Mechanism and kinetics of the subtransition in hydrated L-dipalmitoylphosphatidylcholine

Freeze-Fracture Electron Microscopy on Domains in Lipid Mono- and Bilayer on Nano-Resolution Scale

Freeze-fracture electron microscopy (FFEM) as a cryofixation, replica, and transmission electron microscopy technique is unique in membrane bilayer and lipid monolayer research because it enables us to excess and visualize pattern such as domains in the hydrophobic center of lipid bilayer as well as the lipid/gas interface of lipid monolayer. Since one of the preparation steps of this technique includes fracturing the frozen sample and since during this fracturing process the fracture plane follows the area of weakest forces, these areas are exposed allowing us to explore pattern built up by lipids and/or intrinsic proteins but also initiated by peptides, drugs, and toxins reaching into these normally hard to access areas. Furthermore, FFEM as a replica technique is applicable to objects of a large size range and combines detailed imaging of fine structures down to nano-resolution scale within images of larger biological or artificial objects up to several tens of micrometers in size.Biological membranes consist of a multitude of components which can self-organize into rafts or domains within the fluid bilayer characterized by lateral inhomogeneities in chemical composition and/or physical properties. These domains seem to play important roles in signal transduction and membrane traffic. Furthermore, lipid domains are important in health and disease and make an interesting target for pharmacological approaches in cure and prevention of diseases such as Alzheimer, Parkinson, cardiovascular and prion diseases, systemic lupus erythematosus, and HIV. As a cryofixation technique, FFEM is a very powerful tool to capture such domains in a probe-free mode and explore their dynamics on a nano-resolution scale.

Time-resolved x-ray diffraction and calorimetric studies at low scan rates: I. Fully hydrated dipalmitoylphosphatidylcholine (DPPC) and DPPC/water/ethanol phases

The phase transitions in fully hydrated dipalmitoylphosphatidylcholine (DPPC) and DPPC/water/ethanol phases have been studied by lowangle time-resolved x-ray diffraction under conditions similar to those employed in calorimetry (scan rates 0.05-0.5 degrees C/min and uniform temperature throughout the samples). This approach provides more adequate characterization of the equilibrium transition pathways and allows for close correlations between structural and thermodynamic data. No coexistence of the rippled gel (P(beta')) and liquid-crystalline (L(alpha)) phases was found in the main transition of DPPC; rather, a loss of correlation in the lamellar structure, observed as broadening of the lamellar reflections, takes place in a narrow temperature range of approximately 100 mK at the transition midpoint. Formation of a long-living metastable phase, denoted by P(beta')(mst), differing from the initial P(beta') was observed in cooling direction by both x-ray diffraction and calorimetry. No direct conversion of P(beta')(mst) into P(beta') occurs for over 24 h but only by way of the phase sequence P(beta')(mst) --> L(beta') --> P(beta'). According to differential scanning calorimetry (DSC), the enthalpy of the P(beta')(mst)-L(alpha) transition is by approximately 5% lower than that of the P(beta')-L(alpha) transition. The effects of ethanol (Rowe, E. S. 1983. Biochemistry. 22:3299-3305; Simon, S. A., and T. J. McIntosh. 1984. Biochim. Biophys. Acta 773:169-172) on the mechanism and reversibility of the DPPC main transition were clearly visualized. At ethanol concentrations inducing formation of interdigitated gel phase, the main transition proceeds through a coexistence of the initial and final phases over a finite temperature range. During the subtransition in DPPC recorded at scan rate 0.3 degrees C/min, a smooth monotonic increase of the lamellar spacing from its subgel (L(c)) to its gel (L(beta')) phase value takes place. The width of the lamellar reflections remains unchanged during this transformation. This provides grounds to propose a "sequential" relaxation mechanism for the subgel-gel transition which is not accompanied by growth of domains of the final phase within the initial one.

