Alcohol effects on rapid kinetics of water transport through lipid membranes and location of the main barrier

The effect of 1-alkanols (from 1-butanol up to 1-dodecanol) on the water permeability of dimyristoylphosphatidylcholine vesicle membranes was studied by measuring the osmotic swelling rate as functions of 1-alkanol concentrations and temperatures above the gel-to-liquid-crystalline phase transition. For 1-butanol and 1-hexanol, the activation energy for water permeation was invariant with the addition of alkanols, whereas for 1-octanol, 1-decanol and 1-dodecanol, the activation energy decreased depending on the alkanol concentration, and the extent of the decrease was larger for alkanol with a longer hydrocarbon chain. These results suggests that hydrocarbon moiety beyond seven or eight carbon atoms from the head group in phospholipid molecules constitutes the main barrier for water permeation through the dimyristoylphosphatidylcholine vesicle membrane. The relative volume change of the vesicle due to osmotic swelling increased with the addition of 1-alkanols. Presumably, the membrane structural strength is weakened by the presence of 1-alkanols in the membrane. Contrary to the dependence of the swelling rate upon the alkanol carbon-chain length, no significant difference in the effect on the relative volume changes was seen among the 1-alkanols. This result suggests that weakening of the membrane structure is caused by perturbation of the membrane/water interface induced by incorporation of 1-alkanols into the membrane.     

High pressure antagonism of alcohol effects on the main phase-transition temperature of phospholipid membranes: biphasic response.

The combined effects of high pressure (up to 300 bar) and a homologous series of 1-alkanols (ethanol C2 to 1-tridecanol C13) were studied on the main phase-transition temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes. It is known that short-chain alkanols depress and long-chain alkanols elevate the main transition temperature. The crossover from depression to elevation occurs at the carbon-chain length about C10-C12 in DPPC vesicle membranes coinciding with the cutoff chain-length where anesthetic potency suddenly disappears. Alkanols shorter than C8 linearly decreased the transition temperature and high pressure antagonized the temperature depression. Alkanols longer than C10 showed biphasic dose-response curves. High pressure enhanced the biphasic response. In addition, alkanols longer than the cutoff length depressed the transition temperature under high pressure at the low concentration range. These non-anesthetic alkanols may manifest anesthetic potency under high pressure. At higher concentrations, the temperature elevatory effect was accentuated by pressure. This biphasic effect of long-chain alkanols is not related to the 'interdigitation' associated with short-chain alkanols. The increment of the transition temperature by pressure was 0.0242 K bar-1 in the absence of alkanols. The volume change of the transition was estimated to be 27.7 cm3 mol-1. This value stayed constant to the limit of the present study of 300 bar.     

Stopped-flow study of anesthetic effect on water-transport kinetics through phospholipid membranes. Interfacial versus lipid core ligands

We have compared ligand effects between polar and apolar anesthetic molecules upon water transport across phospholipid membranes by kinetic analysis of the osmotic swelling rate, using a stopped-flow technique. Chloroform and 1-hexanol were used as interfacial ligands, and carbon tetrachloride and n-hexane were used as their counterparts, representing lipid core action. Because anesthetics transform the solid-gel membrane into a liquid-crystalline state, and because phospholipid membranes display an anomaly in permeability at the phase transition, dimyristoylphosphatidylcholine vesicles were studied at temperatures above the main phase transition to avoid this anomaly. All these molecules increased the osmotic swelling rate. However, a significant difference was observed in the activation energy, delta Ep, between polar and apolar molecules; delta Ep was almost unaltered by the addition of polar molecules (chloroform and 1-hexanol), whereas it was decreased by apolar molecules (carbon tetrachloride and n-hexane). The obtained results were analyzed in terms of the dissolution-diffusion mechanism for water permeation across the lipid membrane. It is suggested that polar molecules affect water permeability by altering the partition of water between the membrane interior and water phase, and apolar molecules affect it by altering both the partition and the diffusion of water within the membrane interior.     

