Buffering agents modify the hydration landscape at charged interfaces

Advancing Albumin Isolation from Human Serum with Graphene Oxide and Derivatives: A Novel Approach for Clinical Applications

This study introduces a novel, environmentally friendly albumin isolation method using graphene oxide (GO). GO selectively extracts albumin from serum samples, leveraging the unique interactions between GO's oxygen-containing functional groups and serum proteins. This method achieves high purification efficiency without the need for hazardous chemicals. Comprehensive characterization of GO and reduced graphene oxide (rGO) through techniques such as X-ray diffraction (XRD) analysis, Raman spectroscopy, scanning electron microscopy (SEM), and Fourier transform infrared spectroscopy (FTIR) confirmed the structural and functional group transformations crucial for protein binding. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and mass spectrometry analyses demonstrated over 95% purity of isolated albumin, with minimal contamination from other serum proteins. The developed method, optimized for pH and incubation conditions, showcases a green, cost-effective, and simple alternative for albumin purification, promising broad applicability in biomedical research and clinical applications.

Water and ions in electrified silica nano-pores: a molecular dynamics study

Solid-liquid interfaces (SLIs) are ubiquitous in science and technology from the development of energy storage devices to the chemical reactions occurring in the biological milieu. In systems involving aqueous saline solutions as the liquid, both the water and the ions are routinely exposed to an electric field, whether the field is externally applied, or originating from the natural surface charges of the solid. In the current study a molecular dynamics (MD) framework is developed to study the effect of an applied voltage on the behaviour of ionic solutions located in a ∼7 nm pore between two uncharged hydrophilic silica slabs. We systematically investigate the dielectric properties of the solution and the organisation of the water and ions as a function of salt concentration. In pure water, the interplay between interfacial hydrogen bonds and the applied field can induce a significant reorganisation of the water orientation and densification at the interface. In saline solutions, at low concentrations and voltages the interface dominates the whole system due to the extended Debye length resulting in a dielectric constant lower than that for the bulk solution. An increase in salt concentration or voltage brings about more localized interfacial effects resulting in dielectric properties closer to that of the bulk solution. This suggests the possibility of tailoring the system to achieve the desired dielectric properties. For example, at a specific salt concentration, interfacial effects can locally increase the dielectric constant, something that could be exploited for energy storage.

Phase separation in a ternary DPPC/DOPC/POPC system with reducing hydration

The maintenance of plasma membrane structure is vital for the viability of cells. Disruption of this structure can lead to cell death. One important example is the macroscopic phase separation observed during dehydration associated with desiccation and freezing, often leading to loss of permeability and cell death. It has previously been shown that the hybrid lipid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) can act as a line-active component in ternary lipid systems, inhibiting macroscopic phase separation and stabilising membrane microdomains in lipid vesicles [1]. The domain size is found to decrease with increasing POPC concentration until complete mixing is observed. However, no such studies have been carried out at reduced hydration. To examine if this phase separation is unique to vesicles in excess water, we have conducted studies on several binary and ternary model membrane systems at both reduced hydration ("powder" type samples and oriented membrane stacks) and in excess water (supported lipid bilayers) at 0.2 mol fraction POPC, in the range where microdomain stabilisation is reported. Differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR) are used to map phase transition temperatures, with X-ray and neutron scattering providing details of the changes in lipid packing and phase information within these boundaries. Atomic force microscopy (AFM) is used to image bilayers on a substrate in excess water. In all cases, macroscopic phase separation was observed rather than microdomain formation at this molar ratio. Thus POPC does not stabilise microdomains under these conditions, regardless of the type of model membrane, hydration or temperature. Thus we conclude that the driving force for separation under these conditions overcomes any linactant effects of the hybrid lipid.

