Identification of the response of protein N–H vibrations in vibrational sum-frequency generation spectroscopy of aqueous protein films

How Does pH Affect the Adsorption of Human Serum Protein in the Presence of Hydrophobic and Hydrophilic Nanoparticles at Air-Water and Lipid-Water Interfaces?

This study investigates interaction between hydrophilic (11-mercaptoundecanoic acid (MUA)) and hydrophobic (1-undecanethiol (UDT)) gold nanoparticles (GNPs) with human serum albumin (HSA) protein on air-water and lipid-water interfaces at pH 3 and 7. Vibrational sum frequency generation (VSFG) spectroscopy is used to analyze changes in the intensity of interfacial water molecules and the C-H group of the protein. At the air-water interface, the hydrophobic interaction between the HSA protein and hydrophobic GNPs at pH 3 leads to their accumulation at the interface, resulting in an increased C-H intensity of the protein with a slight decrease in water intensity. Whereas, at pH 7, where the negative charge of the protein results in the reduced surface activity of the HSA compared to pH 3, the interaction between alkyl chain of the hydrophobic GNPs and alkyl group of the protein results in the adsorption of the protein-capped GNPs at the interface. This leads to an increased intensity of the C-H group of protein and water molecules. However, negatively charged hydrophilic GNPs do not induce significant changes in the interfacial water structure or the C-H group of the protein due to the electrostatic force of repulsion with the negatively charged HSA at pH 7. In contrast, at the lipid-water interface, both hydrophobic and hydrophilic GNPs interact with HSA protein, causing disordering of interfacial water molecules at pH 3 and ordering at pH 7. Interestingly, similar behavior of the protein with both types of GNPs results in comparable ordering/disordering at the interface depending on the pH of solution. Furthermore, the VSFG results obtained with the deuterated lipid suggest that changes in ordering and disorder occur due to increased protein adsorption in the presence of GNPs, causing alterations in the membrane structure. These findings give a better understanding of the mechanisms that govern protein-nanoparticle interaction and their consequential effects on the structure, function, and behavior of molecules at the biological membrane interface, which is crucial for developing safe and effective nanoparticle-based therapeutics.

Fibril formation and ordering of disordered FUS LC driven by hydrophobic interactions

Biomolecular condensates, protein-rich and dynamic membrane-less organelles, play critical roles in a range of subcellular processes, including membrane trafficking and transcriptional regulation. However, aberrant phase transitions of intrinsically disordered proteins in biomolecular condensates can lead to the formation of irreversible fibrils and aggregates that are linked to neurodegenerative diseases. Despite the implications, the interactions underlying such transitions remain obscure. Here we investigate the role of hydrophobic interactions by studying the low-complexity domain of the disordered 'fused in sarcoma' (FUS) protein at the air/water interface. Using surface-specific microscopic and spectroscopic techniques, we find that a hydrophobic interface drives fibril formation and molecular ordering of FUS, resulting in solid-like film formation. This phase transition occurs at 600-fold lower FUS concentration than required for the canonical FUS low-complexity liquid droplet formation in bulk. These observations highlight the importance of hydrophobic effects for protein phase separation and suggest that interfacial properties drive distinct protein phase-separated structures.

Elucidating the Molecular Structure of Hydrophobically Modified Polyethylenimine Nanoparticles and Its Potential Implications for DNA Binding

