Dynamics imaging of lipid phases and lipid-marker interactions in model biomembranes.

Biomembranes are complex systems that regulate numerous biological processes. Lipid phases that constitute these membranes influence their properties and transport characteristics. Here, we demonstrate the potential of short-range dynamics imaging (excited-state lifetime, rotational diffusion, and order parameter) as a sensitive probe of lipid phases in giant unilamellar vesicles (GUVs). Liquid-disordered and gel phases were labeled with Bodipy-PC at room temperature. Two-photon fluorescence lifetime imaging microscopy of single-phase GUVs reveals more heterogeneity in fluorescence lifetimes of Bodipy in the gel phase (DPPC: 3.8+/-0.6 ns) as compared with the fluid phase (DOPC: 5.2+/-0.2 ns). The phase-specificity of excited-state lifetime of Bodipy-PC is attributed to the stacking of ordered lipid molecules that possibly enhances homo-FRET. Fluorescence polarization anisotropy imaging also reveals distinctive molecular order that is phase specific. The results are compared with DiI-C12-labeled fluid GUVs to investigate the sensitivity of our fluorescence dynamics assay to different lipid-marker interactions. Our results provide a molecular perspective of lipid phase dynamics and the nature of their microenvironments that will ultimately help our understanding of the structure-function relationship of biomembranes in vivo. Furthermore, these ultrafast excited-state dynamics will be used for molecular dynamics simulation of lipid-lipid, lipid-marker and lipid-protein interactions.