Chain-melting Transition Research Articles

Calorimetric, volumetric and structural studies of the interaction between chlorogenic acid and dipalmitoylphosphatidylcholine bilayers

Chlorogenic acid (CGA) is the main component of coffee and an antioxidant. CGA has been reported to bear various good health effects. At the same time, it has been found that the addition of CGA induces an undesirable deformation of red blood cells. This fact suggests that CGA may bind to the proteins or/and membrane lipids of red blood cells. This study aimed to examine how CGA binds the bilayers of phosphatidylcholine (PC), one of red blood cells' primary lipids. To this end, we investigated the effect of CGA on the phase behavior and the structure of dipalmitoyl-PC (DPPC) bilayers in the form of multi-lamellar vesicles. Calorimetry and dilatometry measurements showed that the DPPC chain melting transition cooperativity decreases as increasing CGA concentrations. In addition, X-ray diffraction results showed that the lamellar repeat periodicity becomes disordered, and the periodicity disappears completely at high CGA concentrations. Together with these findings, it can be inferred that the CGA molecules do not penetrate inside the DPPC bilayers but bind to their surface in a negatively charged form.

Overlap large polaron sensitivity to phase changes in the first alkali earth organic-inorganic hybrid: [(CH2)7(NH3)2]CaCl4

Crystal structure characterization, lattice potential energy, conductivity relaxation and electric conduction mechanisms of the new alkaline earth metal organic-inorganic hybrid, [(CH2)7(NH3)2] CaCl4 are presented. The new hybrid crystallizes in a triclinic system (S.G. P‾1) with two molecules per asymmetric unit cell (Z = 2). Crystallographic parameters are: a = 7.029 Å, b = 8.003 Å, c = 14.039 Å, α = 65.43°, β = 69.99°, γ = 61.39°, the calculated volume and density are 620.3 A3 and 1.68 Mgm−3 respectively. Room temperature lattice potential energy is ΔUpot = 1932.34 (kJ/mol.) and enthalpy change ΔH = 453.8 (kJ/mol.). DSC results indicated weak peaks at T7 = 239 ± 1 K, T6∼ 258.6 ± 0.5 K, a sharp λ-like peak typical of first order T5∼ 266.5 ± 0.5 K, a commensurate to incommensurate at T4 = 279 K, an incommensurate to unmodulated at T3 = 283 ± 1 K, and a chain melting transition is noted with minor peak at T2 = 326.5 K and major one at T1 = 329.3 K. Dielectric constant measurements in the temperature range (170–420) K and frequency range 60 Hz- 40 kHz confirmed the DSC transitions. It is characterized by a nearly temperature and frequency independent background of low dielectric constant (ε’) except at the transitions and at T ≥ 344 ± 2 K where frequency dispersion takes place. At high temperatures, calculated conductivity relaxation energy and relaxation time, are ΔE = 0.78 eV and (το) = 7.81614 × 10−14 s respectively. Fit of frequency dependent ac-conductivity to super-linear power law σtot = σdc + A1ωs1+ A2ωs2 shows different conduction mechanisms in the different temperature ranges (phases). Overlap large polaron tunneling (OLPT) of small radius dominates in the temperature range (240–290) K, it is very sensitive to the phase transitions in this temperature range where the polaron radius varies as the phase changes, which is reflected as small variations of the fitting parameters. In phase VIII (T < 239 K), phase III (295 < T(K) < 326.5) and phase II (327 < T(K) < 339); quantum mechanical tunneling (QMT) is the conduction mechanism. The lower value of the fitting parameters in phase (II) indicates stronger interaction between mobile ions with the lattice than phase (III). At higher temperatures correlated barrier (CBH) hopping prevails. Comparison to other transition metal hybrids of the same organic chain is discussed.

Vitamin E - phosphatidylethanolamine interactions in mixed membranes with sphingomyelin: Studies by 2H NMR

