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PubMed1987

Phase transitions in sequence matches and nucleic acid structure.

Analyses of phase transitions in biopolymers have previously been restricted to studies of average behavior along macromolecules. Extremal properties, such as longest helical region, can now be studied with a new family of probability distributions [Arratia, R., Gordon, L. & Waterman, M. S. (1986) Ann. Stat. 14, 971-993]. Not only is such extremal behavior analyzed with great precision, but new phase transitions are determined. One phase transition occurs when behavior of the free energy of the longest helical region abruptly changes from proportional to sequence length. The annealing of two single-stranded molecules and the melting of a double helix are both considered. These results, initially suggested by studies of optimal matching of random DNA sequences [Smith, T. F., Waterman, M. S. & Burks, C. (1985) Nucleic Acids Res. 13, 645-656], also have importance for significance tests in comparison of nucleic acid or protein sequences.

Base Sequence↗

PubMed1988

Structures and mechanisms of lipid phase transitions in nonaqueous media. Dipalmitoylphosphatidylethanolamine in fused salt.

The phase transitions for dipalmitoylphosphatidylethanolamine (DPPE) dispersed in water and in N-ethylammonium nitrate (EAN) were examined using differential scanning calorimetry and time-resolved x-ray diffraction. Subgel, pre-, and main-phase transitions were observed for DPPE in water, whereas only the pre- and main transitions were observed for DPPE in EAN. Hysteresis was observed for both dispersions upon cooling. In addition, the lamellar (L alpha) to hexagonal (H alpha) phase transition was observed for DPPE dispersed in EAN when using time-resolved x-ray diffraction but not when using calorimetry. This low enthalpy process occurred at 73-77 degrees C, which is significantly lower than that observed for DPPE in water. The presence of EAN stabilizes the existence of the H alpha phase in DPPE by its influence on the bilayer interfacial properties, primarily on the area per lipid head group.

Calorimetry, Differential Scanning↗

PubMed1987

Dynamic properties of the haptenic site of lipid haptens in phosphatidylcholine membranes. Their relation to the phase transition of the host lattice.

The relation between the dynamic properties of the haptenic site of lipid haptens and the phase transition of the host lattice was investigated using head group spin-labeled phosphatidylethanolamines, that is, spin-label lipid haptens (Brûlet, P., and H. M. McConnell, 1976, Proc. Natl. Acad. Sci. USA., 73:2977-2981; Brûlet, P., and H. M. McConnell, 1977, Biochemistry, 16:1209-1217). The electron spin resonance (ESR) spectra of the lipid haptens in liposomal membranes showed three narrow resonance lines, whose widths and hyperfine splitting values suggested that the haptenic site, i.e., the spin-label moiety, should be exposed in the water phase. The line width of each peak depended on the host lipid species and on the incubation temperature. A temperature study using dipalmitoylphosphatidylcholine (DPPC) liposomes showed that the dynamic properties of the haptenic site were related to the main phase transition and the subphase transition of the host lattice but not to the prephase transition. The angular amplitudes of the tumbling motion of the haptenic site were estimated using oriented multibilayer systems. The angular amplitude of dipalmitoyl-phosphatidyl-N-[[N-(1-oxyl-2,2,6, 6-tetramethyl-4-piperidinyl)-carbamoyl]-methyl]-ethanolamine in DPPC membranes was 63 degrees at 2 degrees C, and it increased slightly with an increase in temperature regardless of the phase transition of the host lattice. The value for egg phosphatidylcholine (PC) at 25 degrees C was the same as for DPPC above its main phase transition temperature. Rotational correlation time analysis showed that the axial rotation of the haptenic site was preferable to the tumbling motion of the rotational axis, and the predominance depended on the phase transition, Lc----L beta' and P beta'----L alpha. Elongation of the spacer arm between the haptenic site and phosphate increased the angular amplitude of the tumbling motion but reduced the effect of the host lattice. Spin-label lipid haptens with unsaturated fatty acyl chains were distributed heterogeneously in DPPC membranes, whereas those with the same fatty acyl chain as the host lattice were distributed randomly. The ESR spectrum of a lipid hapten under its prephase transition temperature showed two components, broad and narrow. This suggests that at least two different domains, a hapten-rich domain and a hapten-poor one, may coexist in membranes. ESR measurements at various temperatures suggested that the haptenic site fraction in the hapten-rich domain decreased in part during the phase transition from L beta' to P beta', and disappeared completely in the La phase. The spatial mobility and lateral diffusion of lipid haptens will be discussed in greater detail.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1998

