Effect of hydrostatic pressure on the bilayer phase behavior of symmetric and asymmetric phospholipids with the same total chain length

Phase Imaging of Phosphatidylcholine Bilayer Membranes by Prodan Fluorescence

Prodan (6-propiponyl-2-(N,N-dimethylamino)naphthalene) is well known as a polarity-sensitive fluorescent probe and has a high capability of detecting structural changes occurring within phospholipid bilayer membranes. In this study, we carried out the fluorescence spectroscopic observation of bilayer phase behavior for a series of symmetric saturated diacylphosphatidylcholines (CnPCs) with different acyl-chain length n (n = 12-15 and 19-22) using Prodan as a membrane probe to confirm the availability of Prodan along with the previous results for the CnPC bilayer membranes (n = 16-18). The results were discussed by constructing spectral three-dimensional (3D) imaging plots for visualizing the change in bilayer phase states with temperature or pressure to verify the functionality of this 3D imaging plot. It was found that the Prodan fluorescence technique is applicable to the detection of the changes in the bilayer phase states of all CnPCs with a few exceptions. One of the most crucial exceptions was that Prodan cannot be used for the detection of the bilayer-gel state of the C21PC bilayer membrane. It was also found that it is only to the CnPC bilayer membranes with n = 15-18 that the 3D imaging plot is adequately and accurately applicable as a useful graphical tool for visually detecting the bilayer phase states. This is a disadvantageous feature of this technique brought about by the high sensitivity of Prodan as a membrane probe. Further detailed studies on the molecular behavior of Prodan will enable us to find a more useful way of utilizing this membrane probe.

Interrogating the Structural Dynamics and Energetics of Biomolecular Systems with Pressure Modulation

High hydrostatic pressure affects the structure, dynamics, and stability of biomolecular systems and is a key parameter in the context of the exploration of the origin and the physical limits of life. This review lays out the conceptual framework for exploring the conformational fluctuations, dynamical properties, and activity of biomolecular systems using pressure perturbation. Complementary pressure-jump relaxation studies are useful tools to study the kinetics and mechanisms of biomolecular phase transitions and structural transformations, such as membrane fusion or protein and nucleic acid folding. Finally, the advantages of using pressure to explore biomolecular assemblies and modulate enzymatic reactions are discussed.

Ligand partitioning into lipid bilayer membranes under high pressure: Implication of variation in phase-transition temperatures

The variation in phase-transition temperatures of dipalmitoylphosphatidylcholine (DPPC) bilayer membrane by adding two membrane-active ligands, a long-chain fatty acid (palmitic acid (PA)) and an inhalation anesthetic (halothane (HAL)), was investigated by light-transmittance measurements and fluorometry. By assuming the thermodynamic colligative property for the bilayer membrane at low ligand concentrations, the partitioning behavior of these ligands into the DPPC bilayer membrane was considered. It was proved from the differential partition coefficients between two phases that PA has strong affinity with the gel (lamellar gel) phase in a micro-molal concentration range and makes the bilayer membrane more ordered, while HAL has strong affinity with the liquid crystalline phase in a milli-molal concentration range and does the bilayer membrane more disordered. The transfer volumes of both ligands from the aqueous solution to each phase of the DPPC bilayer membrane showed that the preferential partitioning of the PA molecule into the gel (lamellar gel) produces about 20% decrease in transfer volume as compared with the liquid crystalline phase, whereas that of the HAL molecule into the liquid crystalline phase does about twice increase in transfer volume as compared with the gel (ripple gel) phase. Furthermore, changes in thermotropic and barotropic phase behavior of the DPPC bilayer membrane by adding the ligand was discussed from the viewpoint of the ligand partitioning. Reflecting the contrastive partitioning of PA and HAL into the pressure-induced interdigitated gel phase among the gel phases, it was revealed that PA suppresses the formation of the interdigitated gel phase under high pressure while HAL promotes it. These results clearly indicate that each phase of the DPPC bilayer membrane has a potential to recognize various ligand molecules.