Modification of membrane lipid: physical properties in relation to fatty acid structure.

Differential scanning calorimetry (DSC) and electron spin resonance (ESR) measurements were made to characterize how modifications in the fatty acid composition of Escherichia coli affected the thermotropic phase transition(s) of the membrane lipd. When the fatty acid composition contained between 20 and 60% saturated fatty acids, the DSC curves for isolated phospholipids and cytoplasmic membranes showed a broad (15-25 degree C) gel to liquid-crystalline phase transition, the position of which depended on the particular fatty acid composition. Utilizing multiple lipid mutants, enrichment of the membrane phospholipids with a single long-chain cis-monoenoic fatty acid in excess of that possible in a fatty acid levels less than 20% and gradually replaced the broad peak as the cis-monoenoic fatty acid content increased. These results were obtained with phospholipids, cytoplasmic membranes, and whole cells. With these same phopholipids, plots of 2,2,6,6-tetramethylpiperidinyl-1-oxy partitioning and ESR order parameters vs. 1/T revealed discontinuities at temperatures 40-60 degrees C above the calorimetrica-ly measured gel to liquid-crystalline phase transitions. Moreover, when the membrane phospholipids were enriched with certain combinations of cis-monenoic fatty acids (e.g., cis-delta 9-16:1 plus cis-delta 11-18:1) the DSC curve showed a broad gel to liquid crystalline phase change below 0 degrees C but the ESR studies revealed no discontinuities at temperatures above those of the gel to liquid-crystalline transition. These results demonstrated that enrichment of the membrane lipids with molecules in which both fatty acyl chains are identical cis-monoenoic residues led to a distinct type of liquid-crystalline phase. Furthermore, a general conclusion from this study is that Escherichia coli normally maintains a heterogeneous mixture of lipid molecules and, by so doing, prevents strong lipid-lipid associations that lead to the formation of lipid domains in the membrane.     

Interaction of short-chain lecithin with long-chain phospholipids: characterization of vesicles that form spontaneously

Stable unilamellar vesicles formed spontaneously upon mixing aqueous suspensions of long-chain phospholipid (synthetic, saturated, and naturally occurring phosphatidylcholine, phosphatidylethanolamine, and sphingomyelin) with small amounts of short-chain lecithin (fatty acid chain lengths of 6-8 carbons) have been characterized by using NMR spectroscopy, negative staining electron microscopy, differential scanning calorimetry, and Fourier transform infrared (FTIR) spectroscopy. This method of vesicle preparation can produce bilayer vesicles spanning the size range 100 to greater than 1000 A. The combination of short-chain lecithin and long-chain lecithin in its gel state at room temperature produces relatively small unilamellar vesicles, while using long-chain lecithin in its liquid-crystalline state produces large unilamellar vesicles. The length of the short-chain lecithin does not affect the size distribution of the vesicles as much as the ratio of short-chain to long-chain components. In general, additional short-chain decreases the average vesicle size. Incorporation of cholesterol can affect vesicle size, with the solubility limit of cholesterol in short-chain lecithin micelles governing any size change. If the amount of cholesterol is below the solubility limit of micellar short-chain lecithin, then the addition of cholesterol to the vesicle bilayer has no effect on the vesicle size; if more cholesterol is added, particle growth is observed. Vesicles formed with a saturated long-chain lecithin and short-chain species exhibit similar phase transition behavior and enthalpy values to small unilamellar vesicles of the pure long-chain lecithin prepared by sonication. As the size of the short-chain/long-chain vesicles decreases, the phase transition temperature decreases to temperatures observed for sonicated unilamellar vesicles. FTIR spectroscopy confirms that the incorporation of the short-chain lipid in the vesicle bilayer does not drastically alter the gauche bond conformation of the long-chain lipids (i.e., their transness in the gel state and the presence of multiple gauche bonds in the liquid-crystalline state).     

