FIGURE 4.

Normalized diffusion coefficient, D_rel = D_lipid(c)/D_lipid (c = 0), of TRITC-DHPE plotted versus the area fraction of obstacles, c = c_tether/(c_tether + c_SOPC). Here c_SOPC is the molar concentration of SOPC in the inner leaflet of the bilayer. The solid curve shows the best fit using the free area model of obstructed diffusion developed by Almeida et al. (1992b). Using a radius per obstacle of R = 4.5 Å and applying Eqs. 6–8, we obtain a coherence length of ξ = 2.3 Å. This small coherence length indicates that tracer molecules (TRITC-DHPE) and obstacles (tethered lipids) mainly interact via hard-core repulsion. The dashed line is included to illustrate how linear the fitted curve is. The error margin of Δc_rel = ±0.01 describes the uncertainty concerning relative changes in c, which determines the linearity of the data curve. The absolute error in c, which is mostly caused by the uncertainty of A_tether, is not shown because it does not influence the linearity of the curve.