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.