The values were approximately 0.4 × 10 -7 cm 2 /s to 1.4 × 10 -7 cm 2 /s depending on the Ca 2+ concentration (Fig. ). As a comparison, previous experimental measurements from luorescence recovery after photobleaching, pulsed NMR, single particle tracking, and luorescence correlation spectroscopy yielded the similar magnitude of the diffusion coeficients of approximately 0.5 × 10 -7 cm 2 /s to 1.0 × 10 -7 cm 2 /s. Interestingly, the presence of Ca 2+ signiicantly reduced the mobility of lipid molecules in the mixed POPC/POPS bilayers, but not to the same extent in the pure POPC bilayers (Fig. ). Therefore, our results showed that membranes containing negatively charged lipids can become signiicantly less diffusive in the presence of Ca 2+ . Again, the two Ca 2+ models did not show signiicant differences in affecting the lipid diffusion coeficient.