Time-resolved fluorescence photophysics of trans-stilbene in a DPPC lipid bilayer: Evidence for a free rotation, location within two sites and a pre-liquid crystalline phase transition

The photophysical properties of the first two members of the diphenylpolyene series are reported in a phospholipid bilayer membrane using time-resolved fluorescence anisotropy. A contrasting photophysical behaviour is noted for the time-resolved fluorescence anisotropy where a free rotation with a long rotational correlation time of similar to 10 ns for trans-stilbene in the gel phase of the bilayer at 35 degrees C is in sharp contrast to the restricted motion observed for trans,trans-diphenylbutadiene (DPB) and other diphenyl-polyenes. For trans-stilbene bi-exponential fluorescence decay components in the gel phase (e.g. 0.40 ns and 1.12 ns at 30 degrees C) indicate two sites within the bilayer. The mean rotational correlation time and fluorescence decay components are found to decrease with increasing temperature prior to the main phase transition, Together these provide valuable information on the changing ''microviscosity'' of a lipid bilayer membrane in the pre-transition region leading to the melting of the main chain. Traditional fluorescence probes offer no information on this region.