Effect of pressure on bilayer phase behavior of N-methylated di-O-hexadecylphosphatidylethanolamines: relevance of head-group modification on the bilayer interdigitation

The phase transitions of N-methylated di-O-hexadecylphosphatidylethanolamines (DHPE, DH-N-methyl-PE (DHMePE) and DH-N,N-dimethyl-PE (DHMe₂PE)) were observed by differential scanning calorimetry (DSC) and fluorometry under atmospheric pressure and by light-transmittance measurements under high pressure. The DSC thermograms showed that the N-methylated DHPE bilayers underwent the phase transition from the gel phase to the liquid crystalline (L_α) phase under atmospheric pressure. The gel phase was identified by fluorometry as the lamellar gel (L_β) phase, and not interdigitated gel (L_βI) phase. The gel/L_α transition temperature increased with pressure while decreased stepwise with increasing polar head-group size. This stepwise depression of the transition temperature may be caused by the inverse-proportional hydrogen-bonding capabilities of the head-group to the head-group size. The thermodynamic quantities of the gel/L_α transition were comparable for the N-methylated DHPE bilayers. The pressure-induced L_βI phase was not found in these bilayers although the bilayer of di-O-hexadecylphosphatidylcholine (DHPC), which is a kind of N-methylated DHPEs, forms the L_βI phase only by hydration under atmospheric pressure. Taking into account that the bilayers of diacyl-homologs of N-methylated DHPEs, N-methylated dipalmitoyl-PEs except for dipalmitoylphosphatidylcholine (DPPC), do not form the L_βI phase in the whole pressure range investigated but the DPPC bilayer forms the L_βI phase under high pressure, we can say that the interdigitation requires weaker interaction between large-sized head groups like the bulky choline group.

How Do Membranes Respond to Pressure?

Bilayers formed by phospholipids are fundamental structures of biological membranes. The mechanical perturbation brought about by pressure significantly affects the membrane states of phospholipid bilayers. In this chapter, we focus our attention on the pressure responsivity for bilayers of some major phospholipids contained in biological membranes. At first, the membrane states and phase transitions of phospholipid bilayers depending on water content, temperature and pressure are explained by using the bilayer phase diagrams of dipalmitoylphosphatidylcholine (DPPC), which is the most familiar phospholipid in model membrane studies. Subsequently, the thermotropic and barotropic bilayer phase behavior of various kinds of phospholipids with different molecular structures is discussed from the comparison of their temperature--pressure phase diagrams to that of the DPPC bilayer. It turns out that a slight change in the molecular structure of the phospholipids produces a significant difference in the bilayer phase behavior. The systematic pressure studies on the phase behavior of the phospholipid bilayers reveal not only the pressure responsivity for the bilayers but also the role and meaning of several important phospholipids existing in real biological membranes.

A methodological combined framework for roadmapping biosensor research: a fault tree analysis approach within a strategic technology evaluation frame

Biosensor technology began in the 1960s to revolutionize instrumentation and measurement. Despite the glucose sensor market success that revolutionized medical diagnostics, and artificial pancreas promise currently the approval stage, the industry is reluctant to capitalize on other relevant university-produced knowledge and innovation. On the other hand, the scientific literature is extensive and persisting, while the number of university-hosted biosensor groups is growing. Considering the limited marketability of biosensors compared to the available research output, the biosensor field has been used by the present authors as a suitable paradigm for developing a methodological combined framework for "roadmapping" university research output in this discipline. This framework adopts the basic principles of the Analytic Hierarchy Process (AHP), replacing the lower level of technology alternatives with internal barriers (drawbacks, limitations, disadvantages), modeled through fault tree analysis (FTA) relying on fuzzy reasoning to count for uncertainty. The proposed methodology is validated retrospectively using ion selective field effect transistor (ISFET) - based biosensors as a case example, and then implemented prospectively membrane biosensors, putting an emphasis on the manufacturability issues. The analysis performed the trajectory of membrane platforms differently than the available market roadmaps that, considering the vast industrial experience in tailoring and handling crystallic forms, suggest the technology path of biomimetic and synthetic materials. The results presented herein indicate that future trajectories lie along with nanotechnology, and especially nanofabrication and nano-bioinformatics, and focused, more on the science-path, that is, on controlling the natural process of self-assembly and the thermodynamics of bioelement-lipid interaction. This retained the nature-derived sensitivity of the biosensor platform, pointing out the differences between the scope of academic research and the market viewpoint.

How does acyl chain length affect thermotropic phase behavior of saturated diacylphosphatidylcholine-cholesterol binary bilayers?

