Electrochemical impedance spectroscopy and fluorescence lifetime imaging of lipid mixtures self-assembled on mercury

Monolayers and bilayers of lipid mixtures self-assembled on mercury form spontaneously gel-phase (solid ordered, s(o)) and liquid-ordered (l(o)) microdomains, thanks to the fluidity imparted to these films by the liquid metal support. The differential capacity of the hydrocarbon tail region of monolayers of mixtures of two lipid components of high and low transition temperature T-m, increases during the transition from the liquid disordered (l(d)) phase to the coexistence of l(d) and s(o) phases. Addition of cholesterol to this binary mixture causes a decrease in differential capacity. This behavior is explained by regarding the capacity as a measure of the total perimeter of the s(o) microdomains, due to the mismatch between these microdomains and the l(d) phase. Cholesterol removes this mismatch by converting the anisotropic s(o) microdomains into isotropic l(o) microdomains (rafts). This allows differential capacity measurements by electrochemical impedance spectroscopy to follow phase transitions in lipid mixtures. The coexistance of l(d), l(o) and s(o) phases is confirmed by images of a distal lipid monolayer self-assembled on top of a thiolipid monolayer tethered to a mercury microcap, by using two-photon fluorescence lifetime imaging microscopy (2P-FLIM).