Correlation between lipid plane curvature and lipid chain order.

The 1-palmitoyl-2-oleoyl-phosphatidylethanolamine: 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPE:POPC) system has been investigated by measuring, in the inverted hexagonal (HII) phase, the intercylinder spacings (using x-ray diffraction) and orientational order of the acyl chains (using 2H nuclear magnetic resonance). The presence of 20 wt% dodecane leads to the formation of a HII phase for the composition range from 0 to 39 mol% of POPC in POPE, as ascertained by x-ray diffraction and 2H nuclear magnetic resonance. The addition of the alkane induces a small decrease in chain order, consistent with less stretched chains. An increase in temperature or in POPE proportion leads to a reduction in the intercylinder spacing, primarily due to a decrease in the water core radius. A temperature increase also leads to a reduction in the orientational order of the lipid acyl chains, whereas the POPE proportion has little effect on chain order. A correlation is proposed to relate the radius of curvature of the cylinders in the inverted hexagonal phase to the chain order of the lipids adopting the HII phase. A simple geometrical model is proposed, taking into account the area occupied by the polar headgroup at the interface and the orientational order of the acyl chains reflecting the contribution of the apolar core. From these parameters, intercylinder spacings are calculated that agree well with the values determined experimentally by x-ray diffraction, for the variations of both temperature and POPE:POPC proportion. This model suggests that temperature increases the curvature of lipid layers, mainly by increasing the area subtended by the hydrophobic core through chain conformation disorder, whereas POPC content affects primarily the headgroup interface contribution. The frustration of lipid layer curvature is also shown to be reflected in the acyl chain order measured in the L alpha phase, in the absence of dodecane; for a given temperature, increased order is observed when the curling tendencies of the lipid plane are more pronounced.     

Lipid lateral segregation driven by diacyl cyclodextrin interactions at the membrane surface

Cyclodextrins are hydrophilic molecular cages with a hydrophobic interior allowing the inclusion of water-insoluble drugs. Amphiphilic cyclodextrins obtained by appending a hydrophobic anchor were designed to improve the cell targeting of the drug-containing cavities through their liposome transportation in the organism. After insertion in model membranes, they were found to induce a lateral phase separation into a pure lipid phase and a fluid cyclodextrin-rich phase (L(CD)) with reduced acyl chain order parameters, as observed with a derivative containing a cholesterol anchor (M. Roux, R. Auzely-Velty, F. Djeda?ni-Pilard, and B. Perly. 2002. Biophysical Journal, 8:813-822). We present another class of amphiphilic cyclodextrins obtained by grafting aspartic acid esterified by two lauryl chains on the oligosaccharide core via a succinyl spacer. The obtained dilauryl-beta-cyclodextrin (betaDLC) was inserted in chain perdeuterated dimyristoylphosphatidylcholine (DMPC-d54) membranes and studied by deuterium NMR ((2)H-NMR). A laterally segregated mixed phase was found to sequester three times more lipids than the cholesteryl derivative (approximately 4-5 lipids per monomer of betaDLC), and a quasipure L(CD) phase could be obtained with a 20% molar concentration of betaDLC. When cooled below the main fluid-to-gel transition of DMPC-d54 the betaDLC-rich phase stays fluid, coexisting with pure lipid in the gel state, and exhibits a sharp transition to a gel phase with frozen DMPC acyl chains at 12.5 degrees C. No lateral phase separation was observed with partially or fully methylated betaDLC, confirming that the stability of the segregated L(CD) phase was governed through hydrogen-bond-mediated intermolecular interactions between cyclodextrin headgroups at the membrane surface. As opposed to native betaDLC, the methylated derivatives were found to strongly increase the orientational order of DMPC acyl chains as the temperature reaches the membrane fluid-to-gel transition. The results are discussed in relation to the "anomalous swelling" of saturated phosphatidylcholine multilamellar membranes known to occur in the vicinity of the main fluid-to-gel transition.     

