Probing protein aggregation at buried interfaces: distinguishing between adsorbed protein monomers, dimers, and a monomer-dimer mixture in situ

Antiadhesive Copolymers at Solid/Liquid Interfaces: Complementary Characterization of Polymer Adsorption and Protein Fouling by Sum Frequency Generation Vibrational Spectroscopy and Quartz-Crystal Microbalance Measurements with Dissipation Monitoring

Amphiphilic copolymers comprising hydrophilic segments of poly(ethylene glycol) and hydrophobic domains that are able to adhere to solid/liquid interfaces have proven to be versatile ingredients in formulated products for various types of applications. Recently, we have reported the successful synthesis of a copolymer designed for modifying the surface properties of polyesters as mimics for synthetic textiles. Using sum frequency generation (SFG) spectroscopy, it was shown that the newly developed copolymer adsorbs effectively on the targeted substrates even in the presence of surfactants as supplied by common detergents. In the present work, these studies were extended to evaluate the ability of the formed copolymer adlayers to passivate polyester surfaces against undesired deposition of bio(macro)molecules, as represented by fibrinogen as model protein foulants. In addition, SFG spectroscopy was used to elucidate the structure of fibrinogen at the interface between polyester and water. To complement the obtained data with an independent technique, analogous experiments were performed using quartz-crystal microbalance with dissipation monitoring for the detection of the relevant interfacial processes. Both methods give consistent results and deliver a holistic picture of brush copolymer adsorption on polyester surfaces and subsequent antiadhesive effects against proteins under different conditions representing the targeted application in home care products.

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.

Monitoring the Molecular Structure of Fibrinogen during the Adsorption Process at the Buried Silicone Oil Interface In Situ in Real Time

Interfacial proteins play important roles in many research fields and applications, such as biosensors, biomedical implants, nonfouling coatings, etc. Directly probing interfacial protein behavior at buried solid/liquid and liquid/liquid interfaces is challenging. We used sum frequency generation vibrational spectroscopy and a Hamiltonian data analysis method to monitor the molecular structure of fibrinogen on silicone oil during the adsorption process in situ in real time. The results showed that the adsorbed fibrinogen molecules tend to adopt a bent structure throughout the entire adsorption process with the same orientation. This is different from the case of adsorbed fibrinogen on CaF₂ with a linear structure or on polystyrene with a bent structure but a different orientation. The method introduced herein is generally applicable for following time-dependent molecular structures of many other proteins and peptides at interfaces in situ in real time at the molecular level.

Determination of protein conformation and orientation at buried solid/liquid interfaces

Protein structures at solid/liquid interfaces mediate interfacial protein functions, which are important for many applications. It is difficult to probe interfacial protein structures at buried solid/liquid interfaces in situ at the molecular level. Here, a systematic methodology to determine protein molecular structures (orientation and conformation) at buried solid/liquid interfaces in situ was successfully developed with a combined approach using a nonlinear optical spectroscopic technique - sum frequency generation (SFG) vibrational spectroscopy, isotope labeling, spectra calculation, and computer simulation. With this approach, molecular structures of protein GB1 and its mutant (with two amino acids mutated) were investigated at the polymer/solution interface. Markedly different orientations and similar (but not identical) conformations of the wild-type protein GB1 and its mutant at the interface were detected, due to the varied molecular interfacial interactions. This systematic strategy is general and can be widely used to elucidate protein structures at buried interfaces in situ.

Governing dynamics and preferential binding of the AXH domain influence the aggregation pathway of Ataxin-1

The present state of understanding the mechanism of Spinocerebellar Ataxia-1, a fatal neurodegenerative disease linked to the protein Ataxin-1 (ATXN1), is baffled by a set of self-contradictory, and hence, inconclusive observations. This fallacy poses a bottleneck to the effective designing of curable drugs as the field is currently missing the specific druggable site. To understand the fundamentals of pathogenesis, we tried to decipher the intricacies of the extremely complicated landscape by targeting the relevant species that supposedly dictate the structure-function paradigm. The atomic-level description and characterization of the dynamism of the systems reveal the existence of structural polymorphism in all the leading stakeholders of the overall system. The very existence of conformational heterogeneity in every species creates numerous possible combinations of favorable interactions because of the variability in segmental cross-talks and hence claims its role in the choice of routes between functional activity and dysfunctional disease-causing aggregation. Despite this emergent configurational diversity, there is a common mode of operative intermolecular forces that dictates the extent of stability of all the multimeric complexes due to the localized population of a specific type of residue. The present research proposes a dynamic switch mechanism between aggregability and functional activity, based on the logical interpretation of the estimated variables, which is practically dictated by the effective concentration of the interacting species involved in the cell.

