Deuterium NMR studies of the interactions of polyhydroxyl compounds and of glycolipids with lipid model membranes

Shiga toxin binding alters lipid packing and the domain structure of Gb3-containing membranes: a solid-state NMR study

We studied the influence of globotriaosylceramide (Gb3) lipid molecules on the properties of phospholipid membranes composed of a liquid ordered (lo)/liquid disordered (ld) phase separated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/N-palmitoyl-d-erythro-sphingosylphosphorylcholine (PSM)/cholesterol mixture (40/35/20, mol/mol/mol) supplemented with 5 mol% of either short acyl chain palmitoyl-Gb3 or long acyl chain lignoceryl-Gb3 using 2H solid-state NMR spectroscopy. To this end, both globotriaosylceramides were chemically synthesized featuring a perdeuterated lipid acyl chain. The solid-state 2H NMR spectra support the phase separation into a POPC-rich ld phase and a PSM/cholesterol-rich lo phase. The long chain lignoceryl-Gb3 showed a rather unusual order parameter profile of the acyl chain, which flattens out for the last ∼6 methylene segments. Such an odd chain conformation can be explained by partial chain interdigitation and/or a very fluid midplane region of the membrane. Possibly, the Gb3 molecules may thus preferentially be localized at the lo/ld phase boundary. In contrast, the short chain palmitoyl-Gb3 was well associated with the PSM/cholesterol-rich lo phase. Gb3 molecules act as membrane receptors for the Shiga toxin (STx) produced by Shigella dysenteriae and by enterohemorrhagic strains of Escherichia coli (EHEC). The B-subunits of STx (STxB) forming a pentameric structure were produced recombinantly and incubated with the membrane mixtures leading to alterations in the lipid packing properties and lateral organization of the membranes. Typically, STxB binding led to a decrease in lipid chain order in agreement with partial immersion of protein segments into the lipid-water interface of the membrane. In the presence of STxB, Gb3 preferentially partitioned into the lo membrane phase. In particular the short acyl chain palmitoyl-Gb3 showed very similar chain order parameters to PSM. In the presence of STxB, all lipid species showed isotropic contributions to the 2H NMR powder spectra; this was most pronounced for the Gb3 molecules. Such isotropic contributions are caused by highly curved membrane structures, which have previously been detected as membrane invaginations in fluorescence microscopy. Our analysis estimated that STxB induced highly curved membrane structures with a curvature radius of less than ∼10 nm likely related to the insertion of STxB segments into the lipid-water interface of the membrane.

Molecular dynamics simulations of the interactions of DMSO, mono- and polyhydroxylated cryosolvents with a hydrated phospholipid bilayer

Molecular dynamics (MD) simulations have been used to investigate the interactions of a variety of hydroxylated cryosolvents (glycerol, propylene glycol and ethylene glycol), methanol and dimethyl sulfoxide (DMSO) in aqueous solution with a 1,2-dipalmitoyl-sn-glycero-3-phosphatidylcholine (DPPC) bilayer in its fluid phase at 323K. Each cryosolvent induced lateral expansion of the membrane leading to thinning of the bilayer and resulting in disordering of the lipid hydrocarbon chains. Propylene glycol and DMSO were observed to exhibit a greater disordering effect on the structure of the membrane than the other three alcohols. Closer examination exposed a number of effects on the lipid bilayer as a function of the molecular size and hydrogen bonding capacity of the cryosolvents. Analyses of hydrogen bonds revealed that increased concentrations of the polyhydroxylated cryosolvents induced the formation of a cross-linked cryosolvent layer across the surface of the membrane bilayer. This effect was most pronounced for glycerol at sufficiently high concentrations, which displayed a comparatively enhanced capacity to induce cross-linking of lipid headgroups resulting in the formation of extensive hydrogen bonding bridges and the promotion of a dense cryosolvent layer across the phospholipid bilayer.

Molecular dynamics simulations of membranes composed of glycolipids and phospholipids

Lipid membranes composed of 1,2-di-(9Z,12Z,15Z)-octadecatrienoyl-3-O-β-D-galactosyl-sn-glycerol or monogalactosyldiacylglycerol (MGDG) and 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) were studied by means of molecular dynamics (MD) computer simulations. Three lipid compositions were considered: 0%, 20%, and 45% MGDG (by mole) denoted as MG-0, MG-20, and MG-45, respectively. The article is focused on the calculation of NMR dipolar interactions, which were confronted with previously reported experimental couplings. Dynamical processes and orientational distributions relevant for the averaging of dipolar interactions were evaluated. Furthermore, several parameters important for characterization of the bilayer structure, molecular organization, and dynamics were investigated. In general, only a minor change in DMPC properties was observed upon the increased MGDG/DMPC ratio, whereas properties related to MGDG undergo a more pronounced change. This effect was ascribed to the fact that DMPC is a bilayer (L(α)) forming lipid, whereas MGDG prefers a reverse hexagonal (H(II)) arrangement.