Freeze-fracture electron microscopy on domains in lipid mono- and bilayer on nano-resolution scale

Freeze-fracture electron microscopy (FFEM) as a cryo-fixation, replica, and transmission electron microscopy technique is unique in membrane bilayer and lipid monolayer research because it enables us, to excess and visualize pattern such as domains in the hydrophobic center of lipid bilayer as well as the lipid/gas interface of the lipid monolayer. Since one of the preparatory steps of this technique includes fracturing the frozen sample and, since during this fracturing process the fracture plane follows the area of weakest forces, these areas are exposed allowing us to explore the pattern built up by lipids and/or intrinsic proteins and which are also initiated by peptides, drugs, and toxins reaching into these normally hard to access areas. Furthermore, FFEM as a replica technique is applicable to objects of a large size range and combines detailed imaging of fine structures down to nano-resolution scale within images of larger biological or artificial objects up to several ten's of micrometers in size.Biological membranes consist of a multitude of components which can self-organize into rafts or domains within the fluid bilayer characterized by lateral inhomogeneities in chemical composition and/or physical properties. These domains seem to play important roles in signal transduction and membrane traffic. Furthermore, lipid domains are important in health and disease and make an interesting target for pharmacological approaches in cure and prevention of diseases such as Alzheimer, Parkinson, cardiovascular and prion diseases, systemic lupus erythematosus and HIV. As a cryofixation technique FFEM is a very powerful tool to capture such domains in a probe-free mode and explore their dynamics on a nano-resolution scale.

New ordered metastable phases between the gel and subgel phases in hydrated phospholipids

Formation of low-temperature ordered gel phases in several fully hydrated phosphatidylethanolamines (PEs) and phosphatidylcholines (PCs) with saturated chains as well as in dipalmitoylphosphatidylglycerol (DPPG) was observed by synchrotron x-ray diffraction, microcalorimetry, and densitometry. The diffraction patterns recorded during slow cooling show that the gel-phase chain reflection cooperatively splits into two reflections, signaling a transformation of the usual gel phase into a more ordered phase, with an orthorhombic chain packing (the Y-transition). This transition is associated with a small decrease (2-4 microl/g) or inflection of the partial specific volume. It is fully reversible with the temperature and displays in heating direction as a small (0.1-0.7 kcal/mol) endothermic event. We recorded a Y-transition in distearoyl PE, dipalmitoyl PE (DPPE), mono and dimethylated DPPE, distearoyl PC, dipalmitoyl PC, diC(15)PC, and DPPG. No such transition exists in dimyristoyl PE and dilauroyl PE where the gel L(beta) phase transforms directly into subgel L(c) phase, as well as in the unsaturated dielaidoyl PE. The PE and PC low-temperature phases denoted L(R1) and SGII, respectively, have different hydrocarbon chain packing. The SGII phase is with tilted chains, arranged in an orthorhombic lattice of two-nearest-neighbor type. Except for the PCs, it was also registered in ionized DPPG. In the L(R1) phase, the chains are perpendicular to the bilayer plane and arranged in an orthorhombic lattice of four-nearest-neighbor type. It was observed in PEs and in protonated DPPG. The L(R1) and SGII phases are metastable phases, which may only be formed by cooling the respective gel L(beta) and L(beta') phases, and not by heating the subgel L(c) phase. Whenever present, they appear to represent an indispensable intermediate step in the formation of the latter phase.

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.

Phase behaviour of distearoylphosphatidylethanolamine in glycerol--a thermal and X-ray diffraction study

The phase behaviour of 1,2-distearoylphosphatidylethanolamine in glycerol has been examined using differential scanning calorimetry and real-time synchrotron X-ray diffraction methods. Dry phospholipid and phospholipid dispersed in glycerol over the concentration range 2.4%-90% (w/w) was equilibrated for 30 min at 20 degrees C and thermal and structural parameters on the temperature range 60 degrees C to 110 degrees C recorded during an initial heating and subsequent reheating. The characteristic feature of the initial heating scan was a direct lamellar crystalline to inverted hexagonal phase transition. In the subsequent cooling scan a lamellar gel structure was formed from the non-lamellar phase which transformed, on reheating, to a lamellar crystalline phase in which the acyl chain packing was titled with respect to the bilayer plane. The mechanism of the formation of the two crystalline phases was examined in the context of a relaxation model, where the liquid-crystal phase below the transition temperature from lamellar crystalline phase is metastable. A binary phase diagram over the temperature range 60 degrees C to 110 degrees C has been constructed.

