Electrochemical and PM-IRRAS characterization of cholera toxin binding at a model biological membrane

Theoretical investigation of hydroxylated analogues of valinomycin as potassium transporter

Valinomycin is a potent ionophore known for its ability to transport potassium ions across biological membranes. The study focuses on the hydroxylated analogues of valinomycin (HyVLMs) and compares their energy profiles and capabilities for transporting potassium ions across phospholipid membranes. Using metadynamics, we investigated the energy profiles of wildtype valinomycin (VLM_1) and its three hydroxylated analogues (VLM_2, VLM_3, and VLM_4). We observed that all analogues exhibited energy maxima in the centre of the membrane and preferred positions below the phospholipid heads. Furthermore, the entry barriers for membrane penetration were similar among the analogues, suggesting that the hydroxyl group did not significantly affect their passage through the membrane. Transition state calculations provided insights into the ability of valinomycin analogues to capture potassium ions, with VLM_4 showing the lowest activation energy and VLM_2 displaying the highest. Our findings contribute to understanding the mechanisms of potassium transport by valinomycin analogues and highlight their potential as ionophores. The presence of the hydroxyl group is of particular importance because it paves the way for subsequent chemical modifications and the synthesis of new antiviral agents with reduced intrinsic toxicity.

A study of cobalt (II) complexes involved in marine biogeochemical processes: Co(II)-1,10-Phenanthroline and Co(II)-1,10-Phenanthroline-L-α-Phosphatidylcholine

The cell membrane is structured so that the surface layer is composed of lipid molecules with selective permeability for micronutrients and organic ligands. Binding of Co (II) to natural lipid phosphatidylcholine (PC) has been studied to identify a possible mechanism of Co (II) entry through the cell membrane of the biota in detail, by voltammetry followed by checking the system at the air-water boundary, by Langmuir method. Binding of cobalt (II) ions to the PC molecules was enabled by the Co(II)-1,10-Phenanthroline (Phen) complex formation as an intermediate. Co(II)-Phen-PC complex reduction was recorded in the pH range from 5 to 9.5. The reduction was identified as a two-electron irreversible reaction at about -1.5 V, with the reactant adsorption followed dissociation (EC mechanism). The Co(II)-Phen-PC complex electrode surface concentration (Γ) was calculated to be (1.45 ± 0.12) × 10-10 mol.cm-2. Conditional stability constants log KCo(II)Phen2PC = 23.02 ± 0.26 and log KCo(II)Phen2PC2 = 29.31 ± 0.17 (Ic = 0.55) were calculated by CLE/ACSV method. Pressure-area (π-A) isotherms obtained at water-air interface by Langmuir monolayer technique indicated penetration of Co(II)-Phen into the PC monolayer, supporting electrochemical results. The equilibrium constants of the Co (II)-PC system (1:1) at the air-water interface was calculated to be K1 = 2.4 × 10-2 m3 mol-1, while for Co(II)-Phen-PC K2 = 4.86 × 1010 m2 mol-1.

Evaluation of the effects in cellular membrane models of antitrypanosomal poly-thymolformaldehyde (PTF) using Langmuir monolayers

The polymerization of bioactive compounds may be interesting because the supramolecular structures formed can boost biological action on microorganism membranes. In the present work, poly-thymolformaldehyde (PTF) activity, prepared by condensation of thymol and formaldehyde, was evaluated against trypomastigote forms of Trypanosoma cruzi and related with the physicochemical changes provided by the incorporation of the compound in protozoan cell membrane models. PTF exhibited an EC₅₀ value of 23.4 μg/mL and no toxicity against mammalian cells (CC₅₀ > 200 μg/mL). To understand the molecular action of PTF as an antiprotozoal candidate, this compound was incorporated in Langmuir monolayers of dipalmitoylphosphatidylglycerol (DPPG) as a model for parasite cell membranes. PTF shifted DPPG surface pressure-area isotherms to higher areas, indicating its incorporation in the lipid films. Additionally, it changed the thermodynamic, compressional, structural, and morphological properties of the floating monolayers, decreasing the collapse pressure, reducing the surface elasticity, and segregating molecules at the interface, forming domains with different reflectivities. Infrared spectroscopy showed that the lipid films with PTF presented an increased rate of gauche/all-trans conformers for the methylene groups from the acyl chains, indicating molecular disorder. Therefore, these results show that PTF alters the physicochemical properties of DPPG monolayers as a model for protozoa cell membranes, which can enhance the comprehension of the parasitic action of PTF against T. cruzi.