Reconciliation of opposing views on membrane-sugar interactions

It is well established that small sugars exert different types of stabilization of biomembranes both in vivo and in vitro. However, the essential question of whether sugars are bound to or expelled from membrane surfaces, i.e., the sign and size of the free energy of the interaction, remains unresolved, and this prevents a molecular understanding of the stabilizing mechanism. We have used small-angle neutron scattering and thermodynamic measurements to show that sugars may be either bound or expelled depending on the concentration of sugar. At low concentration, small sugars bind quite strongly to a lipid bilayer, and the accumulation of sugar at the interface makes the membrane thinner and laterally expanded. Above ∼0.2 M the sugars gradually become expelled from the membrane surface, and this repulsive mode of interaction counteracts membrane thinning. The dual nature of sugar-membrane interactions offers a reconciliation of conflicting views in earlier reports on sugar-induced modulations of membrane properties.

An NMR database for simulations of membrane dynamics

Computational methods are powerful in capturing the results of experimental studies in terms of force fields that both explain and predict biological structures. Validation of molecular simulations requires comparison with experimental data to test and confirm computational predictions. Here we report a comprehensive database of NMR results for membrane phospholipids with interpretations intended to be accessible by non-NMR specialists. Experimental ¹³C-¹H and ²H NMR segmental order parameters (S(CH) or S(CD)) and spin-lattice (Zeeman) relaxation times (T(1Z)) are summarized in convenient tabular form for various saturated, unsaturated, and biological membrane phospholipids. Segmental order parameters give direct information about bilayer structural properties, including the area per lipid and volumetric hydrocarbon thickness. In addition, relaxation rates provide complementary information about molecular dynamics. Particular attention is paid to the magnetic field dependence (frequency dispersion) of the NMR relaxation rates in terms of various simplified power laws. Model-free reduction of the T(1Z) studies in terms of a power-law formalism shows that the relaxation rates for saturated phosphatidylcholines follow a single frequency-dispersive trend within the MHz regime. We show how analytical models can guide the continued development of atomistic and coarse-grained force fields. Our interpretation suggests that lipid diffusion and collective order fluctuations are implicitly governed by the viscoelastic nature of the liquid-crystalline ensemble. Collective bilayer excitations are emergent over mesoscopic length scales that fall between the molecular and bilayer dimensions, and are important for lipid organization and lipid-protein interactions. Future conceptual advances and theoretical reductions will foster understanding of biomembrane structural dynamics through a synergy of NMR measurements and molecular simulations.

Water replacement hypothesis in atomic details: effect of trehalose on the structure of single dehydrated POPC bilayers

We present molecular dynamics (MD) simulations to study the plausibility of the water replacement hypothesis (WRH) from the viewpoint of structural chemistry. A total of 256 2-oleoyl-1-palmitoyl-sn-glycero-3-phosphocholine (POPC) lipids were modeled for 400 ns at 11.7 or 5.4 waters/lipid. To obtain a single dehydrated bilayer relevant to the WRH, simulations were performed in the NP(xy)h(z)T ensemble with h(z) > 8 nm, allowing interactions between lipids in the membrane plane and preventing interactions between neighboring membranes via periodic boundary conditions. This setup resulted in a stable single bilayer in (or near) the gel state. Trehalose caused a concentration-dependent increase of the area per lipid (APL) accompanied by fluidizing the bilayer core. This mechanism has been suggested by the WRH. However, dehydrated bilayers in the presence of trehalose were not structurally identical to fully hydrated bilayers. The headgroup vector was in a more parallel orientation in dehydrated bilayers with respect to the bilayer plane and maintained this orientation in the presence of trehalose in spite of APL increase. The total dipole potential changed sign in dehydrated bilayers and remained slightly positive in the presence of trehalose. The model of a dehydrated bilayer presented here allows the study of the mechanisms of membrane protection against desiccation by different compounds.