X-Ray Diffraction Study of the Lamellar-Hexagonal Phase Transition in Phospholipid/Surfactant Mixtures.

The lamellar-to-hexagonal phase transition of a phospholipid/ surfactant mixed system of 1-palmitoyl-2-oleoyl-sn-glycero-3phosphocholine (POPC) and oligo(ethylene oxide)dodecyl ethers of type C12H25O(CH2CH2O)2H(C12E2) in molar surfactant/phospholipid ratio (RS/L) of 2 at low hydration driven by temperature has been studied by X-ray diffraction. The Lbeta-HII phase transition is a reversible two-state process showing hysteresis at fast temperature scan rates. The obtained hexagonal phase exhibits a temperature dependent structural change. The numbers of bound water molecules per composite particle (WS+L) absorbed in the lamellar and hexagonal phases are nearly the same, changing from WS+L = 5.0 to 4.7 during the phase transition. The fluidity of the alkyl chains on increasing the temperature and the close packing of the hydrophilic molecular parts are the driving parameters of the lamellar-to-hexagonal transformation. Copyright 1998 Academic Press.

Journal Article↗

PubMed1990

A solid-solution theory of anesthetic interaction with lipid membranes: temperature span of the main phase transition.

Anesthetics (or any other small additives) depress the temperature of the main phase transition of phospholipid bilayers. Certain anesthetics widen the temperature span of the transition, whereas others do not. The widening in a first-order phase transition is intriguing. In this report, the effects of additive molecules on the temperature and its span were explained by the solid-solution theory. By assuming coexistence of the liquid-crystal and solid-gel phases of lipid membranes at phase transition, the phase boundary is determined from the distribution of anesthetic molecules between the liquid-crystal membrane versus water and between the solid-gel membrane versus water. The theory shows that when the lipid concentration is large or when the lipid solubility of the drug is large, the width of the transition temperature increases, and vice versa. Highly lipid-soluble molecules, such as long-chain alkanols and volatile anesthetics, increase the width of the transition temperature when the lipid:water ratio is large, whereas highly water-soluble molecules, such as methanol and ethanol, do not. The aqueous phase serves as the reservoir for anesthetics. Depletion of the additive molecules from the aqueous phase is the cause of the widening. When the reservoir capacity is large, the temperature width does not increase. The theory also predicts asymmetry of the specific heat profile at the transition.

Anesthetics↗

PubMed1981

Influence of dicarboxylic phosphatidylcholines on the stability and phase transition of phosphatidylcholine liposomes.

The effect of dicarboxylic phosphatidylcholines (glutaryl phosphatidylcholine) on the stability and phase transition of phosphatidylcholine liposomes is examined by using liposomes prepared with egg phosphatidylcholine or dipalmitoyl phosphatidylcholine and by varying the surface charge by addition of dicetyl phosphate. Light-scattering and osmotic behaviour studies showed that the stability of liposomes containing dicarboxylic phosphatidylcholine is influenced by the charge and the fatty acid saturation of the liposomes. Increasing the glutaryl phosphatidylcholine-to-phosphatidylcholine molar ratio in liposomes caused the formation of mixed glutaryl phosphatidylcholine/phosphatidylcholine micelles. The sensitivity of the lipid bilayers towards glutaryl phosphatidylcholine action increases with the fatty acid saturation of liposomes. Dipalmitoyl phosphatidylcholine liposomes are most sensitive to the dicarboxylic phosphatidylcholine effect. Dicetyl phosphate addition enhances the solubilization of liposomes prepared from saturated phospholipids. The effect of increasing concentrations of glutaryl phosphatidylcholine on the gel-to-liquid crystal thermal transition of dipalmitoyl phosphatidylcholine was observed. Glutaryl phosphatidylcholine modifies the thermal phase transition of the constituents of the liposome. The presence of dicetyl phosphate in liposomes affects the phase transition temperature of these liposomes. It is suggested that the formation of the mixed micelles is responsible for the phase transition modifications. These data show that the solubilization of liposomes by dicarboxylic phosphatidylcholines depends on the fatty acid composition of phosphatidylcholine and on the presence of dicetyl phosphate.

