Free Brownian motion of individual lipid molecules in biomembranes

The mobility of phospolipids in free-standing and supported membranes was i nvestigated on the level of individual molecules. For the analysis of traje ctories a new statistical treatment was developed that permitted us to clea rly distinguish different types of diffusional motion. A freely diffusing s ubfraction of lipids within supported membranes was identified. Its mobilit y was characterized by a mean lateral diffusion constant of D-supp = 4.6 mu m(2)/s. In comparison, the mobility of lipids embedded in "free-standing" planar membranes yielded an increase in the mean diffusion constant by a fa ctor of 4.5, D-free = 20.6 mu m(2)/s. This increase is attributed to the ul trathin (less than or equal to 1 nm) lubricating water layer between membra nes and glass support.