Interaction modes of long-chain fatty acids in dipalmitoylphosphatidylcholine bilayer membrane: contrast to mode of inhalation anesthetics

The effects of long-chain fatty acids (four saturated and two unsaturated fatty acids, one derivative) on phase transitions of dipalmitoylphosphatidylcholine (DPPC) bilayer membranes were examined in the low concentration region, and the results were compared with those for an inhalation anesthetic. The effects of all fatty acids on the pre- and main-transition temperatures of the DPPC bilayer membrane appeared in the concentration range of μM order while that of the anesthetic appeared in the mM order. The appearance modes of these ligand actions were significantly different from one another. The three differential partition coefficients of the ligands between two phases of the DPPC bilayer membrane were evaluated by applying the thermodynamic equation to the variation of the phase-transition temperatures. The DPPC bilayer membranes showed the different receptivity for the ligands; the saturated fatty acids had an affinity for gel phase whereas unsaturated fatty acids and an anesthetic had an affinity for liquid-crystalline phase to the contrary. In particular, the receptivity for the ligands in the gel phase markedly changed depending on kinds of ligands. The interaction modes between the DPPC and fatty acid molecules in the gel phase were considered from the hexagonal lattice model. The disappearance compositions of the pretransition by the fatty acids coincided with the compositions at which the membrane is all covered by the units in each of which two fatty acids molecules are regularly distributed in the hexagonal lattice in a different way, and the distribution depended on the chain length and existence of a double bond for the fatty acids. The interpretation did not hold for the case of the anesthetic at all, which proved that a number of anesthetic molecules act the surface region of the bilayer membrane nonspecifically. The present study clearly implies that DPPC bilayer membranes have high ability to recognize kinds of ligand molecules and can discriminate among them with specific interaction by the membrane states.     

NMR study of the interactions of polymyxin B, gramicidin S, and valinomycin with dimyristoyllecithin bilayers

The interactions of three polypeptide antibiotics (polymyxin B, gramicidin S, and valinomycin) with artificial lecithin membranes were studied by nuclear magnetic resonance (NMR). Combination of 31P and 2H NMR allowed observation of perturbations of the bilayer membrane structure induced by each of the antibiotics in the regions of the polar headgroups and acyl side chains of the phospholipids. The comparative study of the effects of these membrane-active antibiotics and the lipid bilayer structure demonstrated distinct types of antibiotic-membrane interactions in each case. Thus, the results showed the absence of interaction of polymyxin B with the dimyristoyllecithin membranes. In contrast, gramicidin S exhibited strong interaction with the lipid above the gel to liquid-crystalline phase transition temperature: disordering of the acyl side chains was evident. Increasing the concentration of gramicidin S led to disintegration of the bilayer membrane structure. At a molar ratio of 1:16 of gramicidin S to lecithin, the results are consistent with coexistence of gel and liquid-crystalline phases of the phospholipids near the phase transition temperature. Valinomycin decreased the phase transition temperature of the lipids and increased the order parameters of the lipid side chains. Such behavior is consistent with penetration of the valinomycin molecule into the interior of the lipid bilayers.     

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)     

A statistical mechanical analysis of the effect of long-chain alcohols and high pressure upon the phase transition temperature of lipid bilayer membranes.