Liposomes↗

PubMed1979

Antagonism between high pressure and anesthetics in the thermal phase-transition of dipalmitoyl phosphatidylcholine bilayer.

The antagonizing action of hydrostatic pressure against anesthesia is well known. The present study was undertaken to quantitate the effects of hydrostatic pressure and anesthetics upon the phase-transition temperature of dipalmitoyl phosphatidylcholine vesicles. The drugs used to anesthetize the phospholipid vesicles included an inhalation anesthetic, halothane, a dissociable local anesthetic, lidocaine and an undissociable local anesthetic, benzyl alcohol. All anesthetics decreased the phase-transition temperature dose-dependently. In the case of lidocaine, the depression was pH dependent and only uncharged molecules were effective. The application of hydrostatic pressure increased the phase-transition temperature both in the presence and the absence of anesthetics. The temperature-pressure relationship was linear over the entire pressure range studied up to 340 bars. Through the use of Clapeyron-Clausius equation, the volume change accompanying the phase-transition of the membrane was calculated to be 27.0 cm3/mol. Although the anesthetics decreased the phase-transition temperature, the molar volume change accompanying the phase-transition was not altered. The anesthetics displaced the temperature-pressure lines parallel to each other. The mole fraction of the anesthetics in the liquid crystalline membrane, calculated from the van't Hoff equation, was independent of pressure. This implies that pressure does not displace the anesthetics from the liquid membrane, and the partition of these agents remains constant. The volume change of the anesthetized phospholipid membranes is entirely dependent upon the phase-transition and not on the space occupied by the anesthetics.

Anesthetics↗

PubMed1999

Two-photon fluorescence microscopy observation of shape changes at the phase transition in phospholipid giant unilamellar vesicles.

Using the sectioning effect of the two-photon fluorescence microscope, we studied the behavior of phospholipid giant unilamellar vesicles (GUVs) composed of pure diacylphosphatidylcholine phospholipids during the gel-to-liquid crystalline phase transition. We used the well-characterized excitation generalized polarization function (GP(ex)) of 6-dodecanoyl-2-dimethylamine-naphthalene (LAURDAN), which is sensitive to the changes in water content in the lipid vesicles, to monitor the phase transition in the GUVs. Even though the vesicles do not show temperature hysteresis at the main phase transition, we observed different behaviors of the vesicle shape, depending on how the GUV sample reaches the main phase transition. During the cooling cycles, we observed an increase in the vesicle diameter at the phase transition ( approximately 0.5-1%), followed by a decrease in the diameter when the vesicle reached the gel phase. During the heating cycles and close to the phase transition temperature, a surprising behavior is observed, showing a sequence of different vesicle shapes as follows: spherical-polygonal-ellipsoidal. We attribute these changes to the effect of lipid domain coexistence on the macroscopic structure of the GUVs. The "shape hysteresis" in the GUVs is reversible and largely independent of the temperature scan rate. In the presence of 30 mol% of cholesterol the events observed at the phase transition in the GUVs formed by pure phospholipids were absent.

2-Naphthylamine↗

PubMed1998

Investigation of phase transitions in bilayer membranes.