Thermotropic phase behavior of diacylphosphatidylcholine (CnPC)-cholesterol binary bilayers (n=14-16) was examined by fluorescence spectroscopy using 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and differential scanning calorimetry. The former technique can detect structural changes of the bilayer in response to the changes in polarity around Prodan molecules partitioned in a relatively hydrophilic region of the bilayer, while the latter is sensitive to the conformational changes of the acyl chains. On the basis of the data from both techniques, we propose possible temperature T-cholesterol composition Xch phase diagrams for these binary bilayers. A notable feature of our phase diagrams, including our previous results for diheptadecanoylphosphatidylcholine (C17PC) and distearoylphosphatidylcholine (C18PC), is that there is a peritectic-like point around Xch=0.15, which can be interpreted as indicating the formation of a 1:6-complex of cholesterol and CnPCs within the binary bilayer irrespective of the acyl chain length. We could give a reasonable explanation for such complex formation using the modified superlattice view. Our results also showed that the Xch value of the abolition of the main transition is almost constant for n=14-17 (ca. 0.33), while it increases to ca. 0.50 for n=18. By contrast, a biphasic n-dependence of Xch was observed for the abolition of the pretransition, suggesting that there are at least two antagonistic n-dependent factors. We speculate that this could be explained by the enhancement of the van der Waals interaction with increases in n and the weakening of the repulsion between the neighboring headgroups with decreases in n.

Thermotropic and barotropic phase behavior of phosphatidylcholine bilayers

Bilayers formed by phospholipids are frequently used as model biological membranes in various life science studies. A characteristic feature of phospholipid bilayers is to undergo a structural change called a phase transition in response to environmental changes of their surroundings. In this review, we focus our attention on phase transitions of some major phospholipids contained in biological membranes, phosphatidylcholines (PCs), depending on temperature and pressure. Bilayers of dipalmitoylphosphatidylcholine (DPPC), which is the most representative lipid in model membrane studies, will first be explained. Then, the bilayer phase behavior of various kinds of PCs with different molecular structures is revealed from the temperature-pressure phase diagrams, and the difference in phase stability among these PC bilayers is discussed in connection with the molecular structure of the PC molecules. Furthermore, the solvent effect on the phase behavior is also described briefly.

Study on the subgel-phase formation using an asymmetric phospholipid bilayer membrane by high-pressure fluorometry

The myristoylpalmitoylphosphatidylcholine (MPPC) bilayer membrane shows a complicated temperature-pressure phase diagram. The large portion of the lamellar gel (L(β)'), ripple gel (P(β)'), and pressure-induced gel (L(β)I) phases exist as metastable phases due to the extremely stable subgel (L(c)) phase. The stable L(c) phase enables us to examine the properties of the L(c) phase. The phases of the MPPC bilayers under atmospheric and high pressures were studied by small-angle neutron scattering (SANS) and fluorescence spectroscopy using a polarity-sensitive fluorescent probe Prodan. The SANS measurements clearly demonstrated the existence of the metastable L(β)I phase with the smallest lamellar repeat distance. From a second-derivative analysis of the fluorescence data, the line shape for the L(c) phase under high pressure was characterized by a broad peak with a minimum of ca. 460 nm. The line shapes and the minimum intensity wavelength (λ″(min)) values changed with pressure, indicating that the L(c) phase has highly pressure-sensible structure. The λ″(min) values of the L(c) phase spectra were split into ca. 430 and 500 nm in the L(β)I phase region, which corresponds to the formation of a interdigitated subgel L(c) (L(c)I) phase. Moreover, the phase transitions related to the L(c) phase were reversible transitions under high pressure. Taking into account the fluorescence behavior of Prodan for the L(c) phase, we concluded that the structure of the L(c) phase is highly probably a staggered structure, which can transform into the L(c)I phase easily.

Thermotropic and barotropic phase transitions of dialkyldimethylammonium bromide bilayer membranes: effect of chain length