The structural properties of the polyethylenimine (PEI) polymer are generally tuned and selectively modified to reinforce its potential in a broad spectrum of applied domains of medicine, healthcare, material design, sensing, and electronic optimization. The selective modification of the polymer brings about changes in its interfacial characteristics and behavior. The current work involves the synthesis of naphthalimide conjugated polyethylenimine organic nanoparticles (NPEI-ONPs). The interfacial molecular structure of NPEI-ONPs is explored in an aqueous medium at pH 7.4 using surface tensiometry and sum-frequency generation vibrational spectroscopy (SFG-VS). The hydrophobic functionalization rendered a concentration-dependent surface coverage of NPEI-ONPs, where the SFG-VS analysis exhibited the molecular rearrangement of its hydrophobic groups at the interface. The interaction of NPEI-ONPs with double-stranded DNA (dsDNA) is carried out to observe the relevance of the synthesized nanocomposites in the biomedical domain. The bulk-specific studies (i.e., thermal denaturation, viscometry, zeta (ζ) potential, and ATR-FTIR) reveal the condensation of dsDNA in the presence of NPEI-ONPs, making its structure more compact. The interface-sensitive SFG-VS showcased the impact of the dsDNA and NPEI-ONP interaction on the interfacial molecular behavior of NPEI-ONPs at the air-aqueous interface. Our results exhibit the potential of such hydrophobically functionalized ONPs as promising candidates for developing biomedical sealants, substrate coatings, and other biomedical domains.

Water-Induced Restructuring of the Surface of a Deep Eutectic Solvent

We study the molecular-scale structure of the surface of Reline, a DES made from urea and choline chloride, using heterodyne-detected vibrational sum frequency generation (HD-VSFG). Reline absorbs water when exposed to the ambient atmosphere, and following structure-specific changes at the Reline/air interface is crucial and difficult. For Reline (dry, 0 wt %, w/w, water) we observe vibrational signatures of both urea and choline ions at the surface. Upon increase of the water content, there is a gradual depletion of urea from the surface, an enhanced alignment, and an enrichment of the surface with choline cations, indicating surface speciation of ChCl. Above 40% w/w water content, choline cations abruptly deplete from the surface, as evidenced by the decrease of the vibrational signal of the -CH2- groups of choline and the rapid rise of a water signal. Above 60% w/w water content, the surface spectrum of aqueous Reline becomes indistinguishable from that of neat water.

Probing the Bovine Hemoglobin Adsorption Process and its Influence on Interfacial Water Structure at the Air-Water Interface

*These authors contributed equally to this work.The molecular-level insight of protein adsorption and its kinetics at interfaces is crucial because of its multifold role in diverse fundamental biological processes and applications. In the present study, the sum frequency generation (SFG) vibrational spectroscopy has been employed to demonstrate the adsorption process of bovine hemoglobin (BHb) protein molecules at the air-water interface at interfacial isoelectric point of the protein. It has been observed that surface coverage of BHb molecules significantly influences the arrangement of the protein molecules at the interface. The time-dependent SFG studies at two different frequencies in the fingerprint region elucidate the kinetics of protein denaturation process and its influence on the hydrogen-bonding network of interfacial water molecules at the air-water interface. The initial growth kinetics suggests the synchronized behavior of protein adsorption process with the structural changes in the interfacial water molecules. Interestingly, both the events carry similar characteristic time constants. However, the conformational changes in the protein structure due to the denaturation process stay for a long time, whereas the changes in water structure reconcile quickly. It is revealed that the protein denaturation process is followed by the advent of strongly hydrogen-bonded water molecules at the interface. In addition, we have also carried out the surface tension kinetics measurements to complement the findings of our SFG spectroscopic results.

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.

Decoding the molecular water structure at complex interfaces through surface-specific spectroscopy of the water bending mode

The structure of interfacial water determines atmospheric chemistry, wetting properties of materials, and protein folding. The challenge of investigating the properties of specific interfacial water molecules has frequently been confronted using surface-specific sum-frequency generation (SFG) vibrational spectroscopy using the O-H stretch mode. While perfectly suited for the water-air interface, for complex interfaces, a potential complication arises from the contribution of hydroxyl or amine groups of non-water species present at the surface, such as surface hydroxyls on minerals, or O-H and N-H groups contained in proteins. Here, we present a protocol to extract the hydrogen bond strength selectively of interfacial water, through the water bending mode. The bending mode vibrational frequency distribution provides a new avenue for unveiling the hydrogen bonding structure of interfacial water at complex aqueous interfaces. We demonstrate this method for the water-CaF₂ and water-protein interfaces. For the former, we show that this method can indeed single out water O-H groups from surface hydroxyls, and that with increasing pH, the hydrogen-bonded network of interfacial water strengthens. Furthermore, we unveil enhanced hydrogen bonding of water, compared to bulk water, at the interface with human serum albumin proteins, a prototypical bio-interface.