Among the structurally diverse collection of lipids that comprise the membrane lipidome, polyunsaturated phospholipids are particularly vulnerable to oxidation. The role of α-tocopherol (vitamin E) is to protect this influential class of membrane phospholipid from oxidative damage. Whether lipid-lipid interactions play a role in supporting this function is an unanswered question. Here, we compare the molecular organization of polyunsaturated 1-[2H31]palmitoyl-2-docosahexaenoylphosphatidylethanolamine (PDPE-d31) and, as a control, monounsaturated 1-[2H31]palmitoyl-2-oleoylphosphatidylethanolamine (POPE-d31) mixed with sphingomyelin (SM) and α-tocopherol (α-toc) (2:2:1 mol) by solid-state 2H NMR spectroscopy. In both cases the effect of α-toc appears similar. Spectral moments reveal that the main chain melting transition of POPE-d31 and PDPE-d31 is broadened beyond detection. A spectral component attributed to the formation of inverted hexagonal HII phase in coexistence with lamellar Lα phase by POPE-d31 (20 %) and PDPE-d31 (18 %) is resolved following the addition of α-toc. Order parameters in the remaining Lα phase are increased slightly more for POPE-d31 (7%) than PDPE-d31 (4%). Preferential interaction with polyunsaturated phospholipid is not apparent in these results. The propensity for α-toc to form phase structure with negative curvature that is more tightly packed at the membrane surface, nevertheless, may restrict the contact of free radicals with lipid chains on phosphatidylethanolamine molecules that accumulate polyunsaturated fatty acids.

Ordering of Saturated and Unsaturated Lipid Membranes near Their Phase Transitions Induced by an Amphiphilic Cyclodextrin and Cholesterol.

When inserted in membranes of dimyristoyl phosphatidylcholine (DMPC), methylated β-cyclodextrins with one (TrimβMLC) or two (TrimβDLC) lauryl acyl chains grafted onto the hydrophilic cavity exert a "cholesterol-like ordering effect", by straightening the acyl chains in the fluid phase at temperatures near the chain melting transition. This effect may be related to pretransitional events such as the "anomalous swelling" known to occur with saturated phosphatidylcholine membranes. To investigate this model, order profiles and bilayer thicknesses of DMPC and unsaturated 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) membranes containing amphiphilic cyclodextrins or cholesterol were determined by deuterium NMR. The pure lipid membranes display both a qualitatively similar chain ordering upon cooling in the fluid phase, more important at the chain extremity, which gets more pronounced near their fluid-to-gel transitions. Both membranes show a bilayer thickness increase by ∼0.5 Å just above their transition, as observed previously with saturated phosphatidylcholines of various chain lengths. Membrane-insertion of 5% TrimβMLC or cholesterol induces an important ordering of the DMPC acyl chains just above the transition, which is also more pronounced at the chain extremity. There is an additional increase of the bilayer thickness, most probably due to a deep insertion of these amphiphilic molecules, facilitated by increased bilayer softness in the anomalous swelling regime. These effects are more important with TrimβMLC than with cholesterol. By contrast, no enhanced acyl chain ordering was observed when approaching the transition of TrimβMLC-containing POPC membranes, as a possible consequence of an eventual lack of anomalous swelling in unsaturated lipid membranes. Insertion of higher concentrations of TrimβMLC was found to induce a magnetic orientation of the DMPC membranes in the fluid phase with 10% of this derivative, coupled with the appearance of a broad isotropic component when the concentration is raised to 20%. No membrane orientation or isotropic component was detected with TrimβMLC-containing POPC membranes.

Stability of the two-dimensional lattice of bacteriorhodopsin reconstituted in partially fluorinated phosphatidylcholine bilayers

This study aims to investigate bacteriorhodopsin (bR) molecules reconstituted in lipid bilayers composed of di(nonafluorotetradecanoyl)-phosphatidylcholine (F4-DMPC), a partially fluorinated analogue of dimyristoyl-phosphatidylcholine (DMPC) to clarify the effects of partially fluorinated hydrophobic chains of lipids on protein's stability. Calorimetry measurements showed that the chain-melting transition of F4-DMPC/bR systems occurs at 3.5 °C, whereas visible circular dichroism (CD) and X-ray diffraction measurements showed that a two-dimensional (2D) hexagonal lattice formed by bR trimers in F4-DMPC bilayers remains intact even above 30 °C, similar to bR in a native purple membrane. Complete dissociation of the trimers into the monomers detected by visible CD almost coincides with the complete melting of 2D lattice observed by X-ray diffraction, in which both take place at around 65 °C (10 °C lower than that for bR in a native purple membrane). However, it is extremely high in comparison with the bR reconstituted in DMPC bilayers in which the dissociation of bR trimer in DMPC bilayers occurs near the chain-melting transition temperature of DMPC bilayers at approximately 18 °C. In order to explore the rationale behind the difference in stability, a further investigation of the detailed structural features of pure F4-DMPC bilayers was performed by analyzing the lamellar diffraction data using simple electron density models. The results suggested that the perfluoroalkyl groups do not exhibit any conformation change even if the chain-melting transition occurs, which is likely to contribute to the stability of the 2D hexagonal lattice formed by the bR trimers.

