Phase behavior of phosphatidylglycerol bilayers as a function of buffer composition: fluorescence studies using Laurdan probe

Molecular structures of fluid phase phosphatidylglycerol bilayers as determined by small angle neutron and X-ray scattering

We have determined the molecular structures of commonly used phosphatidylglycerols (PGs) in the commonly accepted biologically relevant fluid phase. This was done by simultaneously analyzing small angle neutron and X-ray scattering data, with the constraint of measured lipid volumes. We report the temperature dependence of bilayer parameters obtained using the one-dimensional scattering density profile model - which was derived from molecular dynamics simulations - including the area per lipid, the overall bilayer thickness, as well as other intrabilayer parameters (e.g., hydrocarbon thickness). Lipid areas are found to be larger than their phosphatidylcholine (PC) counterparts, a result likely due to repulsive electrostatic interactions taking place between the charged PG headgroups even in the presence of sodium counterions. In general, PG and PC bilayers show a similar response to changes in temperature and chain length, but differ in their response to chain unsaturation. For example, compared to PC bilayers, the inclusion of a first double bond in PG lipids results in a smaller incremental change to the area per lipid and bilayer thickness. However, the extrapolated lipid area of saturated PG lipids to infinite chain length is found to be similar to that of PCs, an indication of the glycerol-carbonyl backbone's pivotal role in influencing the lipid-water interface.

Fluorescence Study of the Fluidity and Cooperativity of the Phase Transitions of Zwitterionic and Anionic Liposomes Confined in Sol−Gel Glasses

The current work makes use of different fluorescent reporter molecules and fluorescent spectroscopic techniques to characterize the thermotropic, physical, and dynamical properties of large unilamellar liposomes formed from either 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) or 1,2-dimyristoyl-sn-glycero-3-[phospho-rac-glycerol] (DMPG) encapsulated in sol-gel matrixes. In particular, cooperativity of the phase transition is analyzed from steady-state fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH), the interfacial properties are studied by measuring the spectral shift of Laurdan, and the structural organization (heterogeneity) of the lipid bilayer is determined from the fluorescence lifetime of trans-parinaric acid (t-PnA). In addition, information regarding order and dynamical properties in the bulk hydrophobic core is obtained from time-resolved fluorescence anisotropy of t-PnA and 3-(4-(6-phenyl)-1,3,5-hexatrienyl)-phenylpropionic acid (PA-DPH). The spectroscopic study reveals that upon encapsulation, the basic thermodynamic properties as well as the fluidity of the lipid bilayer practically remain intact for DMPG liposomes but not for DMPC liposomes, whose lipid bilayer exhibits large gel-fluid heterogeneity. On the basis of these experimental results, electrostatic interactions between phospholipid polar heads and the porous surface of the host matrix seem to play a capital role for the preservation of the structural integrity of encapsulated bilayer.

Laurdan in fluid bilayers: position and structural sensitivity

Laurdan (2-dimethylamino-6-lauroylnaphthalene) is a hydrophobic fluorescent probe widely used in lipid systems. This probe was shown to be highly sensitive to lipid phases, and this sensitivity related to the probe microenvironment polarity and viscosity. In the present study, Laurdan was incorporated in 1,2-dipalmitoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (DPPG), which has a phase transition around 41 degrees C, and DLPC (1,2-dilauroyl-sn-glycero-3-phosphocholine), which is in the fluid phase at all temperatures studied. The temperature dependence of Laurdan fluorescent emission was analyzed via the decomposition into two gaussian bands, a short- and a long-wavelength band, corresponding to a non-relaxed and a water-relaxed excited state, respectively. As expected, Laurdan fluorescence is highly sensitive to DPPG gel-fluid transition. However, it is shown that Laurdan fluorescence, in DLPC, is also dependent on the temperature, though the bilayer phase does not change. This is in contrast to the rather similar fluorescent emission obtained for the analogous hydrophilic probe, Prodan (2-dimethylamino-6-propionylnaphthalene), when free in aqueous solution, over the same range of temperature. Therefore, Laurdan fluorescence seems to be highly dependent on the lipid bilayer packing, even for fluid membranes. This is supported by Laurdan fluorescence anisotropy and spin labels incorporated at different positions in the fluid lipid bilayer of DLPC. The latter were used both as structural probes for bilayer packing, and as Laurdan fluorescence quenchers. The results confirm the high sensitivity of Laurdan fluorescence emission to membrane packing, and indicate a rather shallow position for Laurdan in the membrane.

