Structural alterations of phospholipid film domain morphology induced by cholesterol

Structures of the monolayer films of dipalmitoylphosphatidylcholine (DPPC) mixed with different amounts of cholesterol were studied at air-water interface using surface pressure-area measurements, epifluorescence microscopy and atomic force microscopy (AFM). Pure DPPC, cholesterol or DPPC-cholesterol mixtures were dissolved in organic solvents with a small amount of fluorescently labeled phospholipid probe (NBD-PC) and spread onto the air-water interface. Surface pressure-area isotherms and epifluorescence microscopy of such films at the air-water interface suggested that DPPC undergoes a gas to fluid to condensed phase transition, while cholesterol undergoes a gas to solid-like transition. A shift of the surface pressure-area curve to lower area per molecule was observed when cholesterol was mixed with DPPC. Epifluorescence microscopy showed the formation of spiral shaped domains for mixed monolayers. Increase in cholesterol content abolished domain characteristics possibly due to fluidizing property of cholesterol. AFM measurements of monolayers, transferred onto freshly cleaved mica by Langmuir-Blodgett technique, revealed the alterations caused by cholesterol on the gel and fluid domains of such films. AFM measurements re-established similar trend in domain characteristics as evidenced in epifluorescence microscopy.