This article described three techniques used to study phase transitions in phospholipid bilayers. The complementarity of the three techniques in characterizing the thermotropic and structural properties of phospholipid bilayers has been demonstrated by describing their use to characterize a series of mixed-chain-length PCs. It has been shown that an understanding of the energetics that govern the packing of phospholipid chains in the gel phase can be used to construct a model to interpret thermodynamic data of the PCs. This model, in turn, provided a framework for designing and interpreting the Raman spectroscopic and X-ray diffraction experiments on this series of phospholipids. The result was a complete description of the phase transitions and gel phase packing properties of the mixed-chain-length PCs. The phase diagram of Fig. 5B has been expanded to include the mixed-chain-length PC series C18C18PC through C18C0PC. Furthermore, the phase diagram and the chain inequivalence parameter have been shown to describe the behavior of any mixed-chain-length PC, irrespective of the lengths of the hydrocarbon chains or the position of the chains on the glycerol backbone. This is demonstrated by the additional mixed-chain-length PCs plotted in Fig. 5B. With minor modifications, the phase diagram also accurately describes the behavior of mixed-chain-length phosphatidylethanolamines, sphingomyelins, and unsaturated PCs. Finally, it has been demonstrated that a correlation exists between the thermodynamic and the Raman spectroscopic parameters determined for the phase transition of phospholipid bilayers. This correlation is based on the common chain energetics being measured by these two techniques.

Calorimetry, Differential Scanning↗

PubMed1979

Studies on human finger tapping neural networks by phase transition curves.

The human finger tapping was studied psychologically by the phase transition curves. We assumed that some oscillatory neural network controls the tapping. One of our future works on the human finger tapping is to devise a method to change the magnitude of perturbation continuously in phase resetting experiments of a constant tapping interval with a same kind of task. If this is achieved, we can obtain a phase transition surface (i.e. the new phase as a function of the old phase and the magnitude of perturbation). From the shape of a phase transition surface, the intrinsic properties of the oscillatory network which controls the tapping can be studied (Kawato et al., 1978).

Conditioning, Psychological↗

PubMed1986

Interaction of surfactants with vesicle membrane of dipalmitoylphosphatidylcholine. Effect on gel-to-liquid-crystalline phase transition of lipid bilayer.

The gel-to-liquid-crystalline phase transition of dipalmitoylphosphatidylcholine (DPPC) vesicle membrane was observed in the presence of various types of surfactants; sodium alkylsulfates, alkyltrimethylammonium bromides, alkanoyl-N-methylglucamides, and hexaethyleneglycol mono n-dodecyl ether. The phase transition was monitored by a change in scattered light intensity of the lipid suspension. For all the surfactants examined, the phase transition temperature was depressed linearly with the surfactant concentration in the measured concentration range, from which the partition coefficient, K, of the surfactant between bulk solution and lipid membrane was estimated. Except alkyltrimethylammonium bromides, log K and log CMC showed a linear relationship, which indicates that the driving force to transfer the surfactant from bulk solution to lipid membrane is a hydrophobic interaction. The addition of surfactants increased the transition width. The extent of widening the transition width was in the order of sodium alkylsulfate greater than alkyltrimethylammonium bromides greater than hexaethyleneglycol mono n-dodecyl ether; in the case of alkanoyl-N-methylglucamides, the transition width was not affected by the addition. These effects on the transition width was interpreted qualitatively in terms of the cooperativity of the transition.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1983

Order-disorder phase transition and lipid dynamics in rabbit small intestinal brush border membranes. Effect of proteins.