Curcumin tautomerization in the mechanism of pentameric amyloid- β42 oligomers disassembly

Alzheimer's disease is a neurologic disorder characterized by the accumulation of extracellular deposits of amyloid-β (Aβ) fibrils in the brain of patients. The key etiologic agent in Alzheimer's disease is not known; however oligomeric Aβ appears detrimental to neuronal functions and increases Aβ fibrils deposition. Previous research has shown that curcumin, a phenolic pigment of turmeric, has an effect on Aβ assemblies, although the mechanism remains unclear. In this study, we demonstrate that curcumin disassembles pentameric oligomers made from synthetic Aβ42 peptides (pentameric oAβ42), using atomic force microscopy imaging followed by Gaussian analysis. Since curcumin shows keto-enol structural isomerism (tautomerism), the effect of keto-enol tautomerism on its disassembly was investigated. We have found that curcumin derivatives capable of keto-enol tautomerization also disassemble pentameric oAβ42, while, a curcumin derivative incapable of tautomerization did not affect the integrity of pentameric oAβ42. These experimental findings indicate that keto-enol tautomerism plays an essential role in the disassembly. We propose a mechanism for oAβ42 disassembly by curcumin based on molecular dynamics calculations of the tautomerism. When curcumin and its derivatives bind to the hydrophobic regions of oAβ42, the keto-form changes predominantly to the enol-form; this transition is associated with structural (twisting, planarization and rigidification) and potential energy changes that give curcumin enough force to act as a torsion molecular-spring that eventually disassembles pentameric oAβ42. This proposed mechanism sheds new light on keto-enol tautomerism as a relevant chemical feature for designing such novel therapeutic drugs that target protein aggregation.

Effect of pH on the Surface Layer of Molecular Crystals at the Solid-Liquid Interface

Dissolution of solid matter into aqueous solution is one of the most challenging physicochemical aspects related to drug development. While influenced by several parameters, the effect of pH remains the most important one to be fully understood. The dissolution process is essentially controlled by activity at the surface of the molecular crystals, which is difficult to characterize experimentally. To address this, a combination of in situ atomic force microscopy (AFM) with molecular dynamics (MD) simulation is reported. AFM allows for direct visualization of the crystal surface of basic and acidic model compounds (carvedilol and ibuprofen) in contact with an aqueous medium with varying pH. A dramatic increase in surface mobility in the solid-liquid interface could be observed experimentally as a function of pH. The in situ AFM approach opens up for a more detailed understanding of the behavior of particulate matter in solution with importance at different levels, ranging from engineering aspects related to crystallization, and biological considerations related to bioavailability of the final drug product.

The Effect of Ethanol on Disassembly of Amyloid-β1-42 Pentamer Revealed by Atomic Force Microscopy and Gel Electrophoresis

The most common type of dementia, Alzheimer’s disease, is associated with senile plaques formed by the filamentous aggregation of hydrophobic amyloid-β (Aβ) in the brains of patients. Small oligomeric assemblies also occur and drugs and chemical compounds that can interact with such assemblies have attracted much attention. However, these compounds need to be solubilized in appropriate solvents, such as ethanol, which may also destabilize their protein structures. As the impact of ethanol on oligomeric Aβ assembly is unknown, we investigated the effect of various concentrations of ethanol (0 to 7.2 M) on Aβ pentameric assemblies (Aβp) by combining blue native-PAGE (BN-PAGE) and ambient air atomic force microscopy (AFM). This approach was proven to be very convenient and reliable for the quantitative analysis of Aβ assembly. The Gaussian analysis of the height histogram obtained from the AFM images was correlated with band intensity on BN-PAGE for the quantitative estimation of Aβp. Our observations indicated up to 1.4 M (8.3%) of added ethanol can be used as a solvent/vehicle without quantitatively affecting Aβ pentamer stability. Higher concentration induced significant destabilization of Aβp and eventually resulted in the complete disassembly of Aβp.

Nanostructure of a deep eutectic solvent at solid interfaces

HYPOTHESIS

Deep eutectic solvents (DESs) are an attractive class of tunable solvents. However, their uptake for relevant applications has been limited due to a lack of detailed information on their structure-property relationships, both in the bulk and at interfaces. The lateral nanostructure of the DES-solid interfaces is likely to be more complex than previously reported and requires detailed, high-resolution investigation.

EXPERIMENTS

We employ a combination of high-resolution amplitude-modulated atomic force microscopy and molecular dynamics simulations to elucidate the lateral nanostructure of a DES at the solid-liquid interface. Specifically, the lateral and near-surface nanostructure of the DES choline chloride:glycerol is probed at the mica and highly-ordered pyrolytic graphite interfaces.

