Fluorescence depolarization study on non-bilayer phases of phosphatidylethanolamine and phosphatidylcholine lipid mixtures

Study of plasma membrane heterogeneity using a phosphatidylcholine derivative of 1,6-diphenyl-1,3,5-hexatriene [2-(3-(diphenylhexatriene)propanoyl)-3-palmitoyl-L-α-phosphatidylcholine]

The fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH) and DPH derivatives have been used to characterize structural and physicochemical properties of specific membrane domains. Steady-state and fluorescence decay measurements of three probes, DPH (1,6-diphenyl-1,3,5-hexatriene), TMA-DPH [1-(4-trimethyl-ammonium-phenyl)-6-phenyl-1,3,5-hexatriene], and a phosphatidylcholine derivative of DPH, DPH-pPC [2-(3-(diphenylhexatriene)propanoyl)-3-pamitoyl-L-α-phosphatidyl choline], have been performed in erythrocyte membranes and in lymphocyte plasma membranes. The steady-state fluorescence polarization of the three probes showed a similar trend in both membranes. In fact either in erythrocyte or in lymphocyte plasma membranes the fluorescence polarization values of DPH-pPC and TMA-DPH were similar, but significantly higher with respect to DPH. A better characterization of erythrocyte and lymphocyte plasma membranes was possible by using fluorescence decay measurements. The data suggest the possible use of different DPH derivatives to characterize specific domains in biological membranes.

Detection and characterization of the onset of bilayer packing defects by nanosecond-resolved intramolecular excimer fluorescence spectroscopy

Bilayer packing defects in binary dilinoleoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylcholine (DLPE/POPC) lipid mixtures have been studied by the use of nanosecond-resolved intramolecular excimer fluorescence spectroscopy. Frequency-domain fluorescence intensity decays of dual-chain labelled dipyrenyl lipids of different chain lengths in DLPE/POPC mixtures were acquired at both the monomer (392 nm) and excimer (475 nm) emission channels and at 20 degrees C. On the basis of a new intramolecular excimer formation kinetic model, the extent of aggregation and the rotational mobility, in terms of the equilibrium constant of the monomer to aggregated state and the excimer association rate constant, respectively, of the intralipid pyrenes were calculated from the frequency-domain data. Within the range of 60-100% DLPE where bilayer defects are known to coexist with bilayer and non-bilayer states, a prominent peak in the equilibrium constant and a concomitant dip in the excimer association constant at approximately 80% DLPE were observed. Our nanosecond-resolved fluorescence results suggest that the intramolecular excimer kinetic parameters of dipyrenyl lipids are very sensitive to the onset of bilayer packing defects in lipid membranes. Moreover, the onset of bilayer defect state is characterized by the greater extent of aggregation and more hindered rotational mobility of the acyl chains as compared with the bilayer (0% DLPE) and non-bilayer inverted hexagonal (100% DLPE) states of the lipid membranes.

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 1. DMPC/cholesterol membranes

The intramolecular dynamics of the excimer forming dipyrenyl lipids (DipynPC) of different chain lengths (n) in ethanol and in dimyristoylphosphatidycholine (DMPC) membranes was investigated by the use of frequency-domain fluorescence intensity decay technique. Based on a 3-state model, the extent of aggregation and rotational rate of the two intralipid pyrene moieties in the dipyrenyl lipids were estimated from the frequency-domain data. In ethanol (20 degrees C), the rotational rate for DipynPC increased progressively as n was varied from 4 to 12. At the gel (L beta)-to-liquid crystalline (L alpha) phase transition of DMPC (approximately 23 degrees C), the rotational rate increased and aggregation decreased significantly for Dipy10PC, whereas only the rotational rate was changed for Dipy4PC. In the presence of 30 mol% cholesterol, significant increases in both the rotational rate and aggregation were observed for Dipy10PC in both L beta and L alpha phases. However, for the case of Dipy4PC, an increase in the rotational rate but a decrease in the aggregation were noticed only in the L beta phase, and no similar changes were detected in the L alpha phase. Our results indicate differential effects of cholesterol on the conformational dynamics of acyl chains at different depths of the membranes.

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 2. DOPE/DOPC membranes

The intramolecular dynamics of the excimer-forming dipyrenyl lipids (DipynPE) of different chain lengths (n) in fully hydrated dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) binary mixtures was investigated by the use of frequency-domain fluorescence intensity dcay technique. Using a 3-state model (see companion paper), the extent of aggregation and rotational rate of the two covalently attached pyrene moieties in DipynPE were estimated from the frequency-domain data. At 1 degrees C, the rotational rate and aggregation for Dipy4PE and Dipy10PE were insensitive to DOPE% of the lipid bilayer. At 27 degrees C, the rotational rate decreased, whereas the aggregation increased steadily for Dipy10PE as the DOPE% of the bilayer increased from 0 to 80. However, an abrupt increase in the rotational rate and a decrease in the aggregation for Dipy10PE were detected as the DOPE% reached 100, at which point the membranes are in the inverted hexagonal (HII) phase. No similar changes were found for Dipy4PE. These results indicate that the presence of PE with large intrinsic-curvature increases the lateral stress at the region near the center of the bilayer, and that this stress can be relieved as the membranes enter the highly curved HII phase.

