Spectroelectrochemical studies of bilayers of phospholipids in gel and liquid state on Au(111) electrode surface

Differential capacity, chronocoulometry and Polarization Modulation Fourier Transform Infrared Reflection Absorption Spectroscopy (PM FTIRRAS) were employed to investigate spreading of small unilamellar vesicles (SUVs) of DOPC and DMPC onto a Au(111) electrode surface. The electrochemical experiments demonstrated that vesicles fuse onto the electrode surface and at E>−0.5V (SSCE) or at charge densities −10<σM<5 μC cm−2 to form defected bilayers, directly in contact with the metal surface. The analysis of PM FTIRRA spectra in the CH stretching region showed that the DOPC bilayer is in the liquid crystalline state while the bilayer of DMPC is in the gel or in the ripple phase. The spectra were also used to determine the tilt of the acyl chains in the bilayer of DMPC as a function of the electrode potential. At E<−0.5 V (SSCE), where the bilayer is detached from the metal surface, the chains are tilted at ∼29° with respect to surface normal. When the bilayer is adsorbed at the metal surface at E>−0.5 V (SSCE), the tilt angle increases to ∼42°. The increase of the tilt angle is discussed in terms of a change in the packing of the polar head of the phospholipids molecules in the bilayer adsorbed at the electrode surface.