FINDINGS

The lateral nanostructure of the DES-solid interface is heterogeneous and well-ordered in both systems. At the mica interface, the DES is strongly ordered via polar interactions. The adsorbed layer has a distinct rhomboidal symmetry with a repeat spacing of ~0.9 nm comprising all DES species. At the highly ordered pyrolytic graphite interface, the adsorbed layer appears distinctly different, forming an apolor-driven row-like structure with a repeat spacing of ~0.6 nm, which largely excludes the chloride ion. The interfacial nanostructure results from a delicate balance of substrate templating, liquid-liquid interactions, species surface affinity, and packing constraints of cations, anions, and molecular components within the DES. For both systems, distinct near-surface nanostructural layering is observed, which becomes more pronounced close to the substrate. The surface nanostructures elucidated here significantly expand our understanding of DES interfacial behavior and will enhance the optimization of DES systems for surface-based applications.

Complete aggregation pathway of amyloid β (1-40) and (1-42) resolved on an atomically clean interface

To visualize amyloid β (Aβ) aggregates requires an uncontaminated and artifact-free interface. This paper demonstrates the interface between graphene and pure water (verified to be atomically clean using tunneling microscopy) as an ideal platform for resolving size, shape, and morphology (measured by atomic force microscopy) of Aβ-40 and Aβ-42 peptide assemblies from 0.5 to 150 hours at a 5-hour time interval with single-particle resolution. After confirming faster aggregation of Aβ-42 in comparison to Aβ-40, a stable set of oligomers with a diameter distribution of ~7 to 9 nm was prevalently observed uniquely for Aβ-42 even after fibril appearance. The interaction energies between a distinct class of amyloid aggregates (dodecamers) and graphene was then quantified using molecular dynamics simulations. Last, differences in Aβ-40 and Aβ-42 networks were resolved, wherein only Aβ-42 fibrils were aligned through lateral interactions over micrometer-scale lengths, a property that could be exploited in the design of biofunctional materials.

Length and Charge of Water-Soluble Peptoids Impact Binding to Phospholipid Membranes

In this study, we provide a quantitative description of the adsorption of water-soluble N-substituted glycine oligomers (peptoids) to supported lipid bilayers that mimic mammalian plasma membranes. We prepared a small array of systematically varied peptoid sequences ranging in length from 3 to 15 residues. Using the nonlinear optical method second harmonic generation (SHG), we directly monitored adsorption of aqueous solutions of 3- and 15-residue peptoids to phospholipid membranes of varying physical phase, cholesterol content, and head group charge in physiologically relevant pH buffer conditions without the use of extrinsic labels. Equilibrium binding constants and relative surface coverages of adsorbed peptoids were determined from fits to the Langmuir model. Three- and 15-residue peptoids did not interact with cholesterol-containing lipids or charged lipids in the same manner, suggesting that a peptoid's adsorption mechanism changes with sequence length. In a comparison of four three-residue peptoids, we observed a correlation between equilibrium binding constants and calculated log D_7.4 values. Cationic charge modulated surface coverage. Principles governing how peptoid sequence and membrane composition alter peptoid-lipid interactions may be extended to predict physiological effects of peptoids used as therapeutics or as coatings in medical devices.

Atomic Force Spectroscopy on Ionic Liquids

Ionic liquids have become of significant relevance in chemistry, as they can serve as environmentally-friendly solvents, electrolytes, and lubricants with bespoke properties. In particular for electrochemical applications, an understanding of the interface structure between the ionic liquid and an electrified interface is needed to model and optimize the reactions taking place on the solid surface. As with ionic liquids, the interplay between electrostatic forces and steric effects leads to an intrinsic heterogeneity, as the structure of the ionic liquid above an electrified interface cannot be described by the classical electrical double layer model. Instead, a layered solvation layer is present with a structure that depends on the material combination of the ionic liquid and substrate. In order to experimentally monitor this structure, atomic force spectroscopy (AFS) has become the method of choice. By measuring the force acting on a sharp microfabricated tip while approaching the surface in an ionic liquid, it has become possible to map the solvation layers with sub-nanometer resolution. In this review, we provide an overview of the AFS studies on ionic liquids published in recent years that illustrate how the interface is formed and how it can be modified by applying electrical potential or by adding impurities and solvents.