The total lipids extracted from brush border membranes form smectic lamellar phases when dispersed in water. 31P broad-band nuclear magnetic resonance (NMR) shows that between body temperature (37 degrees C) and freezing of the solvent, the extracted lipids form bilayers with the lipid molecules undergoing fast anisotropic motion. This is also true for the lipids present in the brush border membrane. The electron spin resonance (ESR) results obtained with various hydrophobic spin probes incorporated in either brush border vesicle membranes or their extracted lipids are consistent with this interpretation. By use of a variety of chemically different spin-labels, the temperature dependence of brush border membranes and their extracted lipids was probed. The temperature dependence of various ESR spectral parameters shows discontinuities that, by comparison with differential scanning calorimetry, are assigned to a lipid thermotropic phase transition. Differential scanning calorimetry shows that the lipid in brush border membranes undergoes a broad, reversible phase transition of low enthalpy between 10 and 30 degrees C, with a peak temperature of about 25 degrees C. Hence, the brush border membrane of rabbit small intestine functions in the liquid-crystalline state, well above the peak temperature and also above the upper limit of the lipid phase transition. Therefore, in itself, the thermotropic lipid phase transition is unlikely to play a physiological role. The low enthalpy of the lipid phase transition, indicative of a lack of cooperativity, is primarily attributed to the relatively high cholesterol content and to heterogeneity in the lipid composition of this membrane [Hauser, H., Howell, K., Dawson, R. M. C., & Bowyer, D. E. (1980) Biochim. Biophys. Acta 602, 567-577].(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1992

Energetics of a hexagonal-lamellar-hexagonal-phase transition sequence in dioleoylphosphatidylethanolamine membranes.

The phase diagram of DOPE/water dispersions was investigated by NMR and X-ray diffraction in the water concentration range from 2 to 20 water molecules per lipid and in the temperature range from -5 to +50 degrees C. At temperatures above 22 degrees C, the dispersions form an inverse (HII) phase at all water concentrations. Below 25 degrees C, an HII phase occurs at high water concentrations, an L alpha phase is formed at intermediate water concentrations, and finally the system switches back to an HII phase at low water concentrations. The enthalpy of the L alpha-HII-phase transition is +0.3 kcal/mol as measured by differential scanning calorimetry. Using 31P and 2H NMR and X-ray diffraction, we measured the trapped water volumes in HII and L alpha phases as a function of osmotic pressure. The change of the HII-phase free energy as a function of hydration was calculated by integrating the osmotic pressure vs trapped water volume curve. The phase diagram calculated on the basis of the known enthalpy of transition and the osmotic pressure vs water volume curves is in good agreement with the measured one. The HII-L alpha-HII double-phase transition at temperatures below 22 degrees C can be shown to be a consequence of (i) the greater degree of hydration of the HII phase in excess water and (ii) the relative sensitivities with which the lamellar and hexagonal phases dehydrate with increasing osmotic pressure. These results demonstrate the usefulness of osmotic stress measurements to understand lipid-phase diagrams.

Calorimetry, Differential Scanning↗

PubMed1998

Proton induced vesicle fusion and the isothermal lalpha-->HII phase transition of lipid bilayers: a 31P-NMR and titration calorimetry study.

The proton-induced isothermal fusion of unilamellar lipid vesicles (Duzgunes et al., Biochemistry 24 (1985) 3091-3098) is compared with the lamellar (Lalpha)-->hexagonal (HII) phase transition of multilamellar lipid dispersions. Both lipid systems are composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and oleic acid (OA) at a 7:3 molar ratio. Using solid-state phosphorus-31 nuclear magnetic resonance (31P-NMR) it is demonstrated that the multilamellar lipid dispersions are in the bilayer state at physiological pH and undergo a Lalpha-->HII phase transition between pH 6.3 and 5.7. This phase transition can also be induced at constant pH by increasing the temperature. The midpoint of the temperature-induced Lalpha-->HII transition is Th=56 degrees C (at pH 7.4) and the corresponding transition enthalpy is DeltaH=0. 7+/-0.1 kcal/mol as determined with differential scanning calorimetry. Both the proton-induced and the temperature-induced phase transition can be completely inhibited by addition of 30 mol% of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (LPC). In a second set of experiments unilamellar vesicles are prepared either by sonication or by extrusion through polycarbonate filters at pH 7. 4 and are titrated into buffer at pH 5.7. The proton-induced fusion of the lipid vesicles is monitored with isothermal titration calorimetry, light scattering and fluorescence spectroscopy. The fusion reaction is characterized by an endothermic enthalpy of DeltaH=0.5+/-0.2 kcal/mol (at 28 degrees C). The fusion enthalpy is independent of the vesicle diameter and is only slightly reduced by an increase in temperature to 50 degrees C. Vesicle fusion is accompanied by an increase in light scattering, indicating the formation of larger lipid structures. The transition from unilamellar vesicles to fused lipid structures occurs in the same narrow pH range of 6.3-5.7 as observed for the Lalpha-->HII transition of multilamellar dispersions. Vesicle fusion can be inhibited with 30% LPC. The virtually identical set of parameters found for the Lalpha-->HII phase transition and the vesicle fusion reaction suggests that vesicle fusion also entails a Lalpha-->HII phase transition.