Impedance spectroscopic and phase transition study of a new Organic –Inorganic alkaline earth hybrid

The new organic– inorganic hybrid 1,7-heptanediammonium-calcium tetrachloride, with molecular formula [(CH2)7(NH3)2] CaCl4, crystallized in a monoclinic Pm space group, a= 4.625 A˚ , b = 11.205 A˚ , c = 12.391 A˚ ,  = 900  = 86.82 0,  = 900, V = 641.2 A˚3, density= 1.63 mgm-3 and Z= 2. Differential thermal scanning of the new organic–inorganic hybrid showed five phase changes; chain melting transitions at T1= 329.2 K and T2 = 326.49 K and three transitions at T3= 283.6 K, T4= 278.8 K and T5= 260.8 K associated with solid– solid phase changes. Permittivity and ac conductivity as a function of temperature (170 K- 425 K) and frequency (0.06 kHz

Phase Behavior Controlled by the Addition of Long-Chain n-Alcohols in Systems of Cationic Surfactant/Anionic Polyion Complex Salts and Water.

Phase behavior of surfactants in water may be affected by the addition of a third component, and the present study discusses how long-chain n-alcohols affect phase transitions of systems formed by the surfactant hexadecyltrimethylammonium bromide, C16TAB, or its complex salts formed with polyacrylate, C16TAPA30, as well as other previously reported complex salts/water/alcohol systems. Structural characterization by X-ray diffraction patterns at small and wide angles and different temperatures was performed for samples containing n-decanol, n-dodecanol, or n-tetradecanol. Differential scanning calorimetry (DSC) was also used to study the phase transition. The results allowed us to observe and understand the coexistence of lamellar gel (Lβ) and lamellar liquid-crystal (Lα) phases, elucidating the structure of a previously reported mesophase, proposing an alternative assignment. Whereas the chain-melting transition is well-known to be sharp for lipids, we have found that it is broader for C16TAB and C16TAPA in the presence of these n-alcohols. We have investigated the effects of their composition and chain length on the temperature and enthalpy of transition. This elucidates why the addition of n-alcohols with chains slightly shorter than that of the surfactants leads to the formation of an ordered gel-like lamellar phase (Lβ). n-Alcohols act as neutral cosurfactants, leading to more packing, and all of the factors converge to a limit situation, associated with a common critical area occupied by each alkyl chain. We compared our results with other mesophase systems from the literature, demonstrating that the same trends of phase behavior occur for complex salts of other polyelectrolytes with alkyltrimethylammonium surfactants.

Multivariate Analysis of Mixed Lipid Aggregate Phase Transitions Monitored Using Raman Spectroscopy.

The phase behavior of aqueous 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC)/1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC) mixtures between 8.0 ℃ and 41.0 ℃ were monitored using Raman spectroscopy. Temperature-dependent Raman matrices were assembled from series of spectra and subjected to multivariate analysis. The consensus of pseudo-rank estimation results is that seven to eight components account for the temperature-dependent changes observed in the spectra. The spectra and temperature response profiles of the mixture components were resolved by applying a variant of the non-negative matrix factorization (NMF) algorithm described by Lee and Seung (1999). The rotational ambiguity of the data matrix was reduced by augmenting the original temperature-dependent spectral matrix with its cumulative counterpart, i.e., the matrix formed by successive integration of the spectra across the temperature index (columns). Successive rounds of constrained NMF were used to isolate component spectra from a significant fluorescence background. Five major components exhibiting varying degrees of gel and liquid crystalline lipid character were resolved. Hydrogen-bonded water networks exhibiting varying degrees of organization are associated with the lipid components. Spectral parameters were computed to compare the chain conformation, packing, and hydration indicated by the resolved spectra. Based on spectral features and relative amounts of the components observed, four components reflect long chain lipid response. The fifth component could reflect the response of the short chain lipid, DHPC, but there were no definitive spectral features confirming this assignment. A minor component of uncertain assignment that exhibits a striking response to the DMPC pre-transition and chain melting transition also was recovered. While none of the spectra resolved exhibit features unequivocally attributable to a specific aggregate morphology or step in the gelation process, the results are consistent with the evolution of mixed phase bicelles (nanodisks) and small amounts of worm-like DMPC/DHPC aggregates, and perhaps DHPC micelles, at low temperature to suspensions of branched and entangled worm-like aggregates above the DMPC gel phase transition and perforated multi-lamellar aggregates at high temperature.