Thermodynamic and structural behaviour of equimolar POPC/CnE4 (n=8, 12, 16) mixtures by sorption gravimetry, 2H NMR spectroscopy and X-ray diffraction

The hydration behaviour of equimolar mixtures of phospholipids and nonionic surfactants with different chain length was investigated by gravimetric sorption, NMR spectroscopy and X-ray diffraction. At the most hydration degrees investigated, the incorporation of nonionic surfactants in a phospholipid bilayer leads to an increase of the hydrophilicity, which can be shown by the presence of excess hydration. The increased hydrophilicity could be explained by the excavation of additional water binding sites due to the "dilution" of the dipole field of the phospholipid bilayer. Another related contribution arises from the increase of the accessible surface area due to the increase of gauche conformers that result from the steric mismatch when surfactants are incorporated into the phospholipid matrix. (2)H NMR spectroscopy was used to determine the quadrupolar splitting representing a measure of the order state of water. The swelling behaviour could be assessed by small-angle X-ray diffraction. (31)P NMR spectroscopy was applied for the assignment of phase structures to the respective hydration range.

Water replacement hypothesis in atomic detail--factors determining the structure of dehydrated bilayer stacks

According to the water replacement hypothesis, trehalose stabilizes dry membranes by preventing the decrease of spacing between membrane lipids under dehydration. In this study, we use molecular-dynamics simulations to investigate the influence of trehalose on the area per lipid (APL) and related structural properties of dehydrated bilayers in atomic detail. The starting conformation of a palmitoyloleolylphosphatidylcholine lipid bilayer in excess water was been obtained by self-assembly. A series of molecular-dynamics simulations of palmitoyloleolylphosphatidylcholine with different degrees of dehydration (28.5, 11.7, and 5.4 waters per lipid) and different molar trehalose/lipid ratios (<1:1, 1:1, and >1:1) were carried out in the NPT ensemble. Water removal causes the formation of multilamellar "stacks" through periodic boundary conditions. The headgroups reorient from pointing outward to inward with dehydration. This causes changes in the electrostatic interactions between interfaces, resulting in interface interpenetration. Interpenetration creates self-spacing of the bilayers and prevents gel-phase formation. At lower concentrations, trehalose does not separate the interfaces, and acting together with self-spacing, it causes a considerable increase of APL. APL decreases at higher trehalose concentrations when the layer of sugar physically separates the interfaces. When interfaces are separated, the model confirms the water replacement hypothesis.

NMR investigations of interactions between anesthetics and lipid bilayers

Interactions between anesthetics (lidocaine and short chain alcohols) and lipid membranes formed by dimyristoylphosphatidylcholine (DMPC) were studied using NMR spectroscopy. The orientational order of lidocaine was investigated using deuterium NMR on a selectively labelled compound whereas segmental ordering in the lipids was probed by two-dimensional 1H-13C separated local field experiments under magic-angle spinning conditions. In addition, trajectories generated in molecular dynamics (MD) computer simulations were used for interpretation of the experimental results. Separate simulations were carried out with charged and uncharged lidocaine molecules. Reasonable agreement between experimental dipolar interactions and the calculated counterparts was observed. Our results clearly show that charged lidocaine affects significantly the lipid headgroup. In particular the ordering of the lipids is increased accompanied by drastic changes in the orientation of the P-N vector in the choline group.

NMR studies of membranes composed of glycolipids and phospholipids

Lipid membranes composed of monogalactosyldiacylglycerol (MGDG) and dimyristoylphosphatidylcholine (DMPC) were studied by means of NMR spectroscopy. The macroscopic phase behaviour was investigated by (31)P NMR under stationary conditions, whereas microscopic properties such as segmental ordering were probed by two-dimensional (1)H-(13)C separated local field experiments under magic-angle spinning conditions. Our results clearly show that ordering/disordering effects occur for the headgroups as well as for the acyl chains when the sample composition is varied. In particular, the (1)H-(13)C dipolar couplings within the galactose headgroup of MGDG exhibited significant concentration dependence.