Smoothed acyl chain orientational order parameter profiles in dimyristoylphosphatidylcholine-distearoylphosphatidylcholine mixtures: a 2H-NMR study.

The accommodation of chain-length mismatch in liquid crystal phase bilayers was examined by using deuterium nuclear magnetic resonance to obtain smoothed orientational order parameter profiles for acyl chains of both components in binary lipid mixture bilayers. Mixtures of dimyristoylphosphatidylcholine (DMPC) and distearoylphosphatidylcholine (DSPC) covering a range of compositions were prepared with either DSPC acyl chains or DMPC acyl chains perdeuterated. Orientational order parameters in the plateau regions of the smoothed profiles for both components were found to increase smoothly with increasing DSPC concentration. The orientational order parameters in the DSPC-smoothed profile were found to be slightly higher than corresponding values for DMPC over a wide range of bilayer composition. The shapes of the smoothed profiles for both components were found to be sensitive to bilayer composition. At low DSPC concentration, DSPC methylene deuterons near the bilayer center display a secondary plateau at low orientational order. At high DSPC concentration, the plateau of the DMPC-smoothed profile is stretched slightly. The concentration dependence of the smoothed profiles at low DSPC concentration appears to be consistent with a picture in which the last few segments of the DSPC chain cross the bilayer midplane, on average, but remain very disordered.     

Stages of the bilayer-micelle transition in the system phosphatidylcholine-C12E8 as studied by deuterium- and phosphorous-NMR, light scattering, and calorimetry.

The perturbation of phospholipid bilayer membranes by a nonionic detergent, octaethyleneglycol mono-n-dodecylether (C12E8), was investigated by 2H- and 31P-NMR, static and dynamic light scattering, and differential scanning calorimetry. Preequilibrated mixtures of the saturated phospholipids 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine (DPPC), 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC), and 1,2-dilauroyl-sn-glycero-3-phosphorylcholine (DLPC) with the detergent were studied over a broad temperature range including the temperature of the main thermotropic phase transition of the pure phospholipids. Above this temperature, at a phospholipid/detergent molar ratio 2:1, the membranes were oriented in the magnetic field. Cooling of the mixtures below the thermotropic phase transition temperatures of the pure phospholipids led to micelle formation. In mixtures of DPPC and DMPC with C12E8, a narrow calorimetric signal at the onset temperature of the solubilization suggested that micelle formation was related to the disorder-order transition in the phospholipid acyl chains. The particle size changed from 150 nm to approximately 7 nm over the temperature range of the bilayer-micelle transition. The spontaneous orientation of the membranes at high temperatures enabled the direct determination of segmental order parameters from the deuterium spectra. The order parameter profiles of the phospholipid acyl chains could be attributed to slow fluctuations of the whole membrane and to detergent-induced local perturbations of the bilayer order. The packing constraints in the mixed bilayers that eventually lead to bilayer solubilization were reflected by the order parameters of the interfacial phospholipid acyl chain segments and of the phospholipid headgroup. These results are interpreted in terms of the changing average shape of the component molecules. Considering the decreasing cross sectional areas in the acyl chain region and the increasing hydration of the detergent headgroups, the bilayer-micelle transition is the result of an imbalance in the chain and headgroup repulsion. A neutral or pivotal plane can be defined on the basis of the temperature dependence of the interfacial quadrupolar splittings.     