Long-lived ionic nano-domains can modulate the stiffness of soft interfaces

Metal ions underpin countless processes at bio-interfaces, including maintaining electroneutrality, modifying mechanical properties and driving bioenergetic activity. These processes are typically described by ions behaving as independently diffusing point charges. Here we show that Na+ and K+ ions instead spontaneously form correlated nanoscale networks that evolve over seconds at the interface with an anionic bilayer in solution. Combining single-ion level atomic force microscopy and molecular dynamic simulations we investigate the configuration and dynamics of Na+, K+, and Rb+ at the lipid surface. We identify two distinct ionic states: the well-known direct electrostatic interaction with lipid headgroups and a water-mediated interaction that can drive the formation of remarkably long-lived ionic networks which evolve over many seconds. We show that this second state induces ionic network formation via correlative ion-ion interactions that generate an effective energy well of -0.4kBT/ion. These networks locally reduce the stiffness of the membrane, providing a spontaneous mechanism for tuning its mechanical properties with nanoscale precision. The ubiquity of water-mediated interactions suggest that our results have far-reaching implications for controlling the properties of soft interfaces.

Impact of Electric Fields on the Nanoscale Behavior of Lipid Monolayers at the Surface of Graphite in Solution

The nanoscale organization and dynamics of lipid molecules in self-assembled membranes is central to the biological function of cells and in the technological development of synthetic lipid structures as well as in devices such as biosensors. Here, we explore the nanoscale molecular arrangement and dynamics of lipids assembled in monolayers at the surface of highly ordered pyrolytic graphite (HOPG), in different ionic solutions, and under electrical potentials. Using a combination of atomic force microscopy and fluorescence recovery after photobleaching, we show that HOPG is able to support fully formed and fluid lipid membranes, but mesoscale order and corrugations can be observed depending on the type of the lipid considered (1,2-dioleoyl- sn-glycero-3-phosphocholine, 1,2-dioleoyl- sn-glycero-3-phospho-l-serine (DOPS), and 1,2-dioleoyl-3-trimethylammoniumpropane) and the ion present (Na⁺, Ca²⁺, Cl^-). Interfacial solvation forces and ion-specific effects dominate over the electrostatic changes induced by moderate electric fields (±1.0 V vs Ag/AgCl reference electrode) with particularly marked effects in the presence of calcium, and for DOPS. Our results provide insights into the interplay between the molecular, ionic, and electrostatic interactions and the formation of dynamical ordered structures in fluid lipid membranes.

Lubricating properties of single metal ions at interfaces

The behaviour of ionic solutions confined in nanoscale gaps is central to countless processes, from biomolecular function to electrochemistry, energy storage and lubrication. However, no clear link exists between the molecular-level behaviour of the liquid and macroscopic observations. The problem mainly comes from the difficulty to interrogate a small number of liquid molecules. Here, we use atomic force microscopy to investigate the viscoelastic behaviour of pure water and ionic solutions down to the single ion level. The results show a glassy-like behaviour for pure water, with single metal ions acting as lubricants by reducing the elasticity of the nano-confined solution and the magnitude of the hydrodynamic friction. At small ionic concentration (<20 mM) the results can be quantitatively explained by the ions moving via a thermally-activated process resisted by the ion's hydration water (Prandtl-Tomlinson model). The model breaks down at higher salt concentrations due to ion-ion interaction effects that can no longer be neglected. The correlations are confirmed by direct sub-nanometre imaging of the interface at equilibrium. The results provide a molecular-level basis for explaining the tribological properties of aqueous solutions and suggest that ion-ion interactions create mesoscale effects that prevent a direct link between nanoscale and macroscopic measurements.