Immobilised artificial membrane chromatography coupled with molecular probing. Mimetic system for studying lipid-calcium interactions in nutritional mixtures

Immobilised artificial membrane (IAM) chromatography was utilised to study the interactions of usual membrane probes with grafted phosphatidylcholine silica support, in relation to the presence of calcium ions introduced in the mobile phase as they are present in nutritional mixtures. IAM acts as a mimetic membrane of lipid emulsion globules, a major component of nutritional mixtures. The tested probes were 1,6-diphenyl-1,3,5-hexatriene (DPH), 9-diethylamino-5H-benzo[alpha]phenoxazine-5-one or nile red (NR) and 2-(p-toluidinyl)naphtalene-6-sulfonate (TNS). For each probe, partition coefficients and thermodynamic parameters of transfer from the mobile phase to the IAM stationary phase have been measured. Our results suggested that the interactions of neutral probes (i.e. DPH and NR) with phosphatidylcholine are driven by hydrophobic forces. Addition of calcium chloride to the mobile phase slightly decreased the retention of these neutral probes and dramatically increased that of anionic TNS. Moreover, an enthalpy-entropy compensation study revealed that the mechanism of interaction between TNS and IAM is independent of the calcium concentration. Results argued for the existence of electrostatic repulsion forces exerted by IAM phase towards anionic TNS. Addition of calcium ions into the mobile phase led to the establishment of an ionic double layer at the zwitterionic stationary phase surface weakening the electrostatic barrier and increasing TNS retention. Consequently, it was demonstrated that IAM appears as a suitable model to get a better insight on the lipid-calcium interactions taking place in nutritional mixtures.

Differential effects of oleuropein, a biophenol from Olea europaea, on anionic and zwiterionic phospholipid model membranes

Oleuropein (Ole) is the major phenolic constituent of the olive leaf (Olea europaea) and it is also present in olive oil and fruit. In the last years several compounds from olive tree, oleuropein among them, have shown a variety of biological activities such as antimicrobial or antioxidant. A phospholipid model membrane system was used to study whether the Ole biological effects could be membrane related. Ole showed a significant partition level in phospholipid membranes, i.e. 80%, at lipid-saturating conditions. Moreover, fluorescence quenching experiments indicated a shallow location for Ole in membranes. Ole promoted weak effects on zwiterionic phospholipids such as phosphatidylcholine or phosphatidylethanolamine. In contrast, differential scanning microcalorimetry, light scattering and fluorescence anisotropy pH titration studies revealed strong effects on anionic phospholipids such as phosphatidylglycerol at physiological pH and salt conditions. These effects consisted on perturbations at the phospholipid membrane surface, which might involve specific molecular interactions between Ole and the negatively charged phosphate group and therefore modify the phospholipid/water interface properties. It is proposed that Ole induces lipid structures similar to the gel-fluid intermediate phase (IP) described for PG membranes, in a similar way than low ionic strength does. These effects on phosphatidylglycerol may account for the antimicrobial activity of Ole.

A stability test of liposome preparations using steady-state fluorescent measurements

The stability of liposome preparations under the action of the nonionic detergent Triton X-100 was measured using the fluorescent molecular probe octadecylrhodamine B (R18). The probe inserted in the lipid bilayer shows a self-quenched fluorescence and the degree of quenching depends both on the probe concentration and the phase state of the lipid membrane. The addition of detergent to the liposomes produces a steep decrease in self-quenching caused by dilution of the probe in the bilayer. The curves of steady-state fluorescence intensity show an abrupt change in slope that corresponds to the point at which liposomes break down into lipid-detergent mixed entities that are different from the earlier liposome-monodisperse population. The lytic process was followed in parallel by dynamic light scattering (DLS), and the analysis of the DLS results agree with the interpretation of the fluorescence measurements. The probe R18 therefore is a useful marker to test the stability of liposome preparations. The advantages of the present method are discussed by comparison with other techniques.