The bilayer phase transitions of dialkyldimethylammonium bromides (2C(n)Br; n = 12, 14, 16) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. Under atmospheric pressure, the 2C(12)Br bilayer membrane underwent the stable transition from the lamellar crystal (L(c)) phase to the liquid crystalline (L(α)) phase. The 2C(14)Br bilayer underwent the main transition from the metastable lamellar gel (L(β)) phase to the metastable L(α) phase in addition to the stable L(c)/L(α) transition. For the 2C(16)Br bilayer, moreover, three kinds of phase transitions were observed: the metastable main transition, the metastable transition from the metastable lamellar crystal (L(c(2))) phase to the metastable L(α) phase, and the stable lamellar crystal (L(c(1)))/L(α) transition. The temperatures of all the phase transitions elevated almost linearly with increasing pressure. The temperature (T)-pressure (p) phase diagrams of the 2C(12)Br and 2C(14)Br bilayers were simple, but that of the 2C(16)Br bilayer was complex; that is, the T-p curves for the metastable main transition and the L(c(2))/L(α) transition intersect at ca. 25 MPa, which means the inversion of the relative phase stability between the metastable phases of L(β) and L(c(2)) above and below the pressure. Moreover, the T-p curve of the L(c(2))/L(α) transition was separated into two curves under high pressure, and as a result, the pressure-induced L(c(2P)) phase appeared in between. Thermodynamic quantities for phase transitions of the 2C(n)Br bilayers increased with an increase in alkyl-chain length. The chain-length dependence of the phase-transition temperature for all kinds of transitions observed suggests that the stable L(c(1))/L(α) transition incorporates the metastable L(c(2))/L(α) transition in the bilayers of 2C(n)Br with shorter alkyl chains, and the main-transition of the 2C(12)Br bilayer would occur at a temperature below 0 °C.

Hydrostatic pressure reveals bilayer phase behavior of dioctadecyldimethylammonium bromide and chloride

Bilayer phase transitions of dioctadecyldimethylammonium bromide (2C(18)Br) and chloride (2C(18)Cl) were observed by differential scanning calorimetry and high-pressure light-transmittance measurements. The 2C(18)Br bilayer membrane showed different kinds of transitions depending on preparation methods of samples under atmospheric pressure. Under certain conditions, the 2C(18)Br bilayer underwent three kinds of transitions, the metastable transition from the metastable lamellar crystal (L(c(2))) phase to the metastable lamellar gel (L(β)) phase at 35.4 °C, the metastable main transition from the metastable L(β) phase to the metastable liquid crystalline (L(α)) phase at 44.5 °C, and the stable transition from the stable lamellar crystal (L(c(1))) phase to the stable L(α) phase at 52.8 °C. On the contrary, the 2C(18)Cl bilayer underwent two kinds of transitions, the stable transition from the stable L(c) phase to the stable L(β) phase at 19.7 °C and the stable main transition from the stable L(β) phase to the stable L(α) phase at 39.9 °C. The temperatures of the phase transitions of the 2C(18)Br and 2C(18)Cl bilayers were almost linearly elevated by applying pressure. It was found from the temperature (T)-pressure (p) phase diagram of the 2C(18)Br bilayer that the T-p curves for the main transition and the L(c(1))/L(α) transition intersect at ca. 130 MPa because of the larger slope of the former transition curve. On the other hand, the T-p phase diagram of the 2C(18)Cl bilayer took a simple shape. The thermodynamic properties for the main transition of the 2C(18)Br and 2C(18)Cl bilayers were comparable to each other, whereas those for the L(c(1))/L(α) transition of the 2C(18)Br bilayer showed considerably high values, signifying that the L(c(1)) phase of the 2C(18)Br bilayer is extremely stable. These differences observed in both bilayers are attributable to the difference in interaction between a surfactant and its counterion.

Chain elongation of diacylphosphatidylcholine induces fully bilayer interdigitation under atmospheric pressure

The phase transitions of dibehenoylphosphatidylcholine (C22PC) bilayer membrane were observed by differential scanning calorimetry under atmospheric pressure and light-transmittance measurements under high pressure. The constructed temperature-pressure phase diagram suggests that the gel phase at low temperatures is the interdigitated gel phase. To confirm the phase state, we performed small-angle neutron scattering and fluorescence measurements using a polarity-sensitive probe Prodan for the C22PC bilayer membrane under atmospheric pressure. The peaks obtained in both measurements clearly showed the characteristic patterns of the fully interdigitated gel phase. Taking into account of previous studies on the gel phase for long-chain PC bilayers under atmospheric pressure and our studies on the pressure-induced bilayer interdigitaion of diacyl-PCs, it turned out that the interdigitation of diacyl-PC bilayer membranes occurs when the carbon number of acyl chain reaches at least 22. The present study revealed that the interdigitation of PC bilayer membranes occurs not only by weakening the attractive force of polar head groups but also by strengthening the cohesive force of acyl chains. When dominating the force of acyl chains, the interdigitation can be induced even in a diacyl-PC bilayer membrane by only hydration under atmospheric pressure.