Bacteria as living patchy colloids: Phenotypic heterogeneity in surface adhesion

Understanding and controlling the surface adhesion of pathogenic bacteria is of urgent biomedical importance. However, many aspects of this process remain unclear (for example, microscopic details of the initial adhesion and possible variations between individual cells). Using a new high-throughput method, we identify and follow many single cells within a clonal population of Escherichia coli near a glass surface. We find strong phenotypic heterogeneities: A fraction of the cells remain in the free (planktonic) state, whereas others adhere with an adhesion strength that itself exhibits phenotypic heterogeneity. We explain our observations using a patchy colloid model; cells bind with localized, adhesive patches, and the strength of adhesion is determined by the number of patches: Nonadherers have no patches, weak adherers bind with a single patch only, and strong adherers bind via a single or multiple patches. We discuss possible implications of our results for controlling bacterial adhesion in biomedical and other applications.

Uptake of iron (III)-ethylenediamine-N,N,N',N'-tetraacetic acid complex by phosphatidylcholine lipid film: Part I. Effect of bulk pH

We studied a ternary solutes aqueous solution of NaOH, iron (III)-ethylenediamine-N,N,N',N'-tetraacetic acid complex (Fe-edta), and 1,2-diheptanoyl-sn-glycero-3-phosphatidylcholine (DHPC)/air interface system to clarify the interactions between iron complexes and lipids with a phosphatidylcholine head group. The solution surface tension and pH were measured as functions of the total molality of NaOH, Fe-edta and DHPC, and the mole fractions of NaOH and DHPC. Rigorous thermodynamic equations were derived, in which the overall proton dissociation equilibria of Fe-edta and DHPC were taken into consideration, and applied to experimental data to obtain phase diagram of adsorption. It was found that (1) adsorption of Fe-edta at the solution/air interface with a DHPC monolayer was about 50-130 times higher than that without a DHPC monolayer and (2) when the bulk mole fraction of NaOH was high, Fe-edta tended to be expelled from the adsorbed film. The last finding suggests that the ambient pH significantly affects passive transport of the iron complex through a phospholipid-containing membrane into the cell interior.

Ions Modulate Stress-Induced Nanotexture in Supported Fluid Lipid Bilayers

Most plasma membranes comprise a large number of different molecules including lipids and proteins. In the standard fluid mosaic model, the membrane function is effected by proteins whereas lipids are largely passive and serve solely in the membrane cohesion. Here we show, using supported 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayers in different saline solutions, that ions can locally induce ordering of the lipid molecules within the otherwise fluid bilayer when the latter is supported. This nanoordering exhibits a characteristic length scale of ∼20 nm, and manifests itself clearly when mechanical stress is applied to the membrane. Atomic force microscopy (AFM) measurements in aqueous solutions containing NaCl, KCl, CaCl₂, and Tris buffer show that the magnitude of the effect is strongly ion-specific, with Ca²⁺ and Tris, respectively, promoting and reducing stress-induced nanotexturing of the membrane. The AFM results are complemented by fluorescence recovery after photobleaching experiments, which reveal an inverse correlation between the tendency for molecular nanoordering and the diffusion coefficient within the bilayer. Control AFM experiments on other lipids and at different temperatures support the hypothesis that the nanotexturing is induced by reversible, localized gel-like solidification of the membrane. These results suggest that supported fluid phospholipid bilayers are not homogenous at the nanoscale, but specific ions are able to locally alter molecular organization and mobility, and spatially modulate the membrane’s properties on a length scale of ∼20 nm. To illustrate this point, AFM was used to follow the adsorption of the membrane-penetrating antimicrobial peptide Temporin L in different solutions. The results confirm that the peptides do not absorb randomly, but follow the ion-induced spatial modulation of the membrane. Our results suggest that ionic effects have a significant impact for passively modulating the local properties of biological membranes, when in contact with a support such as the cytoskeleton.