Enhanced Sum Frequency Generation for Monolayers on Au Relative to Silica: Local Field Factors and SPR Effect

Using metals as signal magnified substrates, surface plasmon-enhanced sum frequency generation (SFG) vibrational spectroscopy is a promising technique to probe weak molecular-level signals at surfaces and interfaces. In this study, the vibrational signals of the n-alkane monolayer on the gold (Au) and silica substrates are investigated using the broadband femtosecond SFG. The enhancement factors are discovered to be up to ∼1076 and ∼31 for the methyl symmetric and asymmetric stretching (ss and as) modes of the monolayer, respectively. By systematically analyzing the second-order nonlinear susceptibility tensor components (χ_ijks), the Fresnel coefficients (F_ijks), and the surface plasmon resonance (SPR) effect, we find that the interplay between F_ijk and χ_ijk terms and the SPR effect dominate the SFG signal enhancement. Our study reveals that the relative contributions of different influencing factors (i.e., Fresnel coefficients and SPR) to the SFG signal enhancement provide an approach to interpreting enhanced SFG vibrational signals detected from probe molecules on distinct substrates and may finally guide the design of the experimental methodology to improve the detection sensitivity and signal-to-noise ratio.

Probing Molecular Interactions of Antibody Drugs, Silicone Oil, and Surfactant at Buried Interfaces In Situ

Antibody drugs have been rapidly developed to cure many diseases including COVID-19 infection. Silicone oil is commonly used as a lubricant coating material for devices used in the pharmaceutical industry to store and administer antibody drug formulations. However, the interaction between silicone oil and antibody molecules could lead to the adsorption, denaturation, and aggregation of antibody molecules, impacting the efficacy of antibody drugs. Here, we studied the molecular interactions between antibodies and silicone oil in situ in real time. The effect of the surfactant on such interactions was also investigated. Specifically, the adsorption dynamics of a bispecific antibody (BsAb) onto a silicone oil surface without and with different concentrations of the surfactant PS80 in antibody solutions were monitored. Also the possible lowest effective PS80 concentrations that can prevent the adsorption of BsAb as well as a monoclonal antibody (mAb) onto silicone oil were measured. It was found that different concentrations of PS80 are required for preventing the adsorption of different antibodies. Both BsAB and mAB denature on silicone oil without a surfactant. However, for a low surfactant concentration in the solution, although the surfactant could not completely prevent the antibody from adsorption, it could maintain the native structures of adsorbed BsAb and mAb antibodies on silicone oil. This is important knowledge, showing that to prevent antibody aggregation on silicone oil it is not necessary to add surfactant to a concentration high enough to completely minimize protein adsorption.

Tutorial on the instrumentation of sum frequency generation vibrational spectroscopy: Using a Ti:sapphire based system as an example

Sum frequency generation vibrational spectroscopy (SFG-VS) is an intrinsically surface-selective vibrational spectroscopic technique based on the second-order nonlinear optical process. Since its birth in the 1980s, SFG-VS has been used to solve interfacial structure and dynamics in a variety of research fields including chemistry, physics, materials sciences, biological sciences, environmental sciences, etc. Better understanding of SFG-VS instrumentation is no doubt an essential step to master this sophisticated technique. To address this need, here we will present a Tutorial with respect to the classification, setup layout, construction, operation, and data processing about SFG-VS. We will focus on the steady state Ti:sapphire based broad bandwidth SFG-VS system and use it as an example. We hope this Tutorial is beneficial for newcomers to the SFG-VS field and for people who are interested in using SFG-VS technique in their research.

Morphological integrity of insulin amyloid-like aggregates depends on preparation methods and post-production treatments

Protein aggregates are often varying extensively in their morphological characteristics, which may lead to various biological outcomes, such as increased immunogenicity risk. However, isolation of aggregates with a specific morphology within an ensemble is often challenging. To gain vital knowledge on the effects of aggregate characteristics, samples containing a single morphology must be produced by direct control of the aggregation process. Moreover, the formed aggregates need to be in an aqueous solution suitable for biological assays, while keeping their morphology intact. Here we evaluated the dependence of morphology and integrity of amyloid-like fibrils and spherulites on preparation conditions and post-treatment methods. Samples containing either amyloid-like fibrils or spherulites produced from human insulin in acetic acid solutions are dependent on the presence of salt (NaCl). Moreover, mechanical shaking (600 rpm) inhibits spherulite formation, while only affecting the length of the formed fibrils compared to quiescent conditions. Besides shaking, the initial protein concentration in the formulation was found to control fibril length. Surprisingly, exchanging the solution used for aggregate formation to a physiologically relevant buffer, had a striking effect on the morphological integrity of the fibril and spherulite samples. Especially the secondary structure of one of our spherulite samples presented dramatic changes of the aggregated β-sheet content after exchanging the solution, emphasizing the importance of the aggregate stability. These results and considerations have profound implications on the data interpretation and should be implemented in the workflow for both fundamental characterization of aggregates as well as assays for evaluation of their corresponding biological effects.

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.