Long-chain n-alcohols decrease the main phase-transition temperature of lipid vesicle membranes at low concentrations but increase it at high concentrations. The nonlinear phenomenon is unrelated to the interdigitation and is analyzed by assuming that alcohols form solid solutions with solid as well as liquid phases. The biphasic response originates from the balance of the free energy difference of alcohols in the liquid and solid membranes (delta gA) and the alcohol-lipid interaction free energy difference (delta u) between the two phases. When delta gA less than 0 and delta u greater than 0, or delta gA less than delta u less than 0, the transition temperature decreases monotonously according to the increase in the alcohol concentration. When delta gA greater than 0 and delta u less than 0, or delta gA greater than delta u greater than 0, it increases monotonously. Biphasic response occurs with a minimum temperature when delta u greater than delta gA greater than 0, and with a maximum temperature when delta u less than delta gA less than 0. When the alcohol carbon-chain length becomes closer to the lipid carbon-chain length, delta u is equalized by delta gA, and the temperature minimum of the main transition is shifted to extremely low alcohol concentrations. Hence, long-chain alcohols predominantly elevate the main transition temperature and lose their anesthetic potency. High pressure decreased both delta gA and delta u. Presumably, high pressure improves the packing efficiency of liquid membranes and decreases the difference between the solid and liquid membrane properties.     

Dose-dependent nonlinear response of the main phase-transition temperature of phospholipid membranes to alcohols

Summary The effect of 1-alkanols upon the main phase-transition temperature of phospholipid vesicle membranes between gel and liquid-crystalline phases was not a simple monotonic function of alkanol concentration. For instance, 1-decanol decreased the transition temperature at low concentrations, but increased it at high concentrations, displaying a minimal temperature. This concentration-induced biphasic effect cannot be explained by the van't Hoff model on the effect of impurities upon the freezing point. To explain this nonlinear response, a theory is presented which treats the effect of 1-alkanols (or any additives) on the transition temperature of phospholipid membranes in a three-component mixture. By fitting the experimental data to the theory, the enthalpy of the phase transition H ^* and the interaction energy, _AB ^* between the additive and phospholipid molecules may be estimated. The theory predicts that when _AB ^* >2 (where _AB ^* = _AB,/RT _o,T _o being the transition temperature of phospholipid), both maximum and maximum transition temperatures should exist. When _AB ^* = 2, only one inflection point exists. When _AB ^* < 2, neither maximum nor minimum exists. The alkanol concentration at which the transition temperature is minimum (X _min) depends on the _AB ^* value: the larger the _AB ^* values, the smaller theX _min. When _AB ^* is large enough,X _min values become so small that the plot T vs.X shows positive T in almost all alkanol concentrations. The interaction energy between 1-alkanols and phospholipid molecules increased with the increase in the carbon chain-length of 1-alkanols. In the case of the dipalmitcylphosphatidylcholine vesicle membrane, the carbon chain-length of 1-alkanols that caused predominantly positive T was about 12.     

Study of the cytoplasmic and outer membranes of Escherichia coli by deuterium magnetic resonance.

The cytoplasmic and outer membranes of Escherichia coli were studied between 0 and 40 degrees C by deuterium magnetic resonance quadrupolar echo spectroscopy. The L51 strain of E. coli was used to incorporate perdeuterated palmitic acid into the membrane phospholipids. The cytoplasmic and outer membranes were separated using standard techniques. The spectrum of each membrane preparation was dominated at high temperatures (greater than or equal to 37 degrees C) by the characteristic liquid-crystalline plateau previously observed for perdeuterated palmitate chains in model phospholipid membranes. At low temperatures, the shape and width of the spectrum were characteristic of the gel phase. The relative intensities of the liquid-crystalline and gel features varied systematically with temperature. A quantitative analysis of the acyl chain orientational order was carried out by using the method of moments. The orientational order at each temperature was greater in the outer membrane sample than in that of the cytoplasmic membrane, indicating that the liquid-crystalline-gel transition region in the outer membrane is shifted to higher temperatures than that of the cytoplasmic membrane by about 7 degrees C. It is clear from the results that most of the phospholipid molecules participate in the phase transition.     

Interaction of synthetic glycophospholipids with phospholipid bilayer membranes.