Mechano-capacitive properties of polarized membranes and the application to conductance measurements of lipid membrane patches

Biological membranes are capacitors that can be charged by applying an electric field across the membrane. The charges on the capacitor exert a force on the membrane that leads to electrostriction, i.e., a thinning of the membrane. This effect is especially strong close to chain melting transitions. A consequence is voltage induced pore formation in the lipid membrane. Since the force is quadratic in voltage, negative and positive voltages have an identical influence on the physics of symmetric membranes. This is not the case for a membrane with an asymmetry leading to a permanent electric polarization. Positive and negative voltages of identical magnitude lead to different physical properties. Such an asymmetry can originate from a lipid composition that is different on the two monolayers of the membrane, or from membrane curvature. The latter effect is called flexoelectricity. It was investigated in detail by A.G. Petrov in the recent decades. As a consequence of permanent polarization, the membrane capacitor is discharged at a voltage different from zero. This leads to interesting electrical phenomena such as outward or inward rectification of membrane permeability. The changes in current-voltage relationships are consistent with the known magnitude of the flexoelectric effect.

Phase behaviour of dehydrated phosphatidylcholines

Dehydrated DLPC, DMPC, DPPC and DSPC have been characterised at temperatures below the diacyl carbon chain-melting transition (Tm), using DSC. For the first time, the existence of pre-Tm transition processes, which are, usually, only observed in the colloidal/liposomal state of saturated phospholipids, has been detected for the dehydrated phosphatidylcholines. Temperature-modulated differential scanning calorimetry was used to characterise the several complexes, overlapping pre-Tm transition processes. Kinetic studies of the chain-melting (Tm) transition show the activation energy dependence on α (conversion rate), i.e. activation energy decreases as the transition progresses, pointing to the importance of initial cooperative (intra- and inter-molecular) mobility. Furthermore, the activation energy increases with increase in diacyl chain length of the phosphatidylcholines which supports the finding that greater molecular interactions of the polymer chain and its head groups in the dehydrated solid state lead to enhanced stability of dehydrated phosphatidylcholines.

Crystal structure, phase transition and conductivity study of two new organic – inorganic hybrids: [(CH2)7(NH3)2]X2, X = Cl/Br

Two hybrids 1,7-heptanediammonium di-halide, [(C7H20N2]X2,X = Cl/Br crystallize in monoclinic P21/c, Z = 4. [(C7H20N2]Cl2: a = 4.7838 (2) Å, b = 16.9879 (8) Å, c = 13.9476 (8) Å, β = 97.773 (2)°, V = 1203.58(10) Å3, D = 1.137 g/cm3, λ = 0.71073 Å, R = 0.052 for 1055 reflections with I > 2σ(I), T = 298(2) K. [(C7H20N2]Br2: a = 4.7952 (10) Å, b = 16.9740 (5) Å, c = 13.9281 (5) Å, β = 97.793 (2)°, V = 1203.83(6) Å3, D = 1.612 g/cm3, λ = 0.71073 Å, R = 0.03 for 1959 reflections with I > 2σ(I) T = 298(2) K. Asymmetric unit cell of [(C7H20N2]X2,X = Cl/Br, each consist of one heptane-1,7-diammonium cation and two halide anions. The organic hydrocarbon layers pack in a stacked herring-bone manner, hydrogen bonded to the halide ions. Lattice potential energy is 1568.59 kJ/mol and 1560.78 kJ/mol, and cation molar volumes are 0.295 nm3 and 0.300 nm3 for chloride and bromide respectively. DTA confirmed chain melting transitions for both hybrids below T ∼ 340 K. Dielectric and ac conductivity measurements (290 < T K < 410; 0.080 < f kHz<100) indicated higher conductivity and activation energy of bromide for T > 340 K. Cross over from Jonscher's universal dielectric response at low temperatures T < 340 K to super-linear power law for T > 340 K is observed. At high temperatures halide ion hopping in accordance with the jump relaxation model prevails.

