  The cell membrane is inherently asymmetric and heterogeneous in its
composition, a feature that is crucial for its function. Using atomistic
molecular dynamics simulations, the physical properties of a 3-component
asymmetric mixed lipid bilayer system comprising of an unsaturated POPC
(palmitoyl-oleoyl-phosphatidyl-choline), a saturated SM (sphingomyelin) and
cholesterol are investigated. In these simulations, the initial stages of
liquid ordered, $l_o$, domain formation are observed and such domains are found
to be highly enriched in cholesterol and SM. The current simulations also
suggest that the cholesterol molecules may partition into these SM-dominated
regions in the ratio of $3:1$ when compared to POPC-dominated regions. SM
molecules exhibit a measurable tilt and long range tilt correlations are
observed within the $l_o$ domain as a consequence of the asymmetry of the
bilayer, with implications to local membrane deformation and budding. Tagged
particle diffusion for SM and cholesterol molecules, which reflects spatial
variations in the physical environment encountered by the tagged particle, is
computed and compared with recent experimental results obtained from high
resolution microscopy.