Modulation of the orientational order profile of the lipid acyl chain in the Lα phase

The orientational order profile along the lipid acyl chain has been characterized under several different conditions of polar headgroup composition, temperature, and cholesterol content. Despite the different nature of these factors, the variation of the order is governed by two common trends. First, the relative change of order induced by the variation of these factors is always more pronounced towards the end of the chain than for the methylene groups near the interface. Second, there is, to a first approximation, a distinct correlation between the magnitude of the order parameters and the shape of the order profile. For example when the chain is highly ordered, the relative width of the order distribution is narrow indicating that the plateau region is longer. These conclusions suggest that the orientational order profile depends on only a small number of parameters and demonstrate clearly that the correlation length for changes in orientational order is much greater than one C-C bond length. Our results also show that the reduced temperature is not related in simple terms to orientational order and probably has little theoretical significance. The orientational order profiles of POPC and POPE bilayers are significantly different even when expressed in terms of reduced temperature. The behavior of POPC/cholesterol systems also indicates that the orientational order of the lipid chain and the gel-to-liquid crystalline phase transition temperature are not related in a straightforward manner.Abbreviations POPC 1-palmitoyl-2-oleoyl-phosphatidylcholine - POPE 1-palmitoyl-2-oleoyl-phosphatidylethanolamine - PC phosphatidylcholine - PE phosphatidylethanolamine - NMR nuclear magnetic resonance - EDTA ethylenediaminetetraacetic acid Offprint requests to: M. Bloom     

Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Gel-state disorder in the presence of methyl iso- and anteiso-branched-chain substituents

The hydrocarbon chain orientational order parameters of membranes of Acholeplasma laidlawii B enriched with large quantities of a linear saturated, a methyl iso-branched, or a methyl anteiso-branched fatty acid plus small quantities of various isomeric monofluoropalmitic acid probes were determined via fluorine-19 nuclear magnetic resonance spectroscopy (19F NMR) over a range of temperatures spanning the gel to liquid-crystalline phase transitions (determined by differential scanning calorimetry). Membrane orientational order profiles in the liquid-crystalline state were generally similar regardless of the particular fatty acyl structure, showing a region of relatively constant order preceding a region of progressive decline in order toward the methyl terminus of the acyl chain. In the gel state, the order profile of the linear saturated fatty acid enriched membranes was characteristically flat, with little head to tail gradation of order. In contrast, the methyl iso-branched and the methyl anteiso-branched enriched membranes exhibited a local disordering in the gel phase reflected in a very pronounced head to tail gradient of order, which remained at temperatures below the lipid phase transition. In addition, the methyl iso- and anteiso-branched fatty acid enriched membranes were overall more disordered than the membrane containing only linear saturated fatty acyl groups. Thus, at a constant value of reduced temperature below the lipid phase transition, overall order decreased in the progression 15:0 greater than 16:0i greater than 16:0ai, suggesting that these methyl-branched substituents lower the lipid phase transition by disrupting the gel phase lipid chain packing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Orientational properties of biological pigments in ordered systems studied with polarized light: photosynthetic pigment-protein complexes in membranes

A discussion is presented of the problems involved in the interpretation of linear dichroism and fluorescence depolarization experiments on macroscopically ordered membrane systems. Particular attention has been paid to ordered membranes containing photosynthetic pigment-protein complexes, but the mathematical treatment can equally well be applied to other systems. The information about the orientational properties of the pigments is obtained by the application of the theories developed for the characterization of the molecular orientational order in liquid-crystalline materials. It is shown that while linear dichroism only yields the order parameter S mu of the absorption transition moment, fluorescence depolarization experiments yield in addition the order parameter Sv of the emission transition moment as well as three orientational correlation functions of the two transition moments. It is argued that in general the latter information can only be obtained on utilizing a number of experimental scattering geometries. In particular, the merits of angle-resolved experiments are illustrated.     