Effect of trehalose in low concentration on the binding and transport of porphyrins in liposome-human serum albumin system

The influence of trehalose on the interaction of liposomes with porphyrins and with human serum albumin (HSA) was studied. Small unilamellar liposomes were prepared from 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) and from DMPC/1,2-dimyristoyl-sn-glycero-3-phosphatidylglycerol (DMPG) 19:1 w/w% and incorporated with mesoporphyrin IX (MP) or magnesium mesoporphyrin (MgMP). The fluorescence intensity and anisotropy of porphyrins were measured within the temperature range of 15-33 degrees C, in the presence and in the absence of 3x10(-2) M trehalose, to study the location of the porphyrins inside the liposomes and their partition between the liposomes and HSA. Based on the presented data and our earlier results (I. Bárdos-Nagy, R. Galántai, A.D. Kaposi, J. Fidy, Int. J. Pharm. 175 (1998) 255-267) we conclude that trehalose - even at this relatively low concentration - interacts with the head groups of the liposomes and that the presence of DMPG enhances the effect. This effect seems to hinder the binding of HSA to the liposome surface and influences the location of MgMP within the liposomes. In the case of MP, the porphyrin partition between the liposomes and HSA was affected by trehalose, while for MgMP, trehalose changed the structural conditions of porphyrin binding to the liposomes. The amount of trehalose used did not have a general trend to modify the association constants of porphyrin derivatives either to liposomes or to HSA.

2H-NMR study of two probe-labelled glycosphingolipid-derived signalling modulators in bilayer membranes

We describe here the first report of sphingoid bases bearing non-perturbing 2H probe nuclei. These were produced, by two different routes of partial synthesis, to permit direct assessment of their arrangement and behaviour as minor components in membrane systems. Wideline 2H-NMR spectra of N,N-dimethylsphingosine with deuterated amino-methyl groups ([2H6]dimethylsphingosine), and of lyso-dihydrogalactosylceramide (lyso-GalCer) with deuterium nuclei at C4,C5 of the sphingosine backbone and at C3,C4 of the galactose ring ([2H4]lyso-GalCer), were recorded in unsonicated, cholesterol-containing fluid bilayer membranes. The sphingolipid metabolites behaved as single populations of lipid amphiphiles dispersed uniformly in the membrane and undergoing rapid symmetric motion about their long molecular axes. This was the case throughout the pH ranges examined, which included values generally considered for the cell cytoplasm. Spectra of [2H6]dimethyl sphingosine indicated that the methyl groups are equivalent on the NMR timescale, and that the molecule's orientation and behaviour are largely unaffected by pH over the range, 6 to 10.5. There was no spectral evidence of deprotonation of the tertiary amine function in this range. Similarly, variation of pH between 6.4 and 8.9 had virtually no effect on the average conformation and orientational order of lyso-GalCer at the level of C4,C5 in the sphingosine backbone. pH did, however, exert significant control over the orientation of the galactose residue--the effect being most marked in the region of the sphingoid base pKa. The lyso-glycolipid showed some evidence of being less motionally ordered than the corresponding parent species, presumably as a result of removal of constraints imposed by the fatty acid.

Oligosaccharide behavior of complex natural glycosphingolipids in multicomponent model membranes

Wideline 2H NMR of model membranes was used to consider the molecular consequences of factors often suggested as modulators of complex glycosphingolipid oligosaccharide arrangement and motional characteristics at cell surfaces. GM1, asialo-GM1, and globoside were studied as examples of plasma membrane recognition sites. The experimental approach involved substitution of deuterons (D) for protons at specific locations within the carbohydrate chains. Deuterated glycolipids were then dispersed at 7-10 mol% in unsonicated bilayers of 1-palmitoyl-2-oleoylphosphatidylcholine. Factors tested for their significance to carbohydrate chain conformation and dynamics included glycolipid natural alkyl and acyl chain variability, membrane fluidity, and the presence of cholesterol and a charged sugar residue (neuraminic acid). Effects of Ca2+ and membrane-associated protein were briefly considered. Two distinct strategies were employed in substituting deuterons for selected protons of carbohydrate residues. Neither approach necessitated alteration of the glycolipid natural fatty acid composition. (i) Protons of the exocyclic hydroxymethyl group on the terminal Gal residue of GM1 and asialo-GM1, and on the terminal N-acetylgalactosamine (GalNAc) residue of globoside, were replaced with deuterium (producing -CDHOH) by an enzymatic oxidation/reduction cycle. This represents the first application of such an approach to deuteration of complex neutral glycolipids. Spectral results were compared to those obtained for the similarly-deuterated monoglycosyl lipid, galactosylceramide (GalCer), with natural fatty acid composition. Efficacy of this labeling method may in principle be influenced by structural variations within a given glycolipid family. Also, asymmetric rotation of the deuterated group made it less attractive than the second method for relating spectral features to receptor geometry. (ii) A general synthetic, nonenzymatic method was investigated for replacing amino sugar N-acetyl groups with deuterated acetate (-COCD3). The acetate group of the GalNAc residue of globoside, GM1, and asialo-GM1, as well as that on neuraminic acid in GM1, was replaced with -COCD3. This second method afforded better signal-to-noise--an important consideration for 2H NMR. The NMR technique employed had the potential for detecting changes of as little as 10% in oligosaccharide orientation or motional order. Each glycolipid demonstrated clear evidence of preferred average oligosaccharide conformations in all (fluid) membrane environments examined. The most striking observation was that, in fluid matrices, conformation and motional order of the complex oligosaccharide chains were only modestly influenced by factors tested, including natural variation in the glycolipid hydrocarbon chains, membrane fluidity, temperature, and the presence of cholesterol or the N-acetylneuraminic acid (NeuAc) residue on GM1.(ABSTRACT TRUNCATED AT 400 WORDS)

