Energy Transfer Characteristics of Lipid Bilayer Membranes of Liposomes Examined with Picosecond Time-Resolved Raman...

A number of biochemical reactions proceed inside biomembranes. Since the rate of a chemical reaction is influenced by chemical properties of the surrounding environment, it is important to examine the chemical environment inside the biomembranes. Although the energy transfer characteristics are a basic and important property of a reaction medium, experimental investigation of the thermal conducting capabilities of the biomembranes is a challenging task. We have examined the energy transfer characteristics of lipid bilayer membranes of liposomes, a good model system for the biomembrane, with picosecond time-resolved Raman spectroscopy. The cooling kinetics of the first excited singlet (S₁) state of trans-stilbene solubilized within the lipid bilayer membranes is observed as a peak shift of the 1570 cm^-1 Raman band of S₁ trans-stilbene. The cooling rate constant of S₁ trans-stilbene is obtained in six lipid bilayer membranes formed by phospholipids with different hydrocarbon chains, DSPC, DPPC, DMPC, DLPC, DOPC, and egg-PC. We estimate the thermal diffusivity of the lipid bilayer membranes with a known correlation between the cooling rate constant and the thermal diffusivity of the solvent. The thermal diffusivity estimated for the liquid-crystal-phase lipid bilayer membranes is 8.9 × 10^-8 to 9.4 × 10^-8 m² s^-1, while that for the gel-phase lipid bilayer membranes is 8.4 × 10^-8 to 8.5 × 10^-8 m² s^-1. The difference in thermal diffusivity between the two phases is explained by a one-dimensional diffusion equation of heat.