A series of glycophospholipids synthesized by coupling mono-, di-, or tri-saccharides to dioleoylphosphatidylethanolamine (DOPE) by reductive amination was used to investigate the interaction of glycophospholipids with phospholipid bilayer membranes. These synthetic glycophospholipids functioned as a stabilizer for the formation of DOPE bilayer vesicles. The minimal mol% of glycophospholipid needed to stabilize the DOPE vesicles were as follows: 8% N-neuraminlactosyl-DOPE (NANL-DOPE), 20% N-maltotriosyl-DOPE (MAT-DOPE), 30% N-lactosyl-DOPE (Lac-DOPE), and 42% N-galactosyl-DOPE (Gal-DOPE). The estimated hydration number of glycophospholipid in reverse micelles was 87, 73, 46, and 14 for NANL-DOPE, MAT-DOPE, Lac-DOPE, and Gal-DOPE, respectively. Thus, the hydration intensity of the glycophospholipid was directly related to the ability to stabilize the DOPE bilayer phase for vesicle formation. Glycophospholipids also reduced the transition temperature from gel to liquid-crystalline phase (Tm) of dipalmitoylphosphatidylcholine (DPPC) bilayers. Interestingly, incorporation of NANL-DOPE induced a decrease of membrane fluidity of DPPC bilayers in the gel phase while other glycophospholipids had no effect. Also, low level of NANL-DOPE but not other glycophospholipids increased the transition temperature (TH) from liquid-crystalline to hexagonal phase of dielaidoylphosphatidylethanolamine bilayers. These results showed that NANL-DOPE with a highly hydratable headgroup which provides a strong stabilization activity for the L alpha phase of phospholipid membranes, may also be involved in specific interactions with neighboring phospholipids via its saccharide moiety.     

Impact of free hydroxylated and methyl-branched fatty acids on the organization of lipid membranes

Differential scanning calorimetry (DSC) has been applied to study the effect of free hydroxylated and methyl-branched fatty acids on the physico-chemical properties of lipid membranes. First, the impact of free hydroxy fatty acids (HFAs) on dimyristoylphosphatidylcholine (DMPC) model membranes was monitored only as a function of chain length and position of the attached hydroxyl group. Second, racemic vs. enantiopure anteiso fatty acids (AFAs) and HFAs were investigated to address the question of which role does a fatty acid's chirality play on its membrane pertubing effect. The DSC thermograms revealed that the main gel to liquid-crystalline phase transition of the DMPC bilayers which results in a disordering effect of the lipid hydrocarbon chains was affected in different ways depending on the nature of the incorporated fatty acid. Long-chain 2- and 3-HFAs stabilized the gel phase by reducing the phase transition temperature (T(m)), whereas short-chain HFAs and long-chain HFAs with the hydroxy group remote from the head group stabilized the more disordered liquid-crystalline state. Additionally, we observed that enantiopure (S)-14-methylhexadecanoic acid ((S)-a17:0) and (R)-2-hydroxy octadecanoic acid and the corresponding racemates had contrary effects upon incorporation into DMPC bilayers. In both cases, the pure enantiomers alleviated the liquid-crystalline state of the biological model membrane.     

Deuterium NMR study of the effect of n-alkanol anesthetics on a model membrane system

The effects of 25 mol% incorporation of two anesthetics, 1-octanol and 1-decanol, on a deuterated, saturated phospholipid in 50 wt% aqueous multilamellar dispersions have been studied by 2H-NMR spectroscopy and differential scanning calorimetry (DSC). The phospholipid used is sn-2 substituted '[2H31]-palmitoylphosphatidylcholine' (PC-d31). DSC thermograms demonstrate that PC-d31 has phase behavior qualitatively similar to that of dipalmitoylphosphatidylcholine, with a pretransition at 31 degrees C and a main gel to liquid crystalline transition at 40 degrees C. Analysis of the temperature-dependent 2H-NMR spectra in terms of the first moment, which is extremely sensitive to the phospholipid phase, shows that 1-octanol and 1-decanol depress and broaden the main transition. This is confirmed by DSC, which shows that the pretransition is eliminated by the 1-alkanols. The carbon-deuterium bond order of the phospholipid deuterated acyl chains, in the presence and absence of 1-alkanols, was determined from deuterium quadrupolar splittings. Spectra were analyzed using the depaking technique. A 1-alkanol concentration of 25 mol% had no significant effect on the profile of the carbon-deuterium bond order parameter SCD along the phospholipid acyl chain at 50 degrees C. Thus, it appears that the liquid crystalline phase is able to accommodate large amounts of linear anesthetic molecules without substantial effect on molecular ordering within the membrane bilayer. Preliminary results show that the transverse relaxation rates of the acyl chain segments are significantly decreased by the presence of 1-octanol or 1-decanol.     

