Lateral diffusivity of lipid analogue excimeric probes in dimyristoylphosphatidylcholine bilayers

Abstract

The lateral mobility of pyrenyl phospholipid probes in dimyristoylphosphatidylcholine (DMPC) vesicles was determined from the dependence of the pyrene monomeric and excimeric fluorescence yields on the molar probe ratio. The analysis of the experimental data makes use of the milling crowd model for two-dimensional diffusivity and the computer simulated random walks of probes in an array of lipids. The fluorescence yields for 1-palmitoyl-2-(1'-pyrenedecanoyl)phosphatidylcholine (py10PC) in DMPC bilayers are well fitted by the model both below and above the fluid-gel phase transition temperature (Tc) and permit the evaluation of the probe diffusion rate (f), which is the frequency with which probes take random steps of length L, the host membrane lipid-lipid spacing. The lateral diffusion coefficient is then obtained from the relationship D = fL2/4. In passing through the fluid-gel phase transition of DMPC (Tc = 24 degrees C), the lateral mobility of py10PC determined in this way decrease only moderately, while D measured by fluorescence photobleaching recovery (FPR) experiments is lowered by two or more orders of magnitude in gel phase. This difference in gel phase diffusivities is discussed and considered to be related either to (a) the diffusion length in FPR experiments being about a micrometer or over 100 times greater than that of excimeric probes (approximately 1 nm), or (b) to nonrandomicity in the distribution of the pyrenyl probes in gel phase DMPC. At 35 degrees C, in fluid DMPC vesicles, the diffusion rate is f = 1.8 x 10(8) s-1, corresponding to D = 29 microns2 s-1, which is about three times larger than the value obtained in FPR experiments. The activation energy for lateral diffusion in fluid DMPC was determined to be 8.0 kcal/mol.

Packing of phospholipid vesicles studied by oxygen quenching of Laurdan fluorescence

“…Our results are in line with the FRAP observations and renders the discussion by Sassaroli et al [14] regarding the reasons for the values of the diffusion coefficients obtained using pyrene excimer formation and FRAP technique quite questionable. This discussion is based on the difference between microscopic and macroscopic diffusion, but oxygen diffusion is microscopic in nature.…”

“…The overall quenching effect of the oxygen depends on its local concentration, the diffusion constant in the medium, and the lifetime of the fluorescent molecule. To compare the relative quenching effect on the gel with respect to the liquidcrystalline, we should know the relative solubility of be approximately 18 x 10 -8 cm2/s for pyrene-PC in the liquid-crystalline phase of DMPC at 25 ~ and 9 x 10 -8 cm2/s in the gel phase of DMPC at 15 ~ [14]. Although the absolute value of the diffusion constant of oxygen and pyrene in the two phases cannot be compared due to the different sizes of the molecules, the relative value in the two phases should be similar, Deviations from Stokes-Einstein viscosity relationship has been observed previously for oxygen [16] but this deviation should tend to decrease the difference between the two phases.…”

Modulation and Dynamics of Phase Properties in Phospholipid Mixtures Detected by Laurdan Fluorescence*

“…This model assumes that a Laurdan molecule can diffuse a few Angstroms during its excited-state lifetime. The diffusion coefficients of pyrene in phospholipid vesicles have been recently reported by Vauhkonen et aL2' and by Sassaroli et al 22 to be different in the two phases, being on the order of 4 x 1 O--* cm2/s and of 0.8 x lo-* cm2/s in the liquid-crystalline and gel phases, respectively. Using the Smolukowsky-Einstein relationship and assuming a similar diffusion coefficient for Laurdan in the two dimensions, a motion of about 6-10 A in the membrane plane is possible during approximately 10 ns in the liquid-crystalline phase, whereas no motion should occur in the gel phase.…”

NMR-based molecular ruler for determining the depth of intercalants within the lipid bilayer

“…All 2D experiments (COSY, HMQC, HMBC, and NOESY) were run and processed by Bruker software. NMR spectra were generally run at 25 AE 1 C except in the case of DMPC liposomes, which were run at physiological temperature 37 AE 1 C, above the phase transition temperature (T m ) of DMPC which is 23.6 C (Sassaroli et al, 1990). 1 H and 13 C NMR chemical shifts are expressed in d (ppm) relative to TMS (0 ppm).…”