Electrochemical and PM-IRRAS studies of potential controlled transformations of phospholipid layers on Au(111) electrodes

Abstract

Chronocoulometry and photon polarisation modulation infrared reflection absorption spectroscopy (PM-IRRAS) have been employed to study the fusion of dimyristoylphosphatidylcholine (DMPC) vesicles onto a Au(111) electrode surface. The results show that fusion of the vesicles is controlled by the electrode potential or charge at the electrode surface (s M ). At charge densities of À15 mC cm À2 < s M < 0 mC cm À2 , DMPC vesicles fuse to form a condensed film. When s M < À15 mC cm À2 , de-wetting of the film from the electrode surface occurs. The film is detached from the electrode surface; however, phospholipid molecules remain in its close proximity in an ad-vesicle state. The state of the film can be conveniently changed by adjustment of the potential applied to the gold electrode. PM-IRRAS experiments demonstrated that the potential-controlled transitions between various DMPC states proceed without conformational changes and changes in the packing of the acyl chains of DMPC molecules. However, a remarkable change in the tilt angle of the acyl chains with respect to the surface normal occurs when ad-vesicles spread to form a film at the gold surface. When the bilayer is formed at the gold surface, the acyl chains of DMPC molecules are significantly tilted. The IR spectra have also demonstrated a pronounced change in the hydration of the polar head region that accompanies the spreading of ad-vesicles into the film. For the film deposited at the electrode surface, the infrared results showed that the temperature-controlled phase transition from the gel state to the liquid crystalline state occurs within the same temperature range as that observed for aqueous solutions of vesicles. The results presented in this work show that PM-FTIR spectroscopy, in combination with electrochemical techniques, is an extremely powerful tool for the study of the structure of model membrane systems at electrode surfaces.

Application of Polarization Modulation Infrared Reflection Absorption Spectrosocpy Under Electrochemical Control for Structural Studies of Biomimetic Assemblies

“…In this potential-dependent study of the adsorption process of cyanide ions on a silver electrode showed differences between the species adsorbed directly on the electrode surface and those dissolved in the electrolyte solution [82]. Later, structural aspects of the potential-driven adsorption-desorption process of amphiphilic organic films adsorbed on the gold electrode surface were investigated in detail [29,30,32,51].…”

“…The beam convergence was set to ± 5%; comparable with the beam convergence in commercially available IR spectrometers. The wavenumber of the incoming IR radiation was set to 2900 cm −1 ( = 3.44 μm), corresponding to absorption modes of CH groups, the most often analyzed spectral region in organic molecules adsorbed on solid surfaces [12,32,51,[58][59][60][61]. Figure 2 shows the three-dimensional plot of the MSEFS as a function of the angle of incidence and electrolyte layer thickness.…”

“…However, most lipid and protein molecules contain a large number of these groups making assignments to a particular molecular structure difficult. One approach is the use of single component models [12,32,34,110,114]. However, multicomponent systems can be investigated after the isotopic substitution of the compound of interest [59,66,86,115] For example a selectively per-deuterated hexaethylene glycol-terminated alkane thiol was synthesized to distinguish methylene groups of two parts of the molecule in IR spectroscopy [116].…”

“…Vesicles spreading and combined Langmuir-Blodgett and Langmuir-Schaefer (LB-LS) transfer are predominantly used to fabricate lipid bilayers on electrode surfaces [8, 9, 31-34, 88, 118, 124]. Numerous electrochemical, in situ PM IRRAS and AFM studies show clearly that pure phospholipid bilayers prepared on the Au electrode surface by vesicles spreading have poorly defined structure [8,9,32,120,125]. Defects are present in these bilayers and the hydrocarbon chains at phospholipid molecules adapt a large tilt with respect to the electrode surface [8,120].…”

“…In this paper a brief introduction to IRRAS under electrochemical control (Section 2), limitations of IRRAS and possibilities of the extension of the application of the in situ IRRAS through modifications of the surface properties of the solid substrate, a selection of the optical window and electrolyte solution for studies of biomimetic films are discussed (Section 3). In the fourth section the application of in situ IRRAS for structural studies of potential-driven changes in lipid bilayers [9,[30][31][32][33][34][35][36] and protein films [37,38] is reviewed. Recent studies of the impact of lipid-protein interactions on the structure and stability of the membrane exposed to physiological electric fields are summarized and described [39][40][41][42].…”