  The molecular arrangement of lipids and proteins within biomembranes and
monolayers gives rise to complex film morphologies as well as regions of
distinct electrical surface potential, topographical and electrostatic
nanoscale domains. To probe these nanodomains in soft matter is a challenging
task both experimentally and theoretically. This work addresses the effects of
cholesterol, lipid composition, lipid charge, and lipid phase on the monolayer
structure and the electrical surface potential distribution. Atomic Force
Microscopy (AFM) was used to resolve topographical nanodomains and Kelvin Probe
Force Microscopy (KPFM) to resolve electrical surface potential of these
nanodomains in lipid monolayers. Model monolayers composed of
dipalmitoylphosphatidylcholine (DPPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine
(DOPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC),
1,2-dioleoyl-sn-glycero-3-[phospho-rac-(3-lysyl(1-glycerol))] (DOPG),
sphingomyelin, and cholesterol were studied. It is shown that cholesterol
changes nanoscale domain formation, affecting both topography and electrical
surface potential. The molecular basis for differences in electrical surface
potential was addressed with atomistic molecular dynamics (MD). MD simulations
qualitatively match the experimental results, with 100s of mV difference in
electrostatic potential between liquid-disordered bilayer (Ld, less cholesterol
and lower chain order) and a liquid-ordered bilayer (Lo, more cholesterol and
higher chain order). Importantly, the difference in electrostatic properties
between Lo and Ld phases suggests a new mechanism by which membrane composition
couples to membrane function.
