fermi resonance

We present a theoretical method of studying sum-frequency vibrational spectroscopy for the CH3 group of methanol at interfaces due to Fermi resonance, which provides a novel and untraditional point of view with respect to traditional approaches.