Order parameters and areas in fluid-phase oriented lipid membranes using wide angle X-ray scattering

We used wide angle x-ray scattering (WAXS) from stacks of oriented lipid bilayers to measure chain orientational order parameters and lipid areas in model membranes consisting of mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/cholesterol and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)/cholesterol in fluid phases. The addition of 40% cholesterol to either DOPC or DPPC changes the WAXS pattern due to an increase in acyl chain orientational order, which is one of the main properties distinguishing the cholesterol-rich liquid-ordered (Lo) phase from the liquid-disordered (Ld) phase. In contrast, powder x-ray data from multilamellar vesicles does not yield information about orientational order, and the scattering from the Lo and Ld phases looks similar. An analytical model to describe the relationship between the chain orientational distribution and WAXS data was used to obtain an average orientational order parameter, S_x-ray. When 40% cholesterol is added to either DOPC or DPPC, S_x-ray more than doubles, consistent with previous NMR order parameter measurements. By combining information about the average chain orientation with the chain-chain correlation spacing, we extended a commonly used method for calculating areas for gel-phase lipids to fluid-phase lipids and obtained agreement to within 5% of literature values.     

Comparison of the orientational order of lipid chains in the L alpha and HII phases

The orientational order profile has been determined by using deuterium nuclear magnetic resonance (2H NMR) for POPE in the lamellar liquid-crystalline (L alpha) and the hexagonal (HII) phases and is shown to be sensitive to the symmetry of the lipid phase. In the HII phase, as compared to the L alpha phase, the acyl chains are characterized by a greater motional freedom, and the orientational order is distributed more uniformly along the lipid acyl chain. This is consistent with a change from a cylindrical to a wedge-shaped space available for the lipid chain. 2H NMR studies of POPE dispersions containing tetradecanol or decane, both of which can induce HII phase structure, show very different behavior. Tetradecanol appears to align with the phospholipid chains and experience the L alpha to HII phase transition with a similar change in motional averaging as observed for the phospholipid chains themselves. In contrast, decane is apparently deeply embedded in the lipid structure and exhibits only a small degree of orientation. The L alpha to HII phase transition for systems containing decane leads to a dramatic increase of the motional freedom of decane which is more pronounced than that observed for the lipid chains. This is consistent with a preferential partition of the decane molecules into a disordered environment such as the intercylinder spaces in the HII phase. The presence of decane in the HII phase structure does not modify the order of the lipid chains.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of cholesterol on the orientational order of unsaturated lipids in the membranes of Acholeplasma laidlawii: A 2H-NMR study

We have investigated by ²H-NMR the effects of the incorporation of cholesterol on the orientational order of unsaturated lipid acyl chains in the membranes of Acholeplasma laidlawii B. This is the only ²-NMR study to date of the influence of cholesterol in a biological membrane using specifically labelled fatty acids. We observed the characteristics condensing effect of cholesterol on the lipid acyl chain order in the liquid crystalline phase. In terms of the percentage increase in the quadrupolar splittings, the presence of cholesterol has its greatest effect on the methyl end of the labelled oleoyl chains, with a maximum at the C-14 segment. In absolute terms, the perturbation is greatest in the carboxyl end of the chains. The temperature dependence of the ²H spectra for the cholesterol-containing membranes is very similar to that for the cholesterol-free membranes. The broad phase transition of the membrane lipids, which is characteristic for the samples lacking cholesterol, is apparently little affected by the presence of up to 27 mol% cholesterol. In addition, the temperature of onset of the phase transition is not significantly depressed by the presence of cholesterol.     

The physical properties of glycosyldiacylglycerols. Calorimetric studies of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols

The polymorphic phase behavior of aqueous dispersions of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-glucopyranosyl)-sn-glycerols was studied by differential scanning calorimetry. At fast heating rates, unannealed samples of these lipids exhibit a strongly energetic, lower temperature transition, which is followed by a weakly energetic, higher temperature transition. X-ray diffraction studies have enabled the assignments of these events to a lamellar gel/liquid crystalline (chain-melting) phase transition and a bilayer/nonbilayer phase transition, respectively. Whereas the values for both the temperature and enthalpy of the chain-melting phase transition increase with increasing acyl chain length, those of the bilayer/nonbilayer phase transition show almost no chain-length dependence. However, the nature of the bilayer/nonbilayer transition is affected by the length of the acyl chain. The shorter chain compounds form a nonbilayer 2-D monoclinic phase at high temperature whereas the longer chain compounds from a true inverted hexagonal (HII) phase. Our studies also show that the gel phase that is initially formed on cooling of these lipids is metastable with respect to a more stable gel phase and that prolonged annealing results in a slow conversion to the more stable phase after initial nucleation by incubation at appropriate low temperatures. The formation of these stable gel phases is shown to be markedly dependent upon the length of the acyl chains and whether they contain an odd or an even number of carbon atoms. There is also evidence to suggest that, in the case of the shorter chain compounds at least, the process may proceed via another gel-phase intermediate. In annealed samples of the shorter chain compounds, the stable gel phase converts directly to the L alpha phase upon heating, whereas annealed samples of the longer chain glycolipids convert to a metastable gel phase prior the chain melging.(ABSTRACT TRUNCATED AT 250 WORDS)     

19F nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. Orientational order in the presence of a series of positional isomers of cis-octadecenoic acid

The 19F nuclear magnetic resonance (NMR) spectra of membranes of Acholeplasma laidlawii B enriched with one of a series of positional isomers of cis-octadecenoic acid plus small amounts of one of a number of isomers of monofluoropalmitic acid were interpreted in terms of an orientational order parameter (Smol). The variation of Smol with the position of the fluorine label in the liquid-crystalline state yielded an "order profile" with characteristics similar to those obtained via 2H NMR and which was relatively invariant regardless of the site of cis unsaturation. In the gel state, values of Smol approached the theoretical maximum, and the order profiles in the presence of different isomeric cis-octadecenoic acids displayed distinct dissimilarities. When the cis double bond was located proximal to the methyl terminus of the fatty acyl chain, a gradient of order across the bilayer was still evident in the gel state. When the cis double bond was located near the carbonyl head group, values of Smol were approximately equal at all chain positions. These observations were interpreted as indicating that in the gel state the stringency of packing restrictions is still subject to variation across the width of the bilayer. Relative overall orientational order among all isomers examined (specifically, 18:1c delta 4, delta 5, delta 6, delta 7, delta 8, delta 9, delta 10, delta 11, delta 12, delta 13, delta 14, and delta 15) varied directly as a function of proximity to the lipid gel to liquid-crystalline phase transition (Tm) (determined via differential scanning calorimetry) when compared at a constant temperature.(ABSTRACT TRUNCATED AT 250 WORDS)     

13C NMR studies of the thermal properties of a model high density lipoprotein. Apolipoprotein A-I-dimyristoylphosphatidylcholine complex

The most abundant lipid and protein components of human plasma high density lipoproteins are phosphatidylcholine and apolipoprotein A-I (A-I). Under appropriate conditions, A-I spontaneously associates with dimyristoylphosphatidylcholine (DMPC) to quantitatively form a lipid-protein complex with a DMPC/A-I molar ratio of 100:1. Differential scanning calorimetry of this complex reveals two broad thermal transitions centered at approximately 27 and 72 degrees C. 13C NMR spectra of the complex have been obtained above, at, and below the lower transition temperature. The 13C resonance arising from the 3' carbon of the fatty acyl chains is a doublet, split by approximately 0.2 ppm, suggesting that the 3' carbon nuclei occupy two magnetically inequivalent sites. By replacing the sn-2 fatty acyl chain with myristate selectively 13C-enriched at carbon 3', we have shown that the splitting is, in fact, a result of magnetic inequivalence of the two sites and have assigned the lower field resonance to the 3' carbon nucleus of the sn-2 chain. The temperature dependence of the NMR relaxation rates indicates that the endothermic transition at 27 degrees C is associated with increased motional freedom for the phospholipids within this complex. The temperature dependence of the fatty acyl chain methylene 13C chemical shifts suggests that the population of gauche conformers increases above the transition temperature. These dynamic and conformational changes are characteristic of gel----liquid crystalline phase transitions observed in pure phospholipid systems. For the DMPC-A-I complex at 37 degrees C, the chemical shifts of the fatty acyl C 4'- 11' methylene envelope and of the C 7' and C 13' resonances occur significantly downfield from the corresponding chemical shifts for the DMPC vesicle. These results suggest that the apoprotein rigidifies the acyl chains by increasing their number of trans conformers.     