Rapid reversible formation of a metastable subgel phase in saturated diacylphosphatidylcholines

Formation of well ordered lamellar subgel (SGII) phase in aqueous dispersions of L-dipalmitoylphosphatidylcholine upon cooling from the lamellar gel phase, without low-temperature equilibration, is observed in real time using synchrotron x-ray diffraction. It has the same lamellar repeat period as the gel phase from which it was formed but differs in its wide-angle diffraction pattern. The SGII phase forms at about 7 degrees C upon cooling at 2 degrees C/min. In temperature jump experiments at 1 degree C/s from 50 to -5 degrees C, the relaxation time of the lamellar gel-SGII transition is found to be approximately 15 s. The conversion between the lamellar gel and SGII phase is cooperative and rapidly reversible. Upon heating, it coincides in temperature with an endothermic event with a calorimetric enthalpy of 0.35 kcal/mol, the so-called sub-subtransition. Similar sub-subtransitions are also observed calorimetrically at temperatures approximately 10 degrees C below the subtransition, without low-temperature storage, in aqueous dispersions of L-dimyristoylphosphatidylcholine and L-distearoylphosphatidylcholine, but not in racemic DL-dipalmitoylphosphatidylcholine. The formation of the equilibrium lamellar crystalline Lc phase appears to take place only from within the SGII phase.

Sterols stabilize the ripple phase structure in dihexadecylphosphatidylcholine

The presence of various sterols in mixtures with dihexadecylphosphatidylcholine (DHPC) was studied using static X-ray diffraction of temperature equilibrated samples, and real-time X-ray diffraction of samples undergoing temperature scans. It was found that these sterols eliminate the interdigitation of the alkyl chains in the DHPC sub-gel and gel-state bilayers while stabilizing the ripple gel-state at the expense of the gel-state bilayer phase. The ripple-ripple phase transition previously observed for dipalmitoylphosphatidylcholine in the presence of low molar concentrations of sterols (Wolfe et al. (1992) Phys. Rev. Lett. 68, 1085-1088) was also observed for similar DHPC-sterol mixtures. In addition, we show the first evidence that the presence of 5 alpha-cholestane-3 beta,5,6 beta-triol will cause the lipid mixtures to continue to adopt a ripple mesophase structure even after the DHPC alkyl chain becomes disordered.

Real-time X-ray diffraction study at different scan rates of phase transitions for dipalmitoylphosphatidylcholine in KSCN

Multibilayer arrays of dipalmitoylphosphatidylcholine (DPPC) in 1 M KSCN were characterized using real-time X-ray diffraction and differential scanning calorimetry. A phase transition sequence was observed as a function of increasing temperature which involved changes from the interdigitated subgel (Lc(inter)) to interdigitated gel (L beta(inter)) to disordered (L alpha) bilayer states. The phase transition mechanisms were unambiguously determined by comparison of results from fast and slow scans. The Lc(inter)-->L beta(inter) phase transition was shown to involve a continuous change in acyl chain spacing between the rectangular subgel acyl chain unit cell into an hexagonal gel acyl chain unit cell. The mechanism is similar to that for subgel to gel state transitions involving non-interdigitated DPPC bilayers.

Synchrotron X-ray studies of lipids and membranes: a critique

This review gives a description of techniques, suitable for the study of lipid dispersions and unorientated membranes, that are available at synchrotron facilities to determine either the kinetics of transitional phenomena in the time after a temperature or pressure jump is initiated, or the phases present while a sample undergoes a phase transition. Included in this description is information about synchrotron X-ray sources, sample holders and temperature controllers, detection systems, as well as data reduction. Examples involving lipid dispersions are provided to illustrate the application of these methods using synchrotron radiation.

Phospholipid phase transitions: kinetics and structural mechanisms

A brief review is given on the principles and methods used to investigate structural phase transitions in phospholipid supramolecular structures. The conceptual differences of approaches close to and far from equilibrium are addressed, and the consequences in terms of the limits of interpretation for different types of methods, in particular referring to jump-relaxation and steady-state techniques, are surveyed. With the emphasis on connecting dynamic and structural information, the results obtained so far from different techniques are reviewed, and the open questions addressed. The more recent advances by millisecond time-resolved X-ray diffraction with synchrotron radiation and their main results obtained for transitions triggered by IR-laser temperature jumps are summarized. As a major novel aspect in the field, the necessity of considering martensitic, diffusionless transformation mechanisms and the occurrence of intermediate structures is highlighted.