Effects of pentanol isomers on the phase behavior of phospholipid bilayer membranes

Differential scanning calorimetry (DSC) was used to analyze the thermotropic phase behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers in the presence of pentanol isomers. The concentration of each pentanol isomer needed to induce the interdigitated phase was determined by the appearance of a biphasic effect in the main transition temperatures, the onset of a hysteresis associated with the main transition from the gel-to-liquid crystalline phase, and the disappearance of the pretransition. Lower threshold concentrations were found to correlate with isomers of greater alkyl chain length while branching of the alkyl chain was found to increase biphasic behavior. The addition of a methyl group to butanol systems drastically decreased threshold concentrations. However, as demonstrated in the DPPC/neopentanol system, branching of the alkyl chain away from the -OH group lowers the threshold concentration while maintaining a biphasic effect.

Behavior of plant plasma membranes under hydrostatic pressure as monitored by fluorescent environment-sensitive probes

We monitored the behavior of plasma membrane (PM) isolated from tobacco cells (BY-2) under hydrostatic pressures up to 3.5kbar at 30 degrees C, by steady-state fluorescence spectroscopy using the newly introduced environment-sensitive probe F2N12S and also Laurdan and di-4-ANEPPDHQ. The consequences of sterol depletion by methyl-beta-cyclodextrin were also studied. We found that application of hydrostatic pressure led to a marked decrease of hydration as probed by F2N12S and to an increase of the generalized polarization excitation (GPex) of Laurdan. We observed that the hydration effect of sterol depletion was maximal between 1 and 1.5 kbar but was much less important at higher pressures (above 2 kbar) where both parameters reached a plateau value. The presence of a highly dehydrated gel state, insensitive to the sterol content, was thus proposed above 2.5 kbar. However, the F2N12S polarity parameter and the di-4-ANEPPDHQ intensity ratio showed strong effect on sterol depletion, even at very high pressures (2.5-3.5 kbar), and supported the ability of sterols to modify the electrostatic properties of membrane, notably its dipole potential, in a highly dehydrated gel phase. We thus suggested that BY-2 PM undergoes a complex phase behavior in response to the hydrostatic pressure and we also emphasized the role of phytosterols to regulate the effects of high hydrostatic pressure on plant PM.

Pressure effect on the bilayer phase transition of asymmetric lipids with an unsaturated acyl chain

The bilayer phase transitions of mixed-chain lipids with monounsaturated acyl chain in the sn-2 position, 1-myristoyl-2-oleoyl-sn-glycero-3-phosphocholine (MOPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and 1-stearoyl-2-oleoyl-sn-glycero-3-phosphocholine (SOPC), and with a polyunsaturated acyl chain in the sn-2 position, 1-stearoyl-2-linoleoyl-sn-glycero-3-phosphocholine (SLPC), 1-stearoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (SAPC), and 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (SDPC), were observed by differential scanning calorimetry (DSC) under ambient pressure and by light-transmittance measurements under high pressure. The DSC thermogram for each lipid bilayer showed only one transition between the lamellar gel and liquid crystalline phases. The introduction of one or two cis double bonds into the sn-2 acyl chain caused the significant depression of the main-transition temperature and an obvious decrease of enthalpy and volume changes associated with the transition. These features are attributable to loose packing of saturated and unsaturated acyl chains in the bilayer gel phase. The existence of four or six double bonds in the sn-2 chain produced no further decrease in the transition temperature, and in fact six double bonds caused a slight increase in the transition temperature. Thermodynamic properties associated with the bilayer phase transition were discussed.

A peculiar phase transition of plasmalogen bilayer membrane under high pressure

The bilayer phase transition of plasmalogen, monounsaturated plasmenylcholine 1-O-1'-(Z)-octadecenyl-2-oleoyl-sn-glycero-3-phosphocholine (Plg-SOPC), was examined by differential scanning calorimetry, high-pressure transmittance, and fluorescence techniques. The bilayer properties of Plg-SOPC such as the temperature-pressure phase diagram, the thermodynamic quantities of the transition, and the location of a fluorescent membrane probe in the bilayer, were compared with those of a similar phospholipid 1-stearoyl-2-oleoyl-phosphatidylcholine (SOPC). It turned out that a vinyl-ether bond in the sn-1 position of the glycerol backbone in the Plg-SOPC molecule produces a peculiar phase transition under high pressure and significantly affects the membrane properties.