Phospholipid bilayers at the mercury (Hg)/water interface

Abstract

This study reports on the electrochemical characterisation of dioleoyl phospatidylcholine (DOPC) bilayer structures on a negatively polarised mercury (Hg) electrode. The bilayers are stable on the Hg surface between-1.0 and-1.3 V applied potential. The experimental approaches were:-(i) Rapid cyclic voltammetry to "fingerprint" the bilayers, (ii) Potential step experiments to record Zn2+ reduction and, (ii) Electrochemical impedance. The results show the following. Both the specific capacitance (5 microF cm-2) and the specific resistance of the bilayer are higher and lower respectively than that of a defect-free free standing DOPC bilayer. This indicates the presence of water and ions in the bilayer within an applied negative field. The bilayer's resistance to electrolyte movement decreases with increase in negative potential to a minimum at-1.3 V. The DOPC bilayer is less permeable to Zn2+ ions compared to the DOPC monolayer coated electrode at applied negative potentials and its permeability to Zn2+ increases with an increase in negative applied potential. The specific capacitance of the bilayer increases to about 7.5 microF cm-2 with increase in applied negative potential showing the increasing significance of water in the bilayer commensurate with its increased permeability to ions. Adsorption of SiO2 nanoparticles on the bilayer surface causes a step negative potential shift in the anodic capacitance current bilayer reformation peak indicating an acceleration of the bilayer reformation process.

Influence of the Lipid Backbone on Electrochemical Phase Behavior

“…The hysteresis between the negative-going and positive-going sweeps may be a result of slow kinetics of adsorption and desorption, particularly if there are strong intermolecular interactions. The minimum value of specific capacitance in the positive-going sweep is 6.5 μF cm –2 and in the negative sweep it is 5.7 μF cm –2 , at approximately −0.1 V. These values are similar to those measured for DPPC on Au(111) in our earlier study (8 and 5 μF cm –2 ) and DOPC bilayers on Hg (5 μF cm –2 ) . Using eq , it is possible to estimate a coverage of the surface with SM, θ: θ = C 0 − C C 0 − C 1 where C 0 is the capacitance of the clean surface, C 1 is the capacitance of a perfect bilayer, and C is the measured capacitance.…”

“…The minimum value of specific capacitance in the positive-going sweep is 6.5 μF cm −2 and in the negative sweep it is 5.7 μF cm −2 , at approximately −0.1 V. These values are similar to those measured for DPPC on Au(111) in our earlier study (8 and 5 μF cm −2 ) 56 and DOPC bilayers on Hg (5 μF cm −2 ). 76 Using eq 2, it is possible to estimate a coverage of the surface with SM, θ:…”

Effect of Molecular Structure on Electrochemical Phase Behavior of Phospholipid Bilayers on Au(111)

“…Nelson and co-workers were able to correlate DOPC film thickness on Hg with capacitance to demonstrate a phase transition between a complete monolayer and a partial bilayer on Hg. 40 In another study, Lipkowski and co-workers showed with AFM data that the thickness of DMPC bilayers was lower in the potential region of lowest capacitance and concluded that the changes in capacitance with potential were mainly related to changes in overall permittivity. 51 A similar conclusion has been drawn for DMPS bilayers, which have higher capacity but similar chain tilt angle (and therefore similar bilayer thickness) to DMPE bilayers.…”

“…In the case of capacity data, the effects on capacity from film thickness and from average permittivity cannot be separated so a separate structural method is needed to confirm the interpretation of changes or differences in capacity. Nelson and co-workers were able to correlate DOPC film thickness on Hg with capacitance to demonstrate a phase transition between a complete monolayer and a partial bilayer on Hg . In another study, Lipkowski and co-workers showed with AFM data that the thickness of DMPC bilayers was lower in the potential region of lowest capacitance and concluded that the changes in capacitance with potential were mainly related to changes in overall permittivity .…”

“…Changes in the field across the membrane can result in changes in structure, for example, the reorientation of dipoles or even membrane breakdown at high fields, so the study of these changes is essential for understanding the function of the membrane. Electrochemical measurements provide a means to tune an electric field continuously over a similar range to that found in nature , and have demonstrated the effect of this field on monolayers for various lipids on mercury. − Further in situ structural studies, utilizing techniques such as atomic force microscopy, − scanning tunneling microscopy, , vibrational spectroscopy, ,,,− and neutron reflectivity ,, to investigate bilayers supported on or floating on gold electrodes, have shown changes in ensemble structure as the applied field is varied. Some studies have highlighted the role of lipid molecule structure on ensemble structure and response to the applied field; ,, others have focused on the interaction of peptides and proteins with a lipid bilayer and how charge controls their insertion. ,,− …”