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.     

Molecular areas of phospholipids as determined by 2H NMR spectroscopy. Comparison of phosphatidylethanolamines and phosphatidylcholines.

The role of lipid diversity in biomembranes is one of the major unsolved problems in biochemistry. One parameter of possible importance is the mean cross-sectional area occupied per lipid molecule, which may be related to formation of nonbilayer structures and membrane protein function. We have used 2H NMR spectroscopy to compare the properties of 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE-d62) and 1,2-diperdeuteriopalmitoyl-sn-glycero-3-phosphocholine (DPPC-d62) in the L alpha phase. We find that DPPE has greater segmental order than DPPC, and that this increase in order is related to the smaller area per acyl chain found for DPPE. Values of the mean cross-sectional chain area are calculated using a simple diamond lattice model for the acyl chain configurational statistics, together with dilatometry data. The results obtained for the mean area per molecule are comparable with those from low angle x-ray diffraction studies.     

The effect of chain length and lipid phase transitions on the selective permeability properties of liposomes.

This paper describes experiments showing the importance of the fatty acid chain length on the barrier properties of liposomal bilayers, prepared from saturated lecithins, under conditions of lateral phase separation. 1. Above the gel to liquid crystalline phase transition temperature, liposomes prepared from saturated lecithins with 14 or more carbon atoms per acyl chain exist as stable bilayers, which are practically impermeable to ions. 2. At temperatures well above the transition temperature dilauroyl phosphatidylcholine liposomes exhibited osmotic shrinkage, which was dependent on the ionic size of the solute used to bring about the osmotic gradient, indicating that the permeation through these less stable bilayers takes place mainly via individual diffusion of the permeating ions. 3. An enhanced release of trapped potassium from liposomes was demonstrated in the vicinity of the transition temperature. The extent of the increase, however, depended strongly on the length of the paraffin chain. 4. From measurements of the shrinkage behaviour of liposomes in the vicinity of the transition temperature it is concluded that the increased permeability decreases with increasing diameter of the permeating ion. This finding implies that the increased permeability at the transition temperature cannot be ascribed to "macroscopic" rupture of the liposomal membrane. The maximum permeability in the vicinity of the Tc is discussed in terms of probability and size distribution of statistical pore formation at the boundaries of liquid and solid domains.     

Effect of protein hydration on receptor conformation: decreased levels of bound water promote metarhodopsin II formation.

Neutral solutes were used to investigate the effects of osmotic stress both on the ability of rhodopsin to undergo its activating conformation change and on acyl chain packing in the rod outer segment (ROS) disk membrane. The equilibrium concentration of metarhodopsin II (MII), the conformation of photoactivated rhodopsin, which binds and activates transducin, was increased by glycerol, sucrose, and stachyose in a manner which was linear with osmolality. Analysis of this shift in equilibrium in terms of the dependence of ln(Keq) on osmolality revealed that 20 +/- 1 water molecules are released during the MI-to-MII transition at 20 degrees C, and at 35 degrees C 13 +/- 1 waters are released. At 35 degrees C the average time constant for MII formation was increased from 1.20 +/- 0.09 ms to 1.63 +/- 0.09 ms by addition of 1 osmolal sucrose or glycerol. The effect of the neutral solutes on acyl chain packing in the ROS disk membrane was assessed via measurements of the fluorescence lifetime and anisotropy decay of 1,6-diphenyl-1,3,5-hexatriene (DPH). Analysis of the anisotropy decay of DPH in terms of the rotational diffusion model showed that the angular width of the equilibrium orientational distribution of DPH about the membrane normal was progressively narrowed by increased osmolality. The parameter fv, which is proportional to the overlap between the DPH orientational probability distribution and a random orientational distribution, was reduced by the osmolytes in a manner which was linear with osmolality. This study highlights the potentially opposing interplay between the effect of membrane surface hydration on both the lipid bilayer and integral membrane protein structure. Our results further demonstrate that the binding and release of water molecules play an important role in modulating functional conformational changes for integral membrane proteins, as well as for soluble globular proteins.     

