Heterogeneous Rotational Diffusion in Lipid Monolayers

We have characterized the temperature and pressure dependent scaling of dynamic heterogeneity in a homogenous liquid phase of a lipid monolayer using timeresolved fluorescence anisotropy (TRFA) microscopy. The rotational correlation time of the lipid probe NBD-PC is measured using fluorescence anisotropy for two lipid species. We measure the rotational diffusion in a monolayer of DPPC which displays a phase transition at room temperature from the liquid-expanded to the liquidcondensed phase. The constant rotational diffusion of the probe throughout the phase transition reflects the measurement of dynamics in only the liquid-expanded phase. We contrast the dynamic changes during this phase coexistence to the continuous density increase observed in DMPC at room temperature. We observe a nonexponential decay of the probe diffusion consistent with heterogeneity of the orientational dynamics; as the free-volume is reduced the diffusion becomes increasingly heterogeneous. Rotational diffusion is highly influenced by the interaction of nearest neighbors. DMPC at lower temperatures is in the coexistence of LE-LC phase and for higher temperature it is in the pure liquid phase. Highly correlated motion results in regions of high-density, low mobility lipids, and lowdensity, high mobility lipids; and are observed as the bimodal distribution of rotational correlation times.