Phase-Dependent Lateral Diffusion of α-Tocopherol in DPPC Liposomes Monitored by Fluorescence Quenching

The temperature-dependent fluorescence quenching of an amphiphilic palmitoyl derivative of 2,3-diazabicyclo[2.2.2]oct-2-ene (Fluorazophore-L) by alpha-tocopherol (alpha-Toc) has been determined in liposomes composed of a saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) The mutual lateral diffusion coefficients (D-L) were extracted according to a laterally diffusion-controlled dynamic quenching model Three distinct temperature regimes were identified One between 65 and 39 degrees C. whet e the lateral diffusion coefficients were in the range of 10(-7)cm(-2) s(-1) and the lifetime of the probe was monoexponential in the absence of alpha-Toc, a second one between 39 and 30 degrees C, where the lateral diffusion coefficients were in the range of 10(-8) s(-1) and the lifetime of the probe was biexponential in the absence oirk-Toc, and a third one below 30 degrees C. in which no diffusion was detectable, suggesting DI. < 10 cm-s(-1) These temperature domains were assigned, suppoited by differential scanning calortmetry (DSC) measurements, to the liquid-crystalline, ripple, and solid-gel phases of DPPC liposomes in the pi esence of the two additives The absolute values of the individual lateral diffusion coefficients (taken as 1/2 of the D-L values) of the Fluorazophore-L/alpha-Toc (ca. 25 x 10 s(-1) at 52 degrees C) couple demonstrates that a-Toe does not diffuse at an unexpectedly high rate in comparison to the self-diffusion of DPPC (15 x 10(-7) cm(2) s(-1) at 52 degrees C) 1-lowevei, diffusion in DPPC hposomes is distinctly slower than that in POPC ones (e.g, D-L = 4.9 x 10(-7) cm(2)s(-1) versus 6 4 x 10(-7) cm(2) s(-1) at 50 degrees C), with an activation energy of 49 5 kJ mol(-1) (value for POPC 47 5 kJ mol(-1)). in the temperature range of the liquid-clystalline phase Diffusion in the ripple phase, that is, below the main phase transition temperature, was found to be non-negligible, with an apparent activation energy of 175 +/- 50 kJ mol(-1)