Calorimetry↗

PubMed1984

Changes in thermal phase transition of various membranes during temperature acclimation in Tetrahymena.

Changes in the thermal phase transition temperature of membrane lipids were studied by X-ray wide-angle diffraction during adaptation of Tetrahymena pyriformis to a lower growth temperature. After a shift in growth temperature from 39 to 15 degrees C, the phase transition temperature was lowered gradually in microsomal and pellicular phospholipids, whereas that in mitochondrial phospholipids was unchanged for 10 h after the temperature shift. Only a small decrease in the transition temperature of mitochondrial phospholipids was observed, even after 24 h following the shift. Transition temperatures of microsomal, pellicular and mitochondrial phospholipids reached the growth temperature (15 degrees C) about 6, 10 and 24 h after the temperature shift. The temperature dependence of the solid phase in membrane phospholipids was estimated from the 4.2 A peak of the X-ray diffraction pattern. In the case of the phospholipids extracted from cells grown at 39 degrees C, the solid phase was increased upon lowering temperature in a similar manner in all three membrane fractions: mitochondria, pellicles and microsomes. However, in the case of the phospholipids from cells exposed to a lower growth temperature (15 degrees C) for 10 h, the increase in the solid phase was significantly smaller in mitochondrial phospholipids than in two other membrane fractions. The difference in the thermal behaviour of mitochondrial lipid from pellicular and microsomal lipids is discussed in terms of phase transition and phase separation.

Adaptation, Physiological↗

PubMed1992

Effect of the chirality of the glycerol backbone on the bilayer and nonbilayer phase transitions in the diastereomers of di-dodecyl-beta-D-glucopyranosyl glycerol.

We have studied the physical properties of aqueous dispersions of 1,2-sn- and 2,3-sn-didodecyl-beta-D-glucopyranosyl glycerols, as well as their diastereomeric mixture, using differential scanning calorimetry and low angle x-ray diffraction. Upon heating, both the chiral lipids and the diastereomeric mixture exhibit characteristically energetic L beta/L alpha phase transitions at 31.7-32.8 degrees C and two or three weakly energetic thermal events between 49 degrees C and 89 degrees C. In the diastereomeric mixture and the 1,2-sn glycerol derivative, these higher temperature endotherms correspond to the formation of, and interconversions between, several nonlamellar structures and have been assigned to L alpha/QIIa, QIIa/QIIb, and QIIb/HII phase transitions, respectively. The cubic phases QIIa and QIIb, whose cell lattice parameters are strongly temperature dependent, can be identified as belonging to space groups Ia3d and Pn3m/Pn3, respectively. In the equivalent 2,3-sn glucolipid, the QIIa phase is not observed and only two transitions are seen at 49 degrees C and 77 degrees C, which are identified as L alpha/QIIb and QIIb/HII phase transitions, respectively. These phase transitions temperatures are some 10 degrees C lower than those of the corresponding phase transitions observed in the diastereomeric mixture and the 1,2-sn glycerol derivative. On cooling, all three lipids exhibit a minor higher temperature exothermic event, which can be assigned to a HII/QIIb phase transition. An exothermic L alpha/L beta phase transition is observed at 30-31 degrees C. A shoulder is sometimes discernible on the high temperature side of the L alpha/L beta event, which may originate from a QIIb/L alpha phase transition prior to the freezing of the hydrocarbon chains. None of the lipids show evidence of a QIIa phase on cooling. No additional exothermic transitions are observed on further cooling to -3 degrees C. However, after nucleation at 0 degrees C followed by a short period of annealing at 22 degrees C, the 1,2-sn glucolipid forms an Lc phase that converts to an L alpha phase at 39.5 degrees C on heating. Neither the diastereomeric mixture nor the 2,3-sn glycerol derivative shows such behavior even after extended periods of annealing. Our results suggest that the differences in the phase behavior of these glycolipid isomers may not be attributable to headgroup size per se, but rather to differences in the stereochemistry of the lipid polar/apolar interfacial region, which consequently effects hydrogen-bonding, hydration, and the hydrophilic/hydrophobic balance.

