Pressure effects on submicrosecond phospholipid dynamics using a long-lived fluorescence probe

The effects of applied external hydrostatic pressure on submicrosecond lipi d motions in DPPC4 bilayers have been examined using coronene (a long-lived planar fluorescent molecule) and DPH. Steady-state fluorescence emission a nisotropy (EA) values ([r]) obtained for probe-labeled DPPC SUVs measured a t different fixed temperatures above T-c as a function of increasing hydros tatic pressure reveal pressure-induced lipid phase transition profiles. For coronene-labeled samples, the observed lipid "melt" profiles are broad and shifted to higher midpoint EA pressure values (P-1/2) compared with corres ponding DPH-labeled SUVs at the same temperature. The data suggest lipid mo tions occurring on the submicrosecond time scale, detected only by using a long-lived fluorescence probe, which occur well above the normally reported "fluid-gel" lipid phase transition. Slopes of the pressure-to-temperature equivalence plots (dP(1/2)/dT = 39 bar/K) obtained for DPH-or coronene-labe led DPPC SUVs are identical within experimental error and reflect probe ind ependence. For DPH, the slope of the P-1/2(T) plot provides the expected ph ase transition phospholipid Volume change. However, intercept values (at P- 1/2 = 1 bar) or apparent phase transition temperatures obtained from the eq uivalence plots for the two probes are not equal. Differences appear to ari se due to the very disparate fluorescence lifetime values of the two probes , which result in rotational sensitivity of coronene to gel lipid volume fl uctuations occurring during the extended time window provided by coronene f luorescence.