Lipid Bilayer Membrane of Egg-PC Liposome Evaluated as Chemical Reaction Field with Picosecond Time-Resolved Fluorescence Spectroscopy

Viscosity inside the lipid bilayer of egg-PC (egg yolk phosphatidylcholine) liposome with a diameter of 100 nm is examined with picosecond time-resolved fluorescence spectroscopy. The viscosity is estimated independently from the photoisomerization rate and from the rotational relaxation time of the first excited singlet state of trans-stilbene solubilized within the lipid bilayer. The presence of two solvation environments within the bilayer is suggested from both of the methods. One environment is 50 to 100 times more viscous than the other. The use of trans-stilbene as a probe provides valuable information on the solvation environments inside the lipid bilayer of liposome, which serves as a field for a number of biochemical reactions.