Biophysical Phenomena↗

PubMed1999

Low pH induces an interdigitated gel to bilayer gel phase transition in dihexadecylphosphatidylcholine membrane.

We have investigated the influence of pH on the structures and phase behaviors of multilamellar vesicles of the ether-linked dihexadecylphosphatidylcholine (DHPC-MLV). This phospholipid is known to be in the interdigitated gel (L(beta)I) phase in excess water at 20 degrees C at neutral pH. The results of X-ray diffraction experiments indicate that a phase transition from L(beta)I phase to the bilayer gel phase occurred in DHPC-MLV in 0.5 M KCl around pH 3.9 with a decrease in pH, and that at low pH values, less than pH 2.2, DHPC-MLVs were in L(beta') phase. The results of fluorescence and light scattering method indicate that the gel to liquid-crystalline phase transition temperature (T(m)) of DHPC-MLV increased with a decrease in pH. On the basis of a thermodynamic analysis, we conclude that the main mechanism of the low-pH induced L(beta)I to bilayer gel phase transition in DHPC-MLV and the increase in its T(m) is connected with the decrease in the repulsive interaction between the headgroups of these phospholipids. As pH decreases, the phosphate groups of the headgroups begin to be protonated, and as a result, the apparent positive surface charges appear. However, surface dipoles decrease and the interaction free energy of the hydrophilic segments with water increases. The latter effect dominates the pure electrostatic repulsion between the charged headgroups, and thereby, the total repulsive interaction in the interface decreases.

Calorimetry, Differential Scanning↗

PubMed1993

Cold shock damage is due to lipid phase transitions in cell membranes: a demonstration using sperm as a model.

When cells are cooled to temperatures above the freezing point of water at rates greater than a few degrees per minute, they sustain irreversible injury. Reduction of this "cold shock" damage could increase the survival of animals and plants at low environmental temperatures and improve the cryopreservation of plant and animal cells. Leakage of solutes across membranes, associated with thermotropic phase transitions in membrane lipids, is thought to be responsible, but this hypothesis has not been tested directly. Using Fourier transform infrared spectroscopy (FTIR), we measured the lipid phase transitions in intact, living sperm, the animal cell in which cold shock has been studied most extensively. A shift in the CH2 absorbance peaks indicates the transition from liquid-crystalline to gel phase. The phase transition in sperm membranes occurred at a lower temperature for a marine shrimp than for the pig. In each case, potassium leakage, which is a hallmark of cold shock damage, increased abruptly near the end of the phase transition. Human sperm are quite resistant to cold shock, and an abrupt lipid phase transition was not detected. This phase behavior is typical of membranes containing a high proportion of cholesterol, and human sperm have an unusually high sterol content. High cholesterol levels are known to stabilize membranes during cooling. Overall, the lipid phase behavior was consistent with the temperature range over which cooling was damaging for pig and shrimp sperm, and the with the extent of damage produced in pig and human sperm. This is the first direct evidence that cold shock results from lipid phase transitions in cell membranes.

Animals↗