Hydrocarbon chain packing modes in lipids: effect of altered sub-cell dimensions and chain rotation

The lateral hydrocarbon chain packing modes of lipids have been described in terms of specific hydrocarbon sub-cells as deduced from single crystal structural studies. To understand the changes in hydrocarbon chain packing in lipid bilayers induced by variations in temperature, hydration, ion-binding, etc., we have examined the effect on the calculated X-ray diffraction pattern of (a) systematic variations in the dimensions of the hydrocarbon sub-cell and (b) the effect of chain rotation at fixed lattice sites. For the O perpendicular (orthorhombic) sub-cell, the a and b sub-cell parameters were varied from as = 4.96 to 4.85 A and bs = 7.42 to 8.40 A in six steps and the positions (s = 2 sin theta/lambda) and intensities (Icalc = F2) of the strong sub-cell reflections calculated. In this way, the conversion of the O perpendicular sub-cell (with either fixed chain orientations or simulated chain rotation) to the hexagonal (H) sub-cell (with chain rotation) was followed. Notably, the two strong reflections characteristic of the O perpendicular sub-cell at 4.12 A (110) and 3.71 A (020) show progressive shifts in position and intensity, finally merging to give the strong (O1O) reflection at 4.2 A characteristic of the hexagonal sub-cell. Similar calculations were performed for the orthorhombic (O' perpendicular) and monoclinic (M parallel) sub-cells. This approach can be used to analyze changes in the X-ray diffraction data due to modifications of the hydrocarbon chain packing modes characteristic of simple and complex lipids.

Structure and phase behavior of hydrated mixtures of L-dipalmitoylphosphatidylcholine and palmitic acid. Correlations between structural rearrangements, specific volume changes and endothermic events

Several new features of the phase diagram of L-dipalmitoylphosphatidylcholine (DPPC)/palmitic acid mixtures in excess water were established by means of static and time-resolved X-ray diffraction, densitometry and differential scanning calorimetry (DSC). At low temperatures, palmitic acid has a biphasic effect on the lamellar subgel phases: at concentrations below 5-6 mol%, it prevents formation of the DPPC subgel phase (Lc), while at higher contents (between about 40 and 90 mol%) another subgel phase (Lccom) is formed as a result of lipid co-crystallization at 1 DPPC: 2 palmitic acid stoichiometry. A crystalline palmitic acid phase separates from Lccom above 70-80 mol% of fatty acid. The Lccomphase transforms into a lamellar gel phase (L beta) in an endothermic transition centered at 38 degrees C. At high temperatures, the mixtures form hexagonal liquid-crystalline phase (HII) in the region of 60-70 mol% and an isotropic phase (I) at 90-100 mol% of palmitic acid. No coexistence of HII phase with the fluid lamellar phase of DPPC was observed at intermediate compositions (20 and 50 mol% of palmitic acid) but rather formation of a complex phase with non-periodic geometry characterized by molten chains and a broad, continuous small-angle scattering band. No evidence for fluid phase coexistence was found also at compositions between HII and I phases. The L beta--HII transition at 60-70 mol% of palmitic acids is readily reversible and two-state in both heating and cooling modes. It is characterized by the coexistence of initial and final phases with no detectable intermediates by time-resolved and static X-ray diffraction. The crystalline-isotropic transition in palmitic acid is two-state only in heating direction. On cooling, it is characterized by strong undercooling and gradually relaxing lamellar crystalline structures. The slowly reversible Lccom--L beta transition proceeds continuously through intermediate states. Although clearly discernible by both DSC and X-ray diffraction, it is not accompanied by specific volume changes.

A laser-induced europium (III) ion luminescence study of the interaction of this ion with phospholipid bilayer vesicles above and below the gel to liquid-crystalline phase transition temperature

Laser-induced europium(III) luminescence spectroscopy was used to investigate the formation and integrity of phospholipid bilayer vesicles produced by ultrasonication and detergent dialysis. Eu(III) ion interactions with these model biological membrane systems were explored. 7F0----5D0 spectral and excited-state lifetime data reveal two distinct, temperature-dependent binding sites, one involving a weak, 'superficial' interaction with the phosphate moiety of the phosphatidylcholine head group, the other involving a more tightly bound ion in a relatively dehydrated region of the head group. This latter 'sequestered' species appeared only at temperatures equal to or below that of the gel----liquid-crystalline phase transition. Systems containing various amounts of cholesterol showed a decrease in the formation of the sequestered species, indicative of a decrease in ion permeability. The results of this study demonstrate that the Eu(III) luminescence technique is useful for detecting major phase alterations in phospholipid bilayer vesicles.

