We present direct visualization of pores formed by alamethicin (Alm) in a matrix of phospholipids using electrochemical scanning tunneling microscopy (EC-STM). High-resolution EC-STM images show ...individual peptide molecules forming channels. The channels are not dispersed randomly in the monolayer but agglomerate forming 2D nanocrystals with a hexagonal lattice in which the average channel–channel distance is 1.90 ± 0.1 nm. The STM images suggest that each Alm is shared between the two adjacent channels. Every channel consists of six Alm molecules. Three or four of these molecules have the hydrophilic group oriented toward the center of the channel allowing for water column formation inside the channel. The dimensions of the central pore in the images are consistent with the dimension of the water column in a model of hexameric pore proposed in the literature. The images obtained in this work validate the barrel-stave model of the pore formed in phospholipid membranes by amphiphatic peptides. They also provide direct evidence for cluster formation by such pores.
•Hydration water of lipids polar heads predominates at zero charge.•Monomers and multimers of water molecules are at intermediate charges.•Liquid like water is in the space separating bilayer from ...the metal at very negative charges.
Surface-enhanced infrared absorption spectroscopy (SEIRAS) was employed to study structure of water in a phospholipid bilayer deposited at a gold electrode surface. The technique employs attenuated total reflection (ATR) and an enhancement of the electric field of the IR photon that decays steeply with distance from the metal surface. These conditions allow one to subtract the background from the bulk water and to determine spectra of water in the bilayer or in a confined space between the metal surface and the bilayer. The IR data demonstrated that three types of water are present in the supported bilayer. At potentials close to zero charge the polar heads of the phospholipid molecules retain hydration water. At intermediate charge densities water penetrating deeply into the bilayer and multimers of water molecules were detected in the bilayer. At charge densities more negative than −20μC/cm2 the bilayer is lifted from the metal surface and liquid like water appears in the space separating the bilayer from the metal.
The electrochemical impedance spectroscopy (EIS) and polarization-modulation infrared reflection absorption spectroscopy (PM-IRRAS) techniques were employed to study the ionophore properties of ...valinomycin in a model floating bilayer lipid membrane (fBLM) in perchlorate supporting electrolytes with potassium and sodium cations. Valinomycin decreases the membrane resistance of the fBLM in KClO
4
solution by about 180 times, while it has a negligible effect on the membrane resistivity in NaClO
4
solution. The IR spectra indicate that valinomycin forms a complex with K
+
, but not with Na
+
. The valinomycin-K
+
complex adopts a small tilt angle with respect to the electrode surface normal and is well interdigitated between the acyl chains of the bilayer. The EIS and PM-IRRAS results indicate that valinomycin forms complexes with K
+
and transports K
+
across the lipid bilayer. This transport is potential independent and hence has a passive character.
Combined Langmuir-Blodgett vertical withdrawing and Langmuir-Schaefer horizontal touch (LB-LS) methods were employed to transfer DMPC bilayers onto a Au(111) electrode surface. Charge density ...measurements and photon polarization modulation infrared reflection absorption spectroscopy were employed to investigate electric field induced changes in the structure of the bilayer. The results show that the physical state and the molecular arrangement found in the monolayer at the air-water interface is to a large extent preserved in the bilayer formed by the LB-LS method. This approach provides an opportunity to produce supported bilayers with a well-designed architecture. The properties of the bilayer formed by the LB-LS method were compared to the properties of the bilayer produced by spontaneous fusion of unilamellar vesicles investigated in an earlier study (Bin, X.; Zawisza, I.; Lipkowski, J. Langmuir 2005, 21, 330-347). The tilt angles of the acyl chains are much smaller in the bilayer formed by the LB-LS method and are closer to the angles observed for vesicles and stacked hydrated bilayers. The tilt angles of the phosphate and choline groups are also smaller and are characteristic of an orientation in which the area per DMPC molecule is small. The electric field induced changes of these angles are also less pronounced in the bilayer formed by the LB-LS method. We have shown that these differences are a result of the higher packing density of the phospholipid molecules in the bilayer formed by the LB-LS method.
Cu-based nanomaterials have been widely considered to be promising electrocatalysts for the direct conversion of CO2 to high-value hydrocarbons. However, poor selectivity and slow kinetics have ...hindered the use of Cu-based catalysts for large-scale industrial applications. In this work, we report on a tunable Cu-based synthesis strategy using a dynamic hydrogen bubble template (DHBT) coupled with a sputtered Ag thin film for the electrochemical reduction of CO2 to ethanol. Remarkably, the introduction of Ag into the base of the three-dimensional (3D) Cu nanostructure induced changes in the CO2 reduction reaction (CO2RR) pathway, which resulted in the generation of ethanol with high Faradaic Efficiency (FE). This observation was further investigated through Tafel and electrochemical impedance spectroscopic analyses. The rational design of the electrocatalyst was shown to promote the spillover of formed CO intermediates from the Ag sites to the 3D porous Cu nanostructure for further reduction to C2 products. Finally, challenges toward the development of multi-metallic electrocatalysts for the direct catalysis of CO2 to hydrocarbons were elucidated, and future perspectives were highlighted.
This article describes efforts to build a model biological membrane at a surface of a gold electrode. In this architecture, the membrane may be exposed to static electric fields on the order of 10(7) ...to 10(8) V m(-1). These fields are comparable in magnitude to the static electric field acting on a natural biological membrane. The field may be conveniently used to manipulate organic molecules within the membrane. By turning a knob on the control instrument one can deposit or lift the membrane from the gold surface. Electrochemical techniques can be used to control the physical state of the film while the infrared reflection absorption spectroscopy (IRRAS), surface imaging by STM and AFM and neutron scattering techniques can be employed to study conformational changes of organic molecules and their ordering within the membrane. This is shown on examples of membranes built of a simple zwitterionic phospholipid such as 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and a mixed membrane composed of DMPC and cholesterol. The results illustrate the tremendous effect of cholesterol on the membrane structure. Two methods of membrane deposition at the electrode surface, namely by unilamellar vesicles fusion and using the Langmuir-Blodgett technique, are compared. Applications of these model systems to study interactions of small antibiotic peptides with lipids are discussed.
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