Alzheimer's disease-related amyloid β peptide causes structural disordering of lipids and changes the electric properties of a floating bilayer lipid membrane

Neurodegeneration in Alzheimer's disease is associated with disruption of the neuronal cell membrane by the amyloid β (Aβ) peptide. However, the disruption mechanism and the resulting changes in membrane properties remain to be elucidated. To address this issue, herein the interaction of amyloid β monomers (AβMs) and amyloid β oligomers (AβOs) with a floating bilayer lipid membrane (fBLM) was studied using electrochemical and IR spectroscopy techniques. IR measurements showed that both Aβ forms interacted similarly with the hydrophobic membrane core (lipid acyl chains), causing conformational and orientational changes of the lipid acyl chains, thus decreasing acyl chain mobility and altering the lipid packing unit cell. In the presence of AβOs, these changes were more significant than those in the presence of AβMs. However, respective interactions of AβMs and AβOs with the membrane hydrophilic exterior (lipid heads) were quite different. AβMs dehydrated lipid heads without affecting their orientation while AβOs changed the orientation of lipid heads keeping their hydration level intact. Electrochemical measurements showed that only AβOs porated the fBLM, thus significantly changing the fBLM electrical properties. The present results provide new molecular-level insight into the mechanism of membrane destruction by AβOs and changes in the membrane properties.

Impaired mucociliary motility enhances antigen-specific nasal IgA immune responses to a cholera toxin-based nasal vaccine

Nasal mucosal tissues are equipped with physical barriers, mucus and cilia, on their surface. The mucus layer captures inhaled materials, and the cilia remove the inhaled materials from the epithelial layer by asymmetrical beating. The effect of nasal physical barriers on the vaccine efficacy remains to be investigated. Tubulin tyrosine ligase-like family, member 1 (Ttll1) is an essential enzyme for appropriate movement of the cilia on respiratory epithelium, and its deficiency (Ttll1-KO) leads to mucus accumulation in the nasal cavity. Here, when mice were intra-nasally immunized with pneumococcal surface protein A (PspA, as vaccine antigen) together with cholera toxin (CT, as mucosal adjuvant), Ttll1-KO mice showed higher levels of PspA-specific IgA in the nasal wash and increased numbers of PspA-specific IgA-producing plasma cells in the nasal passages when compared with Ttll1 hetero (He) mice. Mucus removal by N-acetylcysteine did not affect the enhanced immune responses in Ttll1-KO mice versus Ttll1-He mice. Immunohistological and flow cytometry analyses revealed that retention time of PspA in the nasal cavity in Ttll1-KO mice was longer than that in Ttll1-He mice. Consistently, uptake of PspA by dendritic cells was higher in the nasopharynx-associated lymphoid tissue (NALT) of Ttll1-KO mice than that of Ttll1-He mice. These results indicate that the ciliary function of removing vaccine antigen from the NALT epithelial layer is a critical determinant of the efficacy of nasal vaccine.

How Valinomycin Ionophores Enter and Transport K+ across Model Lipid Bilayer Membranes

Valinomycin, a cyclic peptide, was incorporated into a biomimetic lipid membrane tethered to the surface of a gold (111) electrode. Electrochemical impedance spectroscopy was used to study the ionophore properties of the peptide, and polarization modulation infrared reflection absorption spectroscopy was employed to determine the conformation and orientation of valinomycin in the membrane. The combination of these two techniques provided unique information about the ionophore mechanism where valinomycin transports ions across the membrane by creating a complex with potassium ions and forming an ion pair with a counter anion. The ion pair resides within the hydrophobic fragment of the membrane and adopts a small angle of ∼22° with respect to the surface normal. This novel study provides new insights explaining the valinomycin ion transport mechanism in model biological membranes.

Binding of a Myristoylated Protein to the Lipid Membrane Influenced by Interactions with the Polar Head Group Region

Many cytoplasmic proteins contain a hydrophobic acyl chain, which facilitates protein binding to cell membranes. Hydrophobic interactions between the exposed acyl chain of the protein and hydrocarbon chains of lipids in the cell membrane are the driving force for this specific lipid-protein interaction. Recent studies point out that in addition to hydrophobic interactions the charge-charge and charge-dipole interactions between the polar head groups and basic amino acids contribute significantly to the binding process. Recoverin possesses a myristoyl chain at the N-terminus. In the presence of Ca²⁺ ions, the protein undergoes structural rearrangements, leading to the extrusion of the myristoyl chain, facilitating the protein binding to the membrane. In this work, we investigate the impact of interactions between the polar head group region of lipid molecules and recoverin which binds to the model membrane. The interaction with a planar lipid bilayer composed of phosphatidylcholine and cholesterol with myristoylated and nonmyristoylated recoverin is studied by in situ polarization modulation infrared reflection absorption spectroscopy. The binding of recoverin to the lipid bilayer depends on the transmembrane potential, indicating that the orientation of the permanent surface dipole in the supramolecular assembly of the lipid membrane influences the protein attachment to the membrane surface. Analysis of the amide I' mode indicates that the orientation of recoverin bound to the lipid bilayer is independent of the presence of myristoyl chain in the protein and of the folding of the protein into the tense or relaxed state. In contrast, it changes as a function of the membrane potential. At positive transmembrane potentials, the α-helical fragments of recoverin are oriented predominantly parallel to the bilayer surface. This orientation facilitates the insertion of the acyl chain of the protein into the hydrophobic region of the bilayer. At negative transmembrane potentials, the α-helical fragments of recoverin change their orientation with respect to the membrane surface, which is followed by the removal of the myristoyl chain from the membrane.

