Consequences of Transient Heating on the Motional Dynamics of Cholesterol-Containing Phospholipid Vesicles

We have investigated the effects of transient heating on the molecular reorientation dynamics of perylene incorporated into the nonpolar acyl chain region of 100 nm diameter unilamellar vesicles comprised of DMPC and cholesterol as a function of system temperature. We observe distinctly different reorientation behavior for perylene, depending upon whether it is excited to its S(1) or S(2) electronic state. This difference is due to the dissipation of excess excitation energy through nonradiative relaxation from the S(2) to the S(1) manifold. The thermal energy deposited into the lipid bath following S(2)<- S(0) excitation results in faster rotational diffusion than that observed for excitation to the S(1) state. These findings are consistent with an average temperature increase of ca. 7 K for system temperatures <= 278 K and ca. 13 K for system temperatures >278 K. We believe the variation in average temperature change is a result of a lipid phase transition occurring in the vicinity of 278 K.