Anesthetic-protein interaction Random versus helix polylysine monolayers and interaction with 1-alkanols

Penetration of 1-alkanols into monolayers of hydrophobic polypeptides, poly(ε-benzyloxycarbonyl-l-lysine) and poly(ε-benzyloxycarbonyl-dl-lysine), was compared with their adsorption on the air/water interface in the absence of monolayers. The polypeptide prepared from l-lysine is generally considered to be in the α-helical form whereas dl-copolymer polypeptide contains random-coiled portions due to the structural incompatibility between the two isomers. The free energy of adsorption of 1-alkanols on the air/water interface at dilute concentrations was ?0.68 kcal·mol^?1 per methylene group and 0.15 kcal·mol^?1 for the hydroxyl group at 25°C. In the close-packed state, the surface area occupied by each molecule of 1-alkanols of varying carbon chain-lengths showed nearly a constant value of about 27.2 Ų, indicating perpendicular orientation of the alkanol molecules at the interface. About 75% of the water surface was covered by 1-butanol in this close-packed state. The mode of adsorption of 1-alkanols on the vacant air/water interface followed the Gibbs surface excess while the mode on the polypeptide membranes followed the Langmuir adsorption isotherm, indicating that the latter is characterized by the presence of a finite number of binding sites. The free energies of adsorption of 1-alkanols on the l-polymer monolayers were more negative than those on the vacant air/water interface and less negative than those on the dl-copolymer monolayers. Thus, the affinity of 1-alkanols to the interface was in the order of vacant air/water interface <l-polymer <dl-copolymer. The difference between the air/water interface and l-polymer was about 0.54 kcal·mol^?1 and that between l-polymer and dl-copolymer was 0.17 kcal·mol^?1 at 25°C: the adsorption of 1-alkanols to the dl-copolymer was favored compared to the l-polymer. The polar moieties of the backbone of the dl-copolymer may be exposed to the aqueous phase at the disordered portion. Dipole interaction between this portion and 1-alkanol molecules may account for the enhanced adsorption of the alkanols to the dl-copolymer.     

Lipid composition and molecular organization in plasma membrane-enriched fractions from senescing cotyledons

Plasma membrane fractions isolated from cotyledons of Phaseolus vulgaris L. cv. Kinghorn at various stages of senescence showed no significant change in fatty acid saturation with advancing senescence. However, the steroliphospholipid ratio increased by about 400% as senescence intensified. The lipid phase transition temperature of the membranes, which was measured by wide-angle x-ray diffraction, also rose from a point well below the growing temperature for young tissue to about 50°C for membrane from extensively senescent 9-day-old tissue. This means that by day 4 of germination there was a mixture of liquid-crystalline and gel phase phospholipid in the membrane matrices. Crystallinity attributable to sterol-sterol interaction was also apparent in the diffraction patterns for senescent membranes. The co-existence of gel and liquid-crystalline phase phospholipid in the aging membranes as well as the crystalline sterol aggregates presumably render the storage cells of cotyledons leaky and may thus facilitate the translocation of hydrolyzed food reserves into the vascular network.     