Impact of the protein myristoylation on the structure of a model cell membrane in a protein bound state

The neuronal calcium sensor protein recoverin is expressed in retinal rod and cone cells and is involved in the calcium-dependent control of receptor (rhodopsin) phosphorylation and receptor inactivation. In its Ca²⁺-saturated form recoverin is attached to membranes by an exposed myristoyl group and responds to oscillating changes of intracellular Ca²⁺-concentration by performing a so-called Ca²⁺-myristoyl switch. In this work we analyze changes in a liquid lipid bilayer interacting with myristoylated and non-myristoylated recoverin by employing polarization modulation infrared reflection absorption spectroscopy (PM IRRAS) with electrochemical control. The lipid bilayer is transferred onto a polycrystalline gold electrode using Langmuir-Blodgett Langmuir-Schaefer transfer at the surface pressure π = 30 mN m^-1 which ensures, necessary for the lipid-protein interaction, liquid state of the hydrocarbon chains of phospholipids. The model lipid bilayers are adsorbed directly on the Au electrode surface at transmembrane potentials -0.2 < ∆ϕM|S < 0.25 V. The interaction with recoverin leads to a stabilization of the adsorbed state of the lipid bilayer at positive transmembrane potentials. The interaction leads to a decrease in the surface charge density that accumulates on the membrane covered electrode surface, indicating changes in the lateral interactions in the lipid membrane. In situ spectroelectrochemical studies confirm orientation changes in the hydrophobic environment of the model membrane. Insertion of the myristoyl group of recoverin into the membrane is connected with an increase in the tilt of the hydrocarbon chains with respect to the surface normal and decrease in the bilayer thickness. Potential-induced pore formation and desorption of the lipid bilayer from the membrane surface is accompanied by the removal of the acyl chains of recoverin from the membrane.

Surface Chemistry and Spectroscopic Study of a Cholera Toxin B Langmuir Monolayer

In this article, we explored the surface chemistry properties of a cholera toxin B (CTB) monolayer at the air-subphase interface and investigated the change in interfacial properties through in situ spectroscopy. The study showed that the impact of the blue shift was negligible, suggesting that the CTB molecules were minimally affected by the subphase molecules. The stability of the CTB monolayer was studied by maintaining the constant surface pressure for a long time and also by using the compression-decompression cycle experiments. The high stability of the Langmuir monolayer of CTB clearly showed that the driving force of CTB going to the amphiphilic membrane was its amphiphilic nature. In addition, no major change was detected in the various in situ spectroscopy results (such as UV-vis, fluorescence, and IR ER) of the CTB Langmuir monolayer with the increase in surface pressure. This indicates that no aggregation occurs in the Langmuir monolayer of CTB.

Electrochemical surface-enhanced Raman spectroscopy (EC-SERS) study of the interaction between protein aggregates and biomimetic membranes

EC-SERS is used for the first time to characterize protein aggregate–biomembrane interactions.

Copper-phospholipid interaction at cell membrane model hydrophobic surfaces

Detailed investigation of Cu (II) binding with natural lipid phosphatidylglycerol (PG) in aqueous solution was carried out by voltammetric measurements at the mercury drop electrode, complemented by monolayer studies in a Langmuir trough and electrophoretic measurements, all used as models for hydrophobic cell membranes. Penetration of copper ions into the PG layer was facilitated by the formation of hydrophilic Cu-Phenanthroline (Phen) complex in the subphase, followed by the mixed ligand Cu-Phen-PG complex formation at the hydrophobic interface. Electrophoretic measurements indicated a comparatively low abundance of the formed mixed ligand complex within the PG vesicles, resulting it the zeta potential change of +0.83mV, while monolayer studies confirmed their co-existence at the interface. The Cu-Phen-PG complex was identified in the pH range from 6 to 9. The stoichiometry of the complex ([PhenCuOHPG]), as well as its stability and kinetics of formation, were determined at the mercury drop electrode. Cu-Phen-PG reduces quasireversibly at about -0.7V vs. Ag/AgCl including reactant adsorption, followed by irreversible mixed complex dissociation, indicating a two-electron transfer - chemical reaction (EC mechanism). Consequently, the surface concentration (γ) of the adsorbed [PhenCuOHPG] complex at the hydrophobic electrode surface was calculated to be (3.35±0.67)×10^-11molcm^-2. Information on the mechanism of Cu (II) - lipid complex formation is a significant contribution to the understanding of complex processes at natural cell membranes.