Structural rearrangements during crystal-liquid-crystal and gel-liquid-crystal phase transitions in aqueous dispersions of dipalmitoylphosphatidylethanolamine. A time-resolved X-ray diffraction study

The mechanism and kinetics of the crystal-liquid-crystal (Lc----L alpha) and gel-liquid-crystal (L beta----L alpha) transitions of the L-enantiomer and racemic dipalmitoylphosphatidylethanolamine have been examined in temperature scans and jumps using time-resolved X-ray diffraction methods. The Lc----L alpha transformations (at 66 degrees C for L-dipalmitoylphosphatidylethanolamine and 82 degrees C for DL-dipalmitoylphosphatidylethanolamine) were found to be two-state (first-order) processes characterised by co-existence of the initial Lc and final L alpha states during the transition with the absence of any detectable intermediates states. The transition mechanism involves firstly, disordering of the hydrocarbon chains which makes a major contribution to the transition enthalpy and secondly by a transition in the lamellar repeat spacing. The overall relaxation time of the Lc----L alpha transition of L-dipalmitoylphosphatidylethanolamine during temperature jumps of 4.5 degrees C/s was about 10 s. A gradual increase in the gel-state interchain spacing during the L beta----L alpha transitions of L- and DL-dipalmitoylphosphatidylethanolamine preceded a broadening of the wide-angle diffraction peak. There was a concomitant and continuous increase of the lamellar repeat spacing to values typical of the L alpha phase with increasing temperature. This sequence of events is completely reversible on cooling with a temperature hysteresis of 5-6 degrees C. The relaxation times of the L beta---L alpha transitions during jumps of 4.5 degrees C/s were about 2 s in both the heating and cooling directions.

Lamellar gel-lamellar liquid crystal phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from aqueous trehalose solutions. A real-time X-ray diffraction study

The mechanism of the phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from fully hydrated gel phase (L beta') in the presence of trehalose has been investigated by real-time X-ray diffraction methods. Sequential diffraction patterns were recorded with an accumulation time of 3 s during heating and 1.2 s during cooling between about 20 and 80 degrees C. A transition is observed in the range 47-53 degrees C that involves structural events typical of a lamellar gel-lamellar liquid-crystal (L beta--L alpha) transformation. This transition is completely reversible with a temperature hysteresis of 2-3 degrees C and thereby resembles the main phase transition of fully hydrated dipalmitoylphosphatidylcholine multilayers. The mechanism of the transition from L beta to L alpha as seen in the wide-angle scattering profiles show that the sharp peak at about 0.41 nm, characteristic of the gel phase, broadens and shifts progressively to about 0.44 nm towards the end of the transition. A temperature jump of 6C degrees/s through the phase transition region of a freeze-dried dipalmitoylphosphatidylcholine: trehalose mixture (molar ratio 1:1) showed that the phase transition had a relaxation time of about 2 s which is similar to that of the main transition in the fully hydrated lipid. X-ray diffraction studies of the melting of dipalmitoylphosphatidylcholine freeze-dried from the lamellar-gel phase in the absence of trehalose showed a transition at above 70 degrees C. The low-angle diffraction data of phospholipid/trehalose mixtures are consistent with an arrangement of trehalose molecules in a loosely packed 'monolayer' separating bilayers of phospholipid. Trehalose appears to reduce the direct interbilayer hydrogen bond coupling thereby modifying the thermal stability and the phase transition mechanism of the bilayers.

Monovalent ion-phosphatidylcholine interactions: an electron paramagnetic resonance study

The apparent Mn2+ binding constant for L-alpha-dipalmitoylphosphatidylcholine (DPPC) bilayers dispersed in monovalent salt and MnCl2 dispersions was determined as a function temperature using electron paramagnetic resonance (ERP). Reproducibility in the data sets requires the use of a standard salt solution and dual cavity techniques. Changes in the binding constant at different phase states and temperatures were observed and correlated to the influence of monovalent salts on the thermal properties of DPPC. The turning points (i.e. changes in slope) in the curves of the apparent Mn2+ binding constant versus temperature can be understood in terms of differences in ion binding to headgroups with different bilayer surface areas. The influence of Li+ and SCN- on Mn2+ binding is viewed as a function of their presence in the ionic media in contact with the bilayer rather than as a competitive event. Other monovalent ions studied appear to have little effect on the measured apparent Mn2+ binding constants for DPPC headgroups.