Buffers Strongly Modulate Fibrin Self-Assembly into Fibrous Networks

Fibrin is a plasma protein with a central role in blood clotting and wound repair. Upon vascular injury, fibrin forms resilient fibrillar networks (clots) via a multistep self-assembly process, from monomers, to double-stranded protofibrils, to a branched network of thick fibers. In vitro, fibrin self-assembly is sensitive to physicochemical conditions like the solution pH and ionic strength, which tune the strength of the noncovalent driving forces. Here we report a surprising finding that the buffer-which is necessary to control the pH and is typically considered to be inert-also significantly influences fibrin self-assembly. We show by confocal microscopy and quantitative light scattering that various common buffering agents have no effect on the initial assembly of fibrin monomers into protofibrils but strongly hamper the subsequent lateral association of protofibrils into thicker fibers. We further find that the structural changes are independent of the molecular structure of the buffering agents as well as of the activation mechanism and even occur in fibrin networks formed from platelet-poor plasma. This buffer-mediated decrease in protofibril bundling results in a marked reduction in the permeability of fibrin networks but only weakly influences the elastic modulus of fibrin networks, providing a useful tuning parameter to independently control the elastic properties and the permeability of fibrin networks. Our work raises the possibility that fibrin assembly in vivo may be regulated by variations in the acute-phase levels of bicarbonate and phosphate, which act as physiological buffering agents of blood pH. Moreover, our findings add a new example of buffer-induced effects on biomolecular self-assembly to recent findings for a range of proteins and lipids.

An improved method for purification and refolding of recombinant HIV Vif expressed in Escherichia coli

Virion infectivity factor (Vif) is a 23 kDa protein that protects HIV-1 from deamination of its proviral DNA by APOBEC3G. The active form of Vif is a multimer that interacts simultaneously with CBF-beta, the elongin B and C subunits, Cullin 5, and APOBEC3G to form a ubiquitin ligase complex targeting the latter for degradation. Vif clearly represents an attractive target for developing novel antiviral drugs for the therapy of HIV/AIDS, and this goal requires a source of well folded, readily available protein. For that purpose, we have cloned Vif in the pET28a expression vector, expressing the resulting His-tagged recombinant protein in the BL21(DE3) Escherichia coli strain. After lysis, Vif was solubilized from the insoluble fraction with 6 M guanidinium chloride and purified by denaturing immobilized-metal affinity chromatography, refolding the protein afterwards by dialysis. The use of 2-(N-morpholino)ethanesulfonic acid buffer at pH 6.2 and the presence of EDTA improved Vif refolding yields by reducing the formation of insoluble aggregates. The purified protein was bound by two monoclonal antibodies against sequential and conformational epitopes located at the C and N terminus, respectively.

Evaluation of buffers toxicity in tobacco cells: Homopiperazine-1,4-bis (2-ethanesulfonic acid) is a suitable buffer for plant cells studies at low pH

Low pH is an important environmental stressor of plant root cells. Understanding the mechanisms of stress and tolerance to acidity is critical; however, there is no widely accepted pH buffer for studies of plant cells at low pH. Such a buffer might also benefit studies of Al toxicity, in which buffering at low pH is also important. The challenge is to find a buffer with minimal cellular effects. We examined the cytotoxicity and possible metabolic disturbances of four buffers that have adequate pKa values and potential use for studies in the pH range of 4.0-5.0. These were homopipes (homopiperazine-1,4-bis (2-ethanesulfonic acid); pKa1 4.4), 3,3-dimethylglutaric acid (pKa1 3.73), β-alanine (pKa1 3.70) and potassium biphthalate (pKa1 2.95; pKa2 5.41). First, tobacco BY-2 cells were grown in a rich medium containing 10 mM of each buffer or MES (2-(N-morpholino) ethanesulfonic acid) as a control, with the pH initially adjusted to 5.7. β-alanine was clearly toxic and dimethylgluturate and biphthalate were found to be cytostatic, in which no culture growth occurred but cell viability was either unaffected or decreased only after 5 days. Only homopipes allowed normal culture growth and cell viability. Homopipes (10 mM) was then tested in cell cultures with an initial pH of 4.3 ± 0.17 in minimal medium to examine whether its undissociated species (H2A) displayed any cellular effects and no cytotoxic effects were observed. It is possible to conclude that among tested buffers, homopipes is the most suitable for studies at low pH, and may be especially useful for aluminum toxicity experiments.