Temperature-induced fusion of small unilamellar vesicles formed from saturated long-chain lecithins and diheptanoylphosphatidylcholine

Small unilamellar vesicles which form when gel-state long-chain phosphatidylcholines are mixed with micellar short-chain lecithins undergo an increase in size as the long-chain species melts to its liquid-crystalline form. Analysis of the vesicle population with quasi-elastic light scattering shows that the particle size increases from 90-A radius to greater than 5000-A radius. Resonance energy transfer experiments show total mixing of lipid probes with unlabeled vesicles only when the Tm of the long-chain phosphatidylcholine is exceeded. This implies that the large size change represents a fusion process. Aqueous compartments are also mixed during this transition. 31P NMR analysis of the vesicle mixtures above the phase transition shows a great degree of heterogeneity with large unilamellar particles coexisting with oligo- and multilamellar structures. Upon cooling the vesicles below the Tm, the original size distribution (e.g., small unilamellar vesicles) is obtained, as monitored by both quasi-elastic light scattering and 31P NMR spectroscopy. This temperature-induced fusion of unilamellar vesicles is concentration dependent and can be abolished at lower total phospholipid concentrations. It occurs over a wide range of long-chain to short-chain ratios and occurs with 1-palmitoyl-2-stearoylphosphatidylcholine and dimyristoylphosphatidylcholine as well. Characterization of this fusion event is used to understand the anomalous kinetics of water-soluble phospholipases toward these unusual vesicles.     

The Dipole Model and Phase Transitions in Biological Membranes

Assuming the dipole model for a membrane, approximate calculations are made which employ a dipole-dipole interaction energy. The calculations are based upon the assumption of cooperative coupling of membrane polar molecules and make use of the Bragg-Williams approximation. A theoretical estimate is made of the critical temperature at which phase changes might occur in certain biological membranes. Proposals are presented which explain how the dipole transition might relate to the sometimes observed thermal phase transitions in biological membranes.     

Membrane phase behavior of a psychrotrophic and mesophilic bacillus

The phase properties of membranes isolated from the psychrotrophBacillus psychrophilus and the mesophileB. megaterium were examined using wide-angle X-ray diffraction. The temperature at which the transition from liquid-crystalline to crystalline (gel) phase occurred was below −30°C for both microorganisms, regardless of the temperature at which the microbial cells were grown. Thus the membranes for both microorganisms were exclusively liquid-crystalline over the entire growth temperature range. Indeed, the membrane was completely fluid at temperatures where growth of the psychrotroph ceases, thus indicating that the phase transition temperature is not the determinant of the minimum growth temperature.     

Distribution functions, describing the binding of extended ligands with DNA molecules. Possible use for cases of DNA condensation

Due to noncooperative binding of ligands to DNA molecules, DNA molecules are in equilibrium with different numbers of adsorbed ligands. This equilibrium for a given concentration of the free ligand in the solution is characterized by the distribution function, which describes the probability of revealing the DNA molecule with a definite number of adsorbed ligands. If polycations act as ligands, DNA molecules with the number of ligands sufficient for neutralizing the charges on phosphates may undergo a phase transition. One example of this transition is the formation of liquid-crystalline dispersions during the binding of DNA to chitosan. We analyzed the binding of chitosan to DNA on the assumption that this binding is due to equilibrium adsorption. At a definite concentration of chitosan in solution, DNA molecules are in equilibrium with different numbers of adsorbed molecules of chitosan. If the number of adsorbed ligands exceeds some critical value, the DNA molecule covered with chitosan becomes capable of interacting with other DNA molecules. As a result of this interaction (attraction), liquid-crystalline dispersions can form. Equations describing the dependence of the concentration of DNA molecules on the concentration of the ligand in solution were derived. It was shown that, at given parameters of the model, it is possible to describe experimental data characterizing the formation of cholesteric liquid-crystalline dispersions. The analysis of the data makes it possible to reconstitute both the size of the binding site occupied by chitosan on the DNA and the energy of interaction of chitosan with DNA.