(459c) Visualizing Phospholipid-Cholesterol Monolayer Rheology

We present systematic measurements of the surface rheology of monolayers of liquid-condensed(LC) dipalmitoylphosphatidylcholine (DPPC)/cholesterol mixtures at the air/water interface. Using microfabricated, ferromagnetic ‘microbuttons’ as new microrheological probes, we measure the linear viscoelastic moduli of DPPC/cholesterol monolayers as both surface pressure and frequency are varied as a function of cholesterol concentration. Visualization of this interface using fluorescence microscopy reveals that the interlocked, spiral liquid crystalline domains that comprise an LC-DPPC monolayer expand and elongate on addition of cholesterol, and are separated by cholesterol-rich fluid domains, suggesting that cholesterol is line-active.  In addition, the monolayer domains  give rise to a visco-elastic solid response, analogous to a two-dimensional emulsion, and the measured viscoelastic moduli qualitatively agree with predictions based on a 2D concentrated emulsion model.   Surprisingly, 1 - 2 wt% cholesterol decreases the surface viscosity of DPPC monolayers by orders of magnitude; we have correlated this decrease with a log-additive mixing rule, similar to polymer blends, with the cholesterol-rich phase having a very low viscosity and the LC DPPC phases being very viscous.  Atomic Force Microscopy imaging was used to verify the structure of the mixed monolayer at higher surface pressures to show that the spiral domain structure persisted.  These rheological observations have important implications for replacement human lung surfactants in which the role and even presence of cholesterol is hotly debated.