Direct observation of the dynamics of single metal ions at the interface with solids in aqueous solutions

The dynamics of ions adsorbed at the surface of immersed charged solids plays a central role in countless natural and industrial processes such as crystal growth, heterogeneous catalysis, electrochemistry, or biological function. Electrokinetic measurements typically distinguish between a so-called Stern layer of ions and water molecules directly adsorbed on to the solid’s surface, and a diffuse layer of ions further away from the surface. Dynamics within the Stern layer remain poorly understood, largely owing to a lack of in-situ atomic-level insights. Here we follow the dynamics of single Rb⁺ and H₃O⁺ ions at the surface of mica in water using high-resolution atomic force microscopy with 25 ms resolution. Our results suggest that single hydrated Rb⁺ions reside τ₁ = 104 ± 5 ms at a given location, but this is dependent on the hydration state of the surface which evolves on a slower timescale of τ₂ = 610 ± 30 ms depending on H₃O⁺ adsorption. Increasing the liquid’s temperature from 5 °C to 65 °C predictably decreases the apparent glassiness of the interfacial water, but no clear effect on the ions’ dynamics was observed, indicating a diffusion-dominated process. These timescales are remarkably slow for individual monovalent ions and could have important implications for interfacial processes in electrolytes.

Sub-nanometre mapping of the aquaporin-water interface using multifrequency atomic force microscopy

Aquaporins are integral membrane proteins that regulate the transport of water and small molecules in and out of the cell. In eye lens tissue, circulation of water, ions and metabolites is ensured by a microcirculation system in which aquaporin-0 (AQP0) plays a central role. AQP0 allows water to flow beyond the diffusion limit through lens membranes. AQP0 naturally arranges in a square lattice. The malfunction of AQP0 is related to numerous diseases such as cataracts. Despite considerable research into its structure, function and dynamics, the interface between the protein and the surrounding liquid and the effect of the lattice arrangement on the behaviour of water at the interface with the membrane are still not fully understood. Here we use a multifrequency atomic force microscopy (AFM) approach to map both the liquid at the interface with AQP0 and the protein itself with sub-nanometer resolution. Imaging using the fundamental eigenmode of the AFM cantilever probes mainly the interfacial water at the surface of the membrane. The results highlight a well-defined region that surrounds AQP0 tetramers and where water exhibits a higher affinity for the protein. Imaging in the second eigenmode is dominated by the mechanical response of the protein and provides sub-molecular details of the protein surface and the sub-surface structure. The relationship between modes and harmonics is also examined.

Role of Buffers in Protein Formulations

Buffers comprise an integral component of protein formulations. Not only do they function to regulate shifts in pH, they also can stabilize proteins by a variety of mechanisms. The ability of buffers to stabilize therapeutic proteins whether in liquid formulations, frozen solutions, or the solid state is highlighted in this review. Addition of buffers can result in increased conformational stability of proteins, whether by ligand binding or by an excluded solute mechanism. In addition, they can alter the colloidal stability of proteins and modulate interfacial damage. Buffers can also lead to destabilization of proteins, and the stability of buffers themselves is presented. Furthermore, the potential safety and toxicity issues of buffers are discussed, with a special emphasis on the influence of buffers on the perceived pain upon injection. Finally, the interaction of buffers with other excipients is examined.

Effect of temperature on the viscoelastic properties of nano-confined liquid mixtures

The behaviour of fluids confined in nanoscale gaps plays a central role in molecular science and nanofluidics, with applications ranging from biological function to multiscale printing, osmosis and filtration, lab-on-chip technology and friction reduction. Here atomic force microscopy is used to shear five different mixtures of hexadecane and squalane confined between the tip apex and atomically flat graphite. The shearing amplitudes are typically <2 nm, hence reflecting highly localised information at the interface. The evolution of each mixture's viscoelastic properties is studied as a function of temperature, between 20 °C and 100 °C. The results, complemented by sub-nanometre resolution images of the interface, show that spatial organisation of the liquid molecules at the surface of graphite largely dominates the measurements. Squalane presents a higher effective affinity for the surface by forming a robust self-assembled layer in all mixtures. This results in a step-like change of the viscous and elastic response of the confined liquid as the confining pressure increases. In contrast, measurements in pure hexadecane show a continuous and linear increase in the apparent viscosity with pressure at all temperatures. This is interpreted as a more fragile interfacial layer and images show that it can be completely removed at high temperatures. Depending on the mixture composition, measurements can be strongly location-dependent which suggests molecular clustering and nanoscale phase separation at the interface.