Infrared study of the bilayer stability behavior of binary and ternary phospholipid mixtures containing unsaturated phosphatidylethanolamine

The bilayer stability of liposomes containing unsaturated phosphatidylethanolamine (PE) has been investigated by measuring the C = O and CH2 stretching frequencies of the lipids at different lipid compositions and temperatures. Binary mixtures of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) and dilinoleoyl-PE (DLPE) are known to exhibit lamellar liquid crystalline (L alpha), inverted hexagonal (HII) and metastable intermediate (I) phases. Abrupt increases in the C = O and CH2 stretching frequencies at 65-75 and 90-95% PE, respectively, were found as the PE content of the DLPE/POPC mixtures was increased from 0 to 100%. These transitions were associated with the L alpha -I and I-HII phase transitions of the DLPE/POPC mixtures, accordingly. The effects of three lipid perturbants, butylated hydroxytoluene (BHT), diacylglycerol (DG) and cholesterol (CL), on the above L alpha -I and I-HII transitions were also examined. All perturbants were found to be effective in shifting the L alpha -I transition of the DLPC/POPC mixtures to a lower PE% as detected by the C = O stretching frequency measurements. On the other hand, the perturbants appeared to eliminate the I-HII transition of the DLPE/POPC mixtures as detected by the CH2 stretching frequency measurements. The effectiveness of the perturbants in promoting the L alpha -I phase transition of the DLPE/POPC mixtures followed the order of DG > BHT > CL.(ABSTRACT TRUNCATED AT 250 WORDS)

Infrared and time-resolved fluorescence spectroscopic studies of the polymorphic phase behavior of phosphatidylethanolamine/diacylglycerol lipid mixtures

Fourier transform infrared (FTIR) and time-resolved fluorescence spectroscopy have been employed to examine the structural dynamics of lipid fatty acyl chains and lipid/water interfacial region of a binary lipid mixture containing unsaturated phosphatidylethanolamine (PE) and diacylglycerol (DG). Infrared vibrational frequencies of the CH2 symmetric stretching and the C = O stretching bands of the lipids were measured at different lipid compositions and temperatures. For 0% DG, the lamellar gel to lamellar liquid crystalline (L beta-L alpha) and the L alpha to inverted hexagonal (L alpha-HII) phase transitions were observed at approximately 15 degrees and 55 degrees C, respectively. As the DG content increased gradually from 0% to 15%, the L alpha-HII phase transition temperature decreased drastically while the L beta-L alpha phase transition temperature decreased only slightly. At 10% DG, a merge of these two phase transitions was noticed at approximately 10 degrees C. For the composition study at 23 degrees C, the L alpha-HII transition occurred at approximately 6-10% DG as indicated by abrupt increases in both the CH2 and C = O stretching frequencies at those DG contents. Using time-resolved fluorescence spectroscopy, abrupt decreases in both the normalized long time residual and the initial slope of the anisotropy decay function of lipid probes, 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5- hexatrienyl)phenyl]ethyl]carbonyl]-3-sn-phosphatidylcholine, in these PE/DG mixtures were observed at the L alpha-HII phase transition. These changes in the anisotropy decay parameters suggested that the rotational dynamics and orientational packing of the lipids were altered at the composition-induced L alpha-HII transition, and agreed with a previous temperature-induced L alpha-HII transition study on pure unsaturated PE (Cheng (1989) Biophys. J. 55, 1025-1031). The fluorescence lifetime of water soluble probes, 8,1-anilinonapthalenes sulfonate acid, in PE/DG mixtures increased abruptly at the L alpha-HII phase transition, suggesting that the conformation and hydration of the lipid/water interfacial region also undergo significant changes at the L alpha-HII transition.