Comparisons of the relative effects of polyhydroxyl compounds on local versus long-range motions in the mitochondrial inner membrane. Fluorescence recovery after photobleaching, fluorescence lifetime, and fluorescence anisotropy studies

This laboratory has been interested in understanding the relationship between molecular motion and electron transport rates in the mitochondrial inner membrane. We have previously noted a sucrose-induced decrease in both multicomponent electron transport rates and lateral diffusion of redox components. The decreases in lateral diffusion and the related mobile fraction of redox components were greater than expected from hydrodynamic theory. In this report we sought to understand how the presence of increasing aqueous concentrations of polyhydroxyl agents affect short-range motions in different regions of the inner membrane bilayer, frequently expressed in terms of 'viscosity' and order, compared to lateral diffusion. Fluorescence recovery after photobleaching was used to monitor long-range phospholipid and integral protein diffusion. Multifrequency fluorescence lifetime and steady-state fluorescence anisotropy techniques were used to monitor local dynamics of diphenylhexatriene (DPH) and trimethylaminodiphenylhexatriene (TMA-DPH). Light scattering corrections were found to be essential for inner membrane measurements by the latter two techniques. DPH and TMA-DPH each exhibited two-lifetime components. Generally, increasing the aqueous concentration of polyhydroxyl agents decreased the average DPH lifetime and increased the average TMA-DPH lifetime. In general, under the same conditions fluorescence anisotropies increased. Our results indicated that changes in the rotational diffusion coefficient, microviscosity and order were being induced at both the phospholipid headgroup and in the acyl chain regions of the membrane bilayer. Our results suggest that these changes may be due in part to induced changes in the interaction and distribution of water with membranes. Long-range lateral diffusion was found to be significantly retarded by increasing concentrations of polyhydroxyl agents. We conclude that the discrepancies between bulk viscosity predicted decreases in long-range diffusion may result, in part, from the aforementioned membrane/water interactions. We also note an apparent qualitative relationship between long-range lateral diffusion reported diffusion coefficient with local TMA-DPH reported rotational diffusion coefficient and apparent microviscosities.

Increased adhesion between neutral lipid bilayers: interbilayer bridges formed by tannic acid

Tannic acid (TA) is a naturally occurring polyphenolic compound that aggregates membranes and neutral phosolipid vesicles and precipitates many proteins. This study analyzes TA binding to lipid membranes and the ensuing aggregation. The optical density of dispersions of phosphatidylcholine (PC) vesicles increased upon the addition of TA and electron micrographs showed that TA caused the vesicles to aggregate and form stacks of tightly packed disks. Solution calorimetry showed that TA bound to PC bilayers with a molar binding enthalpy of -8.3 kcal/mol and zeta potential measurements revealed that TA imparted a small negative charge to PC vesicles. Monolayer studies showed that TA bound to PC with a dissociation constant of 1.5 microM and reduced the dipole potential by up to 250 mV. Both the increase in optical density and decrease in dipole potential produced by TA could be reversed by the addition of polyvinylpyrrolidone, a compound that chelates TA by providing H-bond acceptor groups. NMR, micropipette aspiration, and x-ray diffraction experiments showed that TA incorporated into liquid crystalline PC membranes, increasing the area per lipid molecule and decreasing the bilayer thickness by 2 to 4%. 2H-NMR quadrupole splitting measurements also showed that TA associated with a PC molecule for times much less than 10(-4) s. In gel phase bilayers, TA caused the hydrocarbon chains from apposing monolayers to fully interdigitate. X-ray diffraction measurements of both gel and liquid crystalline dispersions showed that TA, at a critical concentration of about 1 mM, reduced the fluid spacing between adjacent bilayers by 