Effect of Anesthetics on Action Potential Propagation

Local anesthesia has been attributed to the specific interaction of local anesthetics with (sodium) channel proteins, while the action of general anesthetics still remains unclear. However, already at the beginning of 20th century Meyer and Overton independently found that the critical anesthetic dose of anesthetics, regardless of their types, is linearly proportional to their solubility in olive oil. Based on this finding, in 2005 Heimburg and Jackson proposed that the action potential is a density pulse (soliton) propagating in biological membranes, which explains anesthesia by freezing point depression law that originates from van’t Hoff's law. In this work, we conduct experiments on nerves from lobsters and earthworms to study the effect of anesthetics on compound action potential and action potential from a single neuron. The experimental data are then compared with the simulation results by solving the soliton equations in 1D cylindrical membrane with anesthetics inside the system. Anesthetics move the chain melting transition temperature of membranes far away from the physiological temperature, which requires a higher free energy to induce the phase transition, resulting in a higher stimulation voltage to reach the maximum amplitude of the action potential.

Combination of MD Simulations with Two-State Kinetic Rate Modeling Elucidates the Chain Melting Transition of Phospholipid Bilayers for Different Hydration Levels.

The phase behavior of membrane lipids plays an important role in the formation of functional domains in biological membranes and crucially affects molecular transport through lipid layers, for instance, in the skin. We investigate the thermotropic chain melting transition from the ordered Lβ phase to the disordered Lα phase in membranes composed of dipalmitoylphosphatidylcholine (DPPC) by atomistic molecular dynamics simulations in which the membranes are subject to variable heating rates. We find that the transition is initiated by a localized nucleus and followed by the propagation of the phase boundary. A two-state kinetic rate model allows characterizing the transition state in terms of thermodynamic quantities such as transition state enthalpy and entropy. The extrapolated equilibrium melting temperature increases with reduced membrane hydration and thus in tendency reproduces the experimentally observed dependence on dehydrating osmotic stress.

One-dimensional chain melting in incommensurate potassium

Between 19 and 54 GPa, potassium has a complex composite incommensurate host-guest structure which undergoes two intraphase transitions over this pressure range. The temperature dependence of these host-guest phases is further complicated by the onset of an order-disorder transition in their guest chains. Here, we report single-crystal, quasi-single-crystal, and powder synchrotron x-ray diffraction measurements of this order-disorder phenomenon in incommensurate potassium to 47 GPa and 750 K. The so-called chain melting transition is clearly visible over a 22 GPa pressure range, and there are significant changes in the slope of the phase boundary which divides the ordered and disordered phases, one of which results from the intraphase transitions in the guest structure.

Mixing of oxidized and bilayer phospholipids

Composition and phase dependence of the mixing of 1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), and 1,2-Dioleoyl-sn-glycero-3-phosphocholine (DOPC), with the oxidized phospholipid, 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (PGPC) were investigated by characterizing the aggregation states of DPPC/PGPC and DOPC/PGPC using a fluorescence quenching assay, dynamic light scattering, and time-resolved fluorescence quenching in the temperature range 5–60°C. PGPC forms 3.5nm radii micelles of aggregation number 33. In the gel phase, DPPC and PGPC fuse to form mixed vesicles for PGPC molar fraction, XPGPC≤0.3 and coexisting vesicles and micelles at higher XPGPC. Data suggest that liquid phase DPPC at 50°C forms mixed vesicles with segregated or hemi fused DPPC and PGPC for XPGPC≤0.3. At 60°C, DPPC and PGPC do not mix, but form coexisting vesicles and micelles. DOPC and PGPC do not mix in any proportion in the liquid phase. Two dissimilar aggregates of the sizes of vesicles and PGPC micelles were observed for all XPGPC for T≥22°C. DOPC–PGPC and DPPC–PGPC mixing is non-ideal for XPGPC>0.3 in both gel and fluid phases resulting in exclusion of PGPC from the bilayer. Formation of mixed vesicles is favored in the gel phase but not in the liquid phase for XPGPC≤0.3. For XPGPC≤0.3, aggregation states change progressively from mixed vesicles in the gel phase to component segregated mixed vesicles in the liquid phase close to the chain melting transition temperature to separated coexisting vesicles and micelles at higher temperatures.