Immobilized enzymes: understanding enzyme – surface interactions at the molecular level

Interactions between immobilized enzymes and supporting surfaces are complex and context-dependent and can significantly alter enzyme structure, stability and activity.

Physicochemical Studies on Orientation and Conformation of a New Bacteriocin BacSp222 in a Planar Phospholipid Bilayer

The behavior, secondary structure, and orientation of a recently discovered bacteriocin-like peptide BacSp222 in a lipid model system supported at a gold electrode was investigated by chronocoulometry, polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), and attenuated total reflectance infrared (ATR-IR) spectroscopy. The IR spectra show that the secondary structure of BacSp222 is predominantly α-helical. Analysis of the spectra in the amide I region shows that the α-helical fragment of the peptide is inserted into bilayer at the potential range at which the bilayer is stable and attached to the Au(111) surface, i.e., from -0.5 to 0.3 V vs Ag/AgCl. Insertion of BacSp222 to the membrane significantly changes the conformation of the acyl chains of lipid molecules, from all-trans to partially melted; however, the chains become less tilted. Based on these results, we propose that BacSp222 interacts with the DMPC bilayer through the barrel-stave pore formation. In this model, α-helix of BacSp222 inserts into the membrane with an angle between the α-helix axis and membrane normal equal to ∼18°. The changes in orientation of the α-helical fragment of the peptide indicate that the orientation of BacSp222 with respect to the bilayer surface is potential-dependent. The peptide is inserted into the membrane driven by the electrostatic field generated by negative charge at the metal surface. It is not inserted at negative potentials where the membrane is detached from the metal and no longer exposed to the electrostatic field of the metal.

Dependence of interfacial film organization on lipid molecular structure

Combination of surface analytical techniques was employed to investigate the interfacial behavior of the two designed lipids-N-stearoylglycine (1) and its bulky neutral headgroup-containing derivative N-stearoylvaline ethyl ester (2)-at the air-solution interface and as transferred layers on different substrates. Formation of monolayers at the air-water interface was monitored on pure water and on aqueous solutions of different pH. Crystallization effects were visualized at pure water by recording the hystereses in the Langmuir-Blodgett (LB) isotherms and by transferring the layers onto mica, gold (111), and ITO (indium-tin oxide on glass) electrodes. Subphase pH affects the morphology and patch formation in monolayers of 1, as evidenced by BAM measurements. At pH 8.2, formation of well-ordered crystallites is observed, which upon compression elongate according to predominantly 1-D growth mechanism to form a dense layer of crystallites. This effect is not observed in monolayers of 2, whose headgroup is not protonated. The orientation of layers of 1 transferred to the solid supports is also pH dependent, and their stability can be related to formation of a hydrogen-bonded networks. AFM images of 1 exhibited platelets of multilayer phase. The IR spectra of the ITO substrates covered by 1 indicated formation of hydrogen bonds between the amide groups. The nature of the adsorption layer and its organization as a function of potential were studied in-depth by EC STM using Au(111) as the substrate. A model showing the arrangement of hydrogen bonds between adsorbed molecules is presented and related to the observed organization of the layer.

Vibrational spectroscopy for probing molecular-level interactions in organic films mimicking biointerfaces

Investigation into nanostructured organic films has served many purposes, including the design of functionalized surfaces that may be applied in biomedical devices and tissue engineering and for studying physiological processes depending on the interaction with cell membranes. Of particular relevance are Langmuir monolayers, Langmuir-Blodgett (LB) and layer-by-layer (LbL) films used to simulate biological interfaces. In this review, we shall focus on the use of vibrational spectroscopy methods to probe molecular-level interactions at biomimetic interfaces, with special emphasis on three surface-specific techniques, namely sum frequency generation (SFG), polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS) and surface-enhanced Raman scattering (SERS). The two types of systems selected for exemplifying the potential of the methods are the cell membrane models and the functionalized surfaces with biomolecules. Examples will be given on how SFG and PM-IRRAS can be combined to determine the effects from biomolecules on cell membrane models, which include determination of the orientation and preservation of secondary structure. Crucial information for the action of biomolecules on model membranes has also been obtained with PM-IRRAS, as is the case of chitosan removing proteins from the membrane. SERS will be shown as promising for enabling detection limits down to the single-molecule level. The strengths and limitations of these methods will also be discussed, in addition to the prospects for the near future.