Interrogating the role of liposome size in mediating the dynamics of a chromophore in the acyl chain region of a phospholipid bilayer

We have examined the temperature-dependent reorientation dynamics of perylene imbedded in bilayers of 1,2-dimyristoyl-sn-phosphatidylcholine (DMPC), where the bilayers exist in the form of unilamellar vesicles. Previous work using 100-nm diameter DMPC vesicles has shown that the phase transition from the gel phase to the fluid phase can be detected using the reorientation dynamics of perylene. In this work we explore the vesicle size dependence of the perylene reorientation dynamics in DMPC vesicles. The size of the vesicles is determined by extrusion and the reorientation dynamics of perylene are measured as a function of vesicle size between 100-nm and 5-microm diameter. We find that, while the phase transition for DMPC is seen in smaller vesicles, perylene becomes insensitive to the phase transition for vesicles larger than ca. 800-nm diameter. We also find a discontinuous change in perylene reorientation dynamics with increasing vesicle size, and we consider this result in the context of the location of perylene within the bilayer.