Fluorine-19 nuclear magnetic resonance studies of lipid fatty acyl chain order and dynamics in Acholeplasma laidlawii B membranes. A direct comparison of the effects of cis and trans cyclopropane ring and double-bond substituents on orientational order

The hydrocarbon chain orientational order parameters of membranes of Acholeplasma laidlawii B, enriched with large quantities of fatty acids containing either a cis or a trans cyclopropane ring or a cis or trans double bond, plus small quantities of one of an isomeric series of monofluoropalmitic acids, were determined via fluorine-19 nuclear magnetic resonance spectroscopy over a range of temperatures spanning the corresponding gel to liquid-crystalline phase transitions (determined via differential scanning calorimetry). Membrane orientational order profiles in the liquid-crystalline state were generally similar, regardless of the particular fatty acid structure present, showing a region of relatively constant order preceding a region of progressively decreasing order toward the methyl terminus of the acyl chain. In the gel state, the order profiles in the presence of either a trans cyclopropane ring or trans double-bond substituent were similar and were characterized by a pronounced head to tail gradient of order at temperatures just below the lipid phase transition, while at temperatures far below the lipid phase transition this gradient was less pronounced, all chain positions showing a more uniformly high degree of orientational ordering. In the gel state, the order profiles in the presence of either a cis cyclopropane ring or a cis double-bond substituent were also similar but were highly unusual in that order first increased and only then subsequently decreased toward the acyl chain methyl terminus. In addition, the substituents in the cis configuration, whether a cyclopropane ring or a double bond, were overall more disordered in the gel state than the corresponding substituents in the trans configuration.(ABSTRACT TRUNCATED AT 250 WORDS)     

Modulation of phospholipid acyl chain order by cholesterol. A solid-state 2H nuclear magnetic resonance study

The effect of cholesterol on the acyl chain order of three glycerophosphocholines with 14, 16, and 18 carbons per acyl chain, namely, di(14:0)PC, di(16:0)PC, and di(18:0)PC, above the gel to liquid-crystalline phase transition temperature was investigated by using 2H nuclear magnetic resonance spectroscopy. Average acyl chain lengths were calculated from the segmental order parameters (Smol) for the sn-1 and the sn-2 chains in the absence of cholesterol and at 3:1, 2:1, and 1:1 mole ratios of phospholipid-cholesterol. The three binary mixtures of cholesterol with phosphatidylcholines are in the liquid-ordered (lo) phase. For all the three phosphatidylcholine-cholesterol systems, the distance from the carbonyl groups to the terminal methyl groups is shorter than the length of the cholesterol molecule. A molecular model for the lo phase consistent with these observations has in a statistical sense a part of each cholesterol molecule in one monolayer extending into the other monolayer. This results in a packing arrangement akin to that in interdigitated systems. On the basis of the effect of cholesterol on phospholipid acyl chain orientational order, it is suggested that the liquid-disordered (ld) phase at low cholesterol concentrations corresponds to a packing mode in which the cholesterol molecule spans the entire transbilayer hydrophobic region. A molecular mechanism is proposed in which increasing the concentration of cholesterol has the effect of stretching the acyl chains of phospholipids by increasing the population of trans conformers up to a stage where the hydrophobic length is considerably longer than the cholesterol molecule. Beyond this concentration, the partially interdigitated phase forms.(ABSTRACT TRUNCATED AT 250 WORDS)