Synthesis, physicochemical characterization and membrane interactions of a homologous series of N-acylserotonins: Bioactive, endogenous conjugates of serotonin with fatty acids

N-Acylserotonins (NASTs), present in the mammalian gastro-intestinal tract and central nervous tissues, exhibit significant biological and pharmacological activities. In the present study, a homologous series of NASTs have been synthesized and characterized. Differential scanning calorimetric studies show that in the dry and hydrated states the transition temperatures, enthalpies, and entropies of NASTs exhibit odd–even alternation. Both odd and even chain length NASTs independently display linear dependence of the transition enthalpies and entropies on the chain length under dry as well as hydrated conditions, suggesting that the molecular packing and intermolecular interactions in each series (odd or even) are likely to be similar for NASTs with different acyl chain lengths in the dry state as well as in the hydrated state. Powder X-ray diffraction studies indicated that the incremental increase in the d-spacing per CH2 group is 1.023Å, suggesting that the lipid acyl chains are most likely packed in an interdigitated fashion. Results of computational studies are consistent with this and suggest that the acyl chains of the NASTs are tilted with respect to the bilayer normal. Incorporation of N-myristoylserotonin (NMST) into dimyristoylphosphatidylcholine (DMPC) membranes did not significantly affect the phase transition properties at low mole fractions (1–5mol%), although distinct decrease in the chain-melting transition temperature and increase in the pretransition temperature were observed at higher contents (7.5–30mol%), suggesting that NMST increases the stability of the tilted gel phase (Lβ′) but destabilizes the ripple phase (Pβ′). These observations provide a thermodynamic basis for understanding the functional role of NASTs in their parent tissues.

Differential scanning calorimetric studies on the thermotropic phase behavior of dry and hydrated forms of N-acyltyramines

In this paper, a homologous series of N-acyltyramine (NATA) – which is biosynthetic precursor for neuroactive lipids, N-acyldopamine – have been synthesized and their thermotropic phase transitions were characterized by differential scanning calorimetry (DSC). NATA undergoes a major sharp endothermic transition with the melting point of the hydrated samples occurs considerably at lower temperature compared to the dry samples. Thermodynamic parameters, transition enthalpy (ΔHt) and transition entropy (ΔSt), associated with the chain-melting phase transitions depends linearly on the number of carbon atoms. The contributions of each methylene unit to the transition enthalpy and entropy and the end contributions arising from the head group and the terminal group was determined and reported. These results are very important to understand the thermodynamics basis of NATA phase properties in other membrane lipids.

Simulation of Influence of Bilayer Melting on Dynamics and Thermodynamics of Interfacial Water

We investigate the effect of bilayer melting transition on thermodynamics and dynamics of interfacial water using molecular dynamics simulation with the two-phase thermodynamic model. We show that the diffusivity of interface water depicts a dynamic crossover at the chain melting transition following an Arrhenius behavior until the transition temperature. The corresponding change in the diffusion coefficient from the bulk to the interface water is comparable with experimental observations found recently for water near 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) vesicles [Phys. Chem. Chem. Phys. 13, 7732 (2011)]. The entropy and potential energy of interfacial water show distinct changes at the bilayer melting transition, indicating a strong correlation in the thermodynamic state of water and the accompanying first-order phase transition of the bilayer membrane.

Chiral hierarchical self-assembly in Langmuir monolayers of diacetylenic lipids

When compressed in the intermediate temperature range below the chain-melting transition yet in the low-pressure liquid phase, Langmuir monolayers made of chiral lipid molecules form hierarchical structures. Using Brewster angle microscopy to reveal this structure, we found that as the liquid monolayer is compressed, an optically anisotropic condensed phase nucleates in the form of long, thin claws. These claws pack closely to form stripes. This appears to be a new mechanism for forming stripes in Langmuir monolayers. In the lower temperature range, these stripes arrange into spirals within overall circular domains, while near the chain-melting transition, the stripes arrange into target patterns. We attributed this transition to a change in boundary conditions at the core of the largest-scale circular domains.

The Capacitance and Electromechanical Coupling of Lipid Membranes Close to Transitions: The Effect of Electrostriction

Biomembranes are thin capacitors with the unique feature of displaying phase transitions in a physiologically relevant regime. We investigate the voltage and lateral pressure dependence of their capacitance close to their chain melting transition. Because the gel and the fluid membrane have different area and thickness, the capacitance of the two membrane phases is different. In the presence of external fields, charges exert forces that can influence the state of the membrane, thereby influencing the transition temperature. This phenomenon is called “electrostriction”. We show that this effect allows us to introduce a capacitive susceptibility that assumes a maximum in the melting transition with an associated excess charge. As a consequence, voltage regimes exist in which a small change in voltage can lead to a large uptake of charge and a large capacitive current. Furthermore, we consider electromechanical behavior such as pressure-induced changes in capacitance, and the application of such concepts in biology.