Motion of the lengthened zwitterionic head groups of C16-lecithin analogues in aqueous solutions as studied by dielectric relaxation measurements

Fatty acid contamination and dielectric relaxation in phospholipid vesicle suspensions

Aqueous vesicle or micelle suspensions from various synthetic lecithins or surfactants - most of them purified by a simple ion-exchange procedure in methanol - were investigated, some with ionic admixtures. The dielectric permittivity '(nu) between 5 kHz and 100 MHz was determined by different time-and frequency-domain methods, with attention given to electrode polarization below 1 MHz. Pure ether lecithins (used to reduce hydrolysis during preparation) as well as ester lecithins showed no dielectric dispersion below 10 MHz (Delta' 3). In contrast, even dilute colloidal solutions containing about 1 mol% (with respect to solute) ionic amphiphiles normally exhibited large dielectric dispersion (10 < Delta' < 700), especially with electrolyte present. This low-frequency dispersion is sensitive to vesicle coagulation or fusion. Underlying relaxation mechanisms are discussed, and the main relaxation is shown to be the same as for other charged colloids. This conclusion suggest a new interpretation of measurements, previously reported by other authors, who gave an interpretation in terms of correlated zwitterionic head group orientation in multilamellar lecithin liposomes. Possible effects from traces of impurities in lipids are discussed.

Occurrence of an Intermediate Relaxation Process in Water-in-Oil Microemulsions below Percolation: The Electrical Modulus Formalism

The dielectric and conductometric spectra of water-in-oil microemulsions below percolation in the frequency range from 1 MHz to 1.8 GHz have been analyzed on the basis of the electrical modulus formalism. In the frequency range investigated, this approach clearly evidences the presence of a particular polarization mechanism, resulting in a well-defined dielectric dispersion, located between that due to the orientational polarization of the bulk aqueous phase and that due to the ionic structure of the interface, usually occurring in heterogeneous systems. This polarization mechanism has been attributed to the "in-phase" correlation displacement of surfactant polar head groups surrounding each water droplet dispersed in the oil phase. This mechanism differs from the usual interfacial Maxwell-Wagner effect. The advantage of the electrical modulus formalism, in comparison with the analysis of the directly measured quantities, the permittivity epsilon'(omega), and the total electrical conductivity sigma(omega), are briefly discussed. Copyright 2001 Academic Press.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

The dielectric permittivity spectrum of aqueous colloidal phospholipid solutions between 1 kHz and 60 GHz

The dielectric permittivity spectrum between 1 kHz and 60 GHz of aqueous colloidal solutions of predominantly zwitterionic phospholipids is presented from results of previous and recent measurements. It shows three dispersion/loss regions around 22 GHz. 80 MHz and below 40 MHz (30 degrees C) which are attributed to rotational diffusion of the water molecules and of the zwitterionic phosphorylcholine groups, and to limited translational diffusion of ionic lipid molecules and/or its counterions. respectively. Merely a few mole percent of ionic lipids cause comparatively large dielectric dispersion. Ignoring the fact that such impurities may be present in zwitterionic phospholipid compounds, which have not been especially purified, this has led to misinterpretation of the dielectric spectrum in the past. An approximate quantitative description of the measured spectra is given for vesicle solutions with only very small additional low-molecular-weight salt content. It reproduces the sensitive dependence of the ionic lipid-induced dielectric dispersion (step height and frequency) on various parameters: phospholipid vesicle size. ionic lipid content, as well as the self-diffusion coefficient of the ionic lipid molecules and of its counterions, moving within the phospholipid bilayers or on their surface, respectively.

A comparison of the headgroup conformation and dynamics in synthetic analogs of dipalmitoylphosphatidylcholine

14N-NMR spectra and relaxation times for dipalmitoylphosphatidylcholine and three analogs were obtained in both the liquid crystal and gel phases. The analogs either changed the PO-4 to N+ (CH3)3 distance (P-N) within the headgroup by increasing the number of CH2 groups from two in the phosphocholine headgroup (PN-2) to six in the phospho-(N',N',N'-trimethyl)hexanolamine headgroup (PN-6), or replaced the ester linkages to the hydrocarbon chains with either linkages. 31P-NMR spectra were obtained for the four samples in the liquid-crystal phase. (1) The 14N- and 31P-NMR spectra and 14N relaxation times all indicate that increasing the P-N distance within the headgroup causes changes in both the average orientation of the C-N bond and its dynamics. (2) The 14N-NMR spectra provide evidence for a change in orientational order of the headgroup as a result of changing the linkage to the acyl chains. On the other hand, the relaxation time measurements indicate that the molecular motion for the headgroup is independent of the type of linkage. (3) The thermal behaviour of the four samples is clearly reflected in the 14N-NMR spectra. The second moments of the spectra show distinct changes at each of the phase transitions. (4) The 14N-NMR spectra show that the average conformation of the headgroups is not significantly altered by the main phase transition. For the PN-2 samples, T2e, the decay of the quadrupolar echo, decreases discontinuously in the P beta, phase, which is evidence for a possible exchange process between two molecular states within this phase.

On the phase transition kinetics of phospholipid bilayers. Relaxation experiments with detection of fluorescent anisotropy

Relaxation experiments were performed on vesicles of dimyristoylphosphatidylcholine in the lipid phase transition region by means of a Joule heating temperature jump technique. The time course of fluorescence anisotropy of the dye 1,6-diphenyl-1,3,5-hexatriene (DPH), incorporated in the bilayer, was observed. Since the dye always exhibits stationary anisotropy in the time range of observation, its anisotropy represents the order of the bilayer during the entire course of the experiment. Two relaxation processes were detected within the 1-100 ms range with maximal time constants at the midpoint of transition. At least one process was faster than the temperature jump dead time. The relaxation times, especially the maximal relaxation times, depend strongly on the bilayer curvature: larger vesicles imply larger time constants. This observation can explain differences between kinetic results of different laboratories. A comparison between the kinetic findings by means of three different dyes located at different sites along the lipid molecules suggests that the slower steps of the lipid phase transition mechanism involve the entire lipid molecule rather than individual parts of the molecule. The results of this present contribution reconfirm the recently published phase transition mechanism which postulates a series of steps of which the first, the kink formation, is fast and noncooperative and the following ones, representing the expansion of the aggregates, are slower and cooperative.

The kinetics of the formation of rotational isomers in the hydrophobic tail region of phospholipid bilayers

Very fast structural changes in dipalmitoyl phosphatidylcholine molecules forming a vesicular bilayer were investigated by means of a laser temperature-jump technique. After temperature increases of about 1 K within 1 ns, the solution turbidity increases with a time constant of about 4 ns. This time constant exhibited no appreciable temperature dependence and represents a noncooperative process. It is interpreted as a local increase in density in the bilayer which results from a shortening of the individual lipid molecule due to formation of rotational isomers (e.g., kinks) without an appropriate expansion of the molecular environment. The final membrane expansion is achieved in consecutive steps with a decrease in turbidity and time constants between 100 mus and several seconds which are maximal in the midpoint of the phospholipid phase transition. These steps represent cooperative processes, namely the molecular interaction leading to the membrane expansion. The rate of kink formation implies that kinks migrate through the membrane by energetic transitions forwarded from lipid to lipid, rather than by hopping of individual lipids thereby carrying a kink.