Effects of lateral diffusion on the fluorescence anisotropy in hexagonal lipid phases. II. An experimental study

The polymorphic phase behavior of unsaturated phosphatidylethanolamine (PE)/diacylglycerol (DG) binary lipid mixtures was investigated by the use of time-resolved fluorescence anisotropy. Using a fluorescent lipid, 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5-hexatrienyl)phenylethyl] carbonyl]3-sn-phosphatidyl-choline (DPH-PC), the orientational order and rotational dynamics of the above lipid mixtures in the liquid crystalline lamellar (L alpha) and inverted hexagonal (HII) phases were studied. By employing a one-exponential model (Cheng, K.H. 1989: Biophys. J. 55:1025-1031) to fit the anisotropy decay data, abrupt decreases in the normalized initial anisotropy decay slope and the residual anisotropy of DPH-PC were observed at approximately 6-8% DG, signifying a L alpha/HII phase transition. Using our new theoretical WOBHOP and P2P4HOP models as described in a preceding paper (Van Der Meer, B.W., K.H. Cheng, and S.Y. Chen. 1990. Biophys. J. 58:000-000), two or more rotational correlation times were required to describe the anisotropy decay behavior of DPH-PC in the HII phase. These rotation correlation times were further related to the second and fourth rank order parameters, and the wobbling and hopping diffusion constants of the fluorescent probe in the highly curved lipid cylindrical tubes of the HII phase. The hopping diffusion constant (DH) equals the lateral diffusion constant (DL) divided by R2 (R = radius of the lipid cylindrical tubes). The value of DL was estimated by measuring the excimer formation rate of 1-palmitoyl-2-[10-(1-pyrenl)decanoyl] phosphatidyl choline (py-PC) in the same PE/DG mixtures. Upon comparing the values of DH and DL, the value of R was determined to be approximately 10-15 A, and agreed with that derived from x-ray diffraction (Tate, M.W., and S.M. Gruner, 1989, Biochemistry. 28:4245-4253; Rand, R.P., N.L. Fuller, S.M. Gruner, and V.A. Parsegian. 1990. Biochemistry. 29:76-87).

Lateral diffusion study of excimer-forming lipids in lamellar to inverted hexagonal phase transition of unsaturated phosphatidylethanolamine

Using multi-frequency cross-correlation fluorometry, the monomer fluorescence lifetime of 1-palmitoyl-2-[10-(1-pyrenyl)decanoyl)phosphatidylcholine (Py-PC) was employed to determine the lateral diffusion constant (DT) of dioleoylphosphatidylethanolamine (DOPE) in both the lamellar (L alpha) and the inverted hexagonal (HII) phases. The values of DT increased with temperature in both phases. However, the rate of increase of DT declined abruptly at approximately 10-13 degrees C (L alpha -HII transition temperature), as indicated by the existence of an inflection point in the log (DT/T) vs. 1/T plot. This observation suggests that the translational motion of lipids in the HII phase is lower than that in the L alpha phase upon temperature extrapolation. Lipid perturbants, cholesterol and diacylglycerol, were found to destabilize the L alpha phase of DOPE. This was demonstrated by a down-shift of the inflection point in the log(DT/T) vs. 1/T plot in the presence of the perturbants. Both cholesterol and 1,2-dioleoyl-sn-glycerol (diolein) decreased the lateral diffusion constant in both phases. Diolein promoted the HII phase more effectively than did the cholesterol. This is explained by an intrinsic wedge-shape geometry of diolein which strongly favors the formation of inverted cylindrical packing of the lipids.

Headgroup hydration and motional order of lipids in lamellar liquid crystalline and inverted hexagonal phases of unsaturated phosphatidylethanolamine--a time-resolved fluorescence study

By the use of frequency domain cross-correlation fluorometry, the fluorescence lifetime of the water soluble probe 8,1-anilinonapthalene sulfonic acid (ANS) in aqueous dispersions of dioleoylphosphatidylethanolamine (DOPE) and phosphatidylethanolamine transphosphatidylated from egg phosphatidylcholine (TPE) was measured. The orientational order parameter and rotational diffusion constant of the lipophilic probe 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) were also determined in TPE dispersions. In agreement with a previous study on DOPE (Cheng (1989) Biophys. J. 55, 1025-1031), abrupt changes in both the order packing and rotational diffusion constant were found at the lamellar liquid crystalline (L alpha) to inverted hexagonal (HII) phase transition of TPE. Owing to the subnanosecond resolution capability of this frequency domain fluorometric technique, the heterogeneous fluorescence decay of ANS was resolved into three distinct components with different decay lifetimes (tau's). They were 0 less than tau less than 0.5 ns, 2 less than tau less than 9 ns and tau greater than 15 ns. These lifetime regions were attributed to the partitioning of ANS into the bulk aqueous medium, the lipid/water interface and the lipid hydrocarbon region, respectively. These classifications of lifetime regions were further supported by the sensitivity of those lifetime components with the solvent isotopic shift of D2O. Similar to the changes of orientational order and rotational diffusion of lipophilic probe, the lifetime and intensity fraction of ANS associated with the lipid/water interfacial region declined abruptly at the L alpha-HII transition of both DOPE and TPE. This observation suggested that a dehydration of the lipid headgroup surface occurs at the L alpha-HII transition. This study provided evidence that both the lipid headgroup surface hydration and the lipid dynamics change drastically as a result of the macroscopic rearrangement of lipids at the L alpha-HII transition.