Single molecules as probes of lipid membrane microenvironments

The single-molecule dynamics of the membrane probe diIC(18) dispersed in we ll-controlled lipid environments were studied. Lipid monolayers and bilayer s of DPPC, transferred onto a solid substrate using the Langmuir-Blodgett t echnique, were utilized to systematically control the environment around si ngle diIC(18) molecules. The single-molecule emission trajectories revealed large intensity fluctuations that were strongly coupled to the lipid envir onment surrounding the probe molecule. For example, as the surface pressure of a DPPC monolayer was increased from the liquid-expanded/liquid-condense d (pi = 5 mN/m) region to the solid condensed (pi = 30 mN/m) region, the ch aracteristic fluorescence fluctuation times increased from approximately 44 0 ms to over 1 s. For bilayer films, we found characteristic fluctuation ti mes on the order of 2 s, regardless of which side of the bilayer the probe molecule resided. The monolayer and bilayer results are most consistent wit h a mechanism for intensity fluctuations driven by small twisting motions i n the diIC(18) probe molecule that modify its emission properties. Comparis on of the measured time scales with results from NMR studies suggest that t he observed single-molecule dynamics are associated with director fluctuati ons arising from collective motions of the lipid tailgroups. These results clearly reveal an environmental dependence in the single-molecule emission trajectories that can provide a new tool for studying membrane microenviron ments and dynamics.