The phase diagram of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/sucrose in the dry state. Sucrose substitution for water in lamellar mesophases

Ion-induced modification of the sucrose network and its impact on melting of freeze-dried liposomes. DSC and molecular dynamics study

Disaccharides play an important role in survival of anhydrobiotic organisms during extreme environmental conditions. A key protection feature is their capability to form the hydrogen bond (HB) network in a similar fashion as the one made by water. Since various ions also affect the HB network in completely hydrated systems, it is of a great interest to understand how they impact preservation when incorporated in a disaccharide network. To address this, we employ a combination of experimental and modeling techniques to study behavior of multilamellar 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) liposomes freeze-dried with sucrose in presence of NaCl or NaH₂PO₄·H₂O at various concentrations (0.01-1M). Differential scanning calorimetry (DSC) was employed in order to determine the cooperative unit size (CUS), the number of lipid molecules that constitute a domain of cooperative motion in the liposome, and the melting temperature (T_m). In the absence of salt CUS was estimated to be 122±12, whereas in the presence of NaCl CUS increases more (347±34 for c=1M) than for NaH₂PO₄·H₂O (193±26 for 1M). When it comes to T_m, the situation is reversed; NaCl induces increase by about 1K, while NaH₂PO₄·H₂O by about 10K. These findings clearly demonstrate how different interaction forces-hydrogen bonding, charge pairing, and van der Waals interactions between acyl chains-affect CUS and T_m. Their interplay and contribution of particular interaction was further analyzed with molecular dynamics (MD) simulations. This analysis demonstrated that the HB network of DMPC and sucrose is partially disrupted in the presence of NaCl ions, and even to a greater extent in the case of NaH₂PO₄·H₂O ions. Notably, H₂PO₄^- ions outcompete and replace the sucrose molecules at the DMPC surface, which in turn alters the nature of the DMPC-surrounding interactions, from a weaker HB-dominated to a stronger CP-dominated interaction network.

Simulation of gel phase formation and melting in lipid bilayers using a coarse grained model

The transformation between a gel and a fluid phase in dipalmitoyl-phosphatidylcholine (DPPC) bilayers has been simulated using a coarse grained (CG) model by cooling bilayer patches composed of up to 8000 lipids. The critical step in the transformation process is the nucleation of a gel cluster consisting of 20-80 lipids, spanning both monolayers. After the formation of the critical cluster, a fast growth regime is entered. Growth slows when multiple gel domains start interacting, forming a percolating network. Long-lived fluid domains remain trapped and can be metastable on a microsecond time scale. From the temperature dependence of the rate of cluster growth, the line tension of the fluid-gel interface was estimated to be 3+/-2 pN. The reverse process is observed when heating the gel phase. No evidence is found for a hexatic phase as an intermediate stage of melting. The hysteresis observed in the freezing and melting transformation is found to depend both on the system size and on the time scale of the simulation. Extrapolating to macroscopic length and time scales, the transition temperature for heating and cooling converges to 295+/-5 K, in semi-quantitative agreement with the experimental value for DPPC (315 K). The phase transformation is associated with a drop in lateral mobility of the lipids by two orders of magnitude, and an increase in the rotational correlation time of the same order of magnitude. The lipid headgroups, however, remain fluid. These observations are in agreement with experimental findings, and show that the nature of the ordered phase obtained with the CG model is indeed a gel rather than a crystalline phase. Simulations performed at different levels of hydration furthermore show that the gel phase is stabilized at low hydration. A simulation of a small DPPC vesicle reveals that curvature has the opposite effect.

An index of lipid phase diagrams

There is a growing awareness of the utility of lipid phase behavior data in studies of membrane-related phenomena. Such miscibility information is commonly reported in the form of temperature-composition (T-C) phase diagrams. The current index is a conduit to the relevant literature. It lists lipid phase diagrams, their components and conditions of measurement, and complete bibliographic information. The main focus of the index is on lipids of membrane origin where water is the dispersing medium. However, it also includes records on acylglycerols, fatty acids, cationic lipids, and detergent-containing systems. The miscibility of synthetic and natural lipids with other lipids, with water, and with biomolecules (proteins, nucleic acids, carbohydrates, etc.) and non-biological materials (drugs, anesthetics, organic solvents, etc.) is within the purview of the index. There are 2188 phase diagram records in the index, the bulk (81%) of which refers to binary (two-component) T-C phase diagrams. The remainder is made up of more complex (ternary, quaternary) systems, pressure-T phase diagrams, and other more exotic miscibility studies. The index covers the period from 1965 through to July, 2001.

Freeze-fracture studies on lipids and membranes

Freeze-fracture electron microscopy is especially useful for investigation of lipid structures by the advantageous fracture course within hydrophobic zones. Freezing is, on the other hand, a restriction because the structures of lamellar and non-lamellar phase states with disordered acyl chains (L(alpha), H(II,) cubic) are difficult to preserve. An important aspect of this method is therefore the lipid structure of phase states with ordered acyl chains (crystal, gel), and with a different degree of hydration. Freeze-fracture of pure lipid systems creates a valid representation of the structure of non-lamellar phases and of the general structure of the "lamellar" lipid bilayer, and lamellar phases with characteristic deformations (ripples, curvatures, plane sectors) can be identified. Fracture through the hydrophobic bilayer centre of biological membranes reveals characteristic protein components, the intramembraneous particles (IMPs). The lateral distribution of the IMPs is a helpful marker for fluid and rigid phase states, also without deformation of the lamella. The overall history and the present state of knowledge concerning the different structures revealed by the freeze-fracture and freeze-etch techniques in lipid systems, and to a limited extent in biological membranes, is reviewed, taking into account studies from our own laboratory.

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.

Temperature change of the lamellar structure of DPPC/disaccharide/water systems with low water content

Temperature change in l-alpha-dipalmitoyl phosphatidylcholine (DPPC)/disaccharide systems with low water content (less than 8 wt. %) was investigated using X-ray diffraction within a range of two transition temperatures. X-ray diffraction above the higher transition temperature showed a broad symmetric peak, indicating the Lalpha phase. Below the higher transition temperature, two overlapping diffraction peaks were observed. After peak separation, temperature change in these systems was analyzed using peak parameters of the two peaks. Peak parameters of the lower angle peak changed continuously up to and above the higher transition temperature, suggesting the systems to be in a liquid crystal phase below the higher transition temperature. Fourier-transform infrared (FT-IR) spectra of the DPPC/trehalose system with 5.5 wt.% water showed the wave number of asymmetric stretching of phosphate groups to change at the lower transition temperature and that of symmetric stretching of CH2 groups, to change between the lower and higher transition temperatures. Thus, below the lower transition temperature, the system is shown to be in a gel phase. Conformational change in phosphate groups occurred at the lower transition temperature. Within the lower and higher transition temperatures, two phases were found to coexist and transition from the gel phase to Lalpha phase to occur continuously. Above the higher transition temperature, the system is in the Lalpha phase.

Effect of water on lamellar structure of DPPC/sugar systems

The ability of two monosaccharides, four disaccharides and one trisaccharide, to lower the transition temperature of L-alpha-dipalmitoyl phosphatidylcholine (DPPC) was investigated using differential scanning calorimetry (DSC) and the ability of these sugars to change the lateral packing of the acyl chains of DPPC was investigated using wide-angle X-ray diffraction. The sugars affected the gel-liquid crystal transition temperature (Tc) of DPPC when the water content of the DPPC/sugar systems was less than 20 wt.%. Specifically, Tc of the DPPC without sugar increased to approximately 106 degrees C, the Tc of the DPPC/monosaccharide system remained almost constant at 43 degrees C and of the DPPC/disaccharide or trisaccharide systems decreased to approximately 24 degrees C. In the dehydrated state, di- and trisaccharides caused looser packing of the DPPC hydrocarbon chains than the monosaccharides did, and the sugars affected the packing mode in different ways. The addition of water caused this difference in the sugars' effects on the packing mode to disappear and further addition of water caused the effect of sugar to almost disappear. Thus, the addition of water to a DPPC/sugar system weakens the interaction between the sugar and lipid and strengthens the DPPC chain packing.

Interactions between soluble sugars and POPC (1-palmitoyl-2-oleoylphosphatidylcholine) during dehydration: vitrification of sugars alters the phase behavior of the phospholipid

The phase behavior of 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) was characterized as a function of hydration in the presence of combinations of sugars representative of sugars found in seed embryos having differing degrees of desiccation tolerance. The tendency of the sugar mixes to vitrify was also monitored as a function of hydration. Using differential scanning calorimetry, it was found that all sugars diminished the increase in the gel-to-fluid phase transition temperature (Tm) of POPC that occurred upon dehydration of the pure lipid. These results are analyzed in terms of the osmotic and volumetric properties of sugars. Also, it was found that in those samples for which the glass transition temperature (Tg) was greater than the Tm of POPC, Tm was lowered by approx. 20 C degrees from the value for the fully hydrated lipid. X-ray diffraction data confirmed that acyl chain freezing was deferred to a lower temperature during cooling of vitrified samples. The significance of these results is discussed in terms of the ability of many organisms to tolerate desiccation.

Protection by sugars against phase transition-induced leak in hydrated dimyristoylphosphatidylcholine liposomes

The disaccharides trehalose and sucrose have small effects on temperature and enthalpy of the pre- and main phase transition in hydrated DMPC bilayers. In contrast, these sugars cause a considerable retention of carboxyfluorescein when large unilamellar vesicles of DMPC are heated through the main transition. This effect is sugar specific, as the monosaccharides glucose and fructose are less effective and ethyleneglycol has no effect at all.