Abstract
Lipid dynamics plays important roles in the maintenance of homeostasis, and understanding and control of these lipid dynamics is a key challenge in biophysics and cell biology. Time-resolved small-angle neutron scattering (TR-SANS) is a powerful technique to determine the rates of intervesicular exchange and flip-flop of lipids in situ and real-time. Time-resolved fluorescence anisotropy determines the order parameter of membrane lipids. Combination of these time-resolved methods will clarify the relationship between static structure and dynamic property of lipid membranes. In this study, we applied these techniques to large (LUVs) and small unilamellar vesicles (SUVs) consisting of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) or 1,2-dimyristoylphosphatidylcholine (DMPC) to elucidate the effect of curvature on the structure and dynamics of lipid membranes.
Time-resolved fluorescence anisotropy revealed that the order parameter (S) of DPH-PC is larger in SUVs than in LUVs. Moreover, TMAP-DPH represented significantly large S value when the probe was located at outer leaflet of SUVs, suggesting the acyl chain ordering.
TR-SANS showed that the rates of the intervesicular exchange and flip-flop of DMPC are higher in SUVs than in LUVs. In addition, both the activation enthalpy and entropy of the intervesicular exchange and flip-flop of DMPC increased in SUVs. These results suggest that the increase in the curvature induces tighter packing and reduces the enthalpy and entropy of lipids in membranes, which facilitates the intervesicular exchange and flip-flop of lipids by making these processes entropically less unfavorable.