Chiral Water Superstructures around Antiparallel β-Sheets Observed by Chiral Vibrational Sum Frequency Generation Spectroscopy

Hydration modulates every aspect of protein structure and function. However, studying water structures in hydration shells remains challenging mostly due to overwhelming background from bulk water. We used vibrational sum frequency generation (SFG) spectroscopy to characterize hydrated films of an antiparallel β-sheet peptide (LK7β) adsorbed on glass slides. The hydrated films give chiral SFG response from water only when the peptide self-assembles into antiparallel β-sheets. Experiments of isotopic labeling, isotopic dilution of water, and H2O-D2O exchange kinetics corroborate the assignments of the chiral SFG response to water stretching modes. Because individual water molecules are achiral, the chiral SFG response indicates formation of chiral superstructures of water around the antiparallel β-sheet, implying that a protein secondary structure can imprint its chirality onto the surrounding water. This result demonstrates chiral SFG spectroscopy as a promising tool for probing water structures in protein hydration and addressing fundamental questions of protein structure-function.

Surface Structure of Solutions of Poly(vinyl alcohol) in Water

We use surface-specific heterodyne-detected vibrational sum-frequency generation spectroscopy (HD-VSFG) and surface tension measurements to investigate the molecular structure of the surface of aqueous solutions of poly(vinyl alcohol) (PVA) polymers with average molecular weights of 10000 and 125000 g/mol. We find that the interfacial water molecules have a preferred orientation with their hydrogen-bonded O-H groups pointing away from the bulk, for both PVA₁₀₀₀₀ and PVA₁₂₅₀₀₀. This observation is explained from the ongoing hydrolysis of the acetyl impurities on the PVA polymer chains. This hydrolysis yields negatively charged acetate ions that have a relatively high surface propensity. For both PVA₁₀₀₀₀ and PVA₁₂₅₀₀₀ the strong positive signal vanishes when the pH is decreased, due to the neutralization of the acetate ions. For solutions with a high concentration of PVA₁₀₀₀₀ the interfacial water signal becomes very small, indicating that the surface gets completely covered with a disordered PVA polymer film. In contrast, for high concentrations of PVA₁₂₅₀₀₀, the strong positive water signal persists at high pH, which shows that the water surface does not get completely covered. The HD-VSFG data combined with surface tension data indicate that concentrated PVA₁₂₅₀₀₀ solutions form a structured surface layer with pores containing a high density of interfacial water.

Two dimensional crowding effects on protein folding at interfaces observed by chiral vibrational sum frequency generation spectroscopy

The crowding effect is prevalent in cellular environments due to high concentrations of biomacromolecules. It can alter the structures and dynamics of proteins and thus impact protein functions. The crowding effect is important not only in 3-dimensional cytoplasm but also for a 2-dimensional (2D) cell surface due to the presence of membrane proteins and glycosylation of membrane proteins and phospholipids. These proteins and phospholipids - with limited translational degrees of freedom along the surface normal - are confined in 2D space. Although the crowding effect at interfaces has been studied by adding crowding agents to bulk solution, the 2D crowding effect remains largely unexplored. This is mostly due to challenges in controlling 2D crowding and synergistic use of physical methods for in situ protein characterization. To address these challenges, we applied chiral vibrational sum frequency generation (SFG) spectroscopy to probe the sp1 zinc finger (ZnF), a 31-amino acid protein, folding into a β-hairpin/α-helix (ββα) motif upon binding to Zn2+. We anchored ZnF at the air/water interface via covalent linkage of ZnF to palmitic acid and controlled 2D crowding by introducing neutral lipid as a spacer. We obtained chiral amide I SFG spectra upon addition of Zn2+ and/or spacer lipid. The chiral SFG spectra show that interfacial crowding in the absence of spacer lipid hinders ZnF from folding into the ββα structure even in the presence of Zn2+. The results establish a paradigm for future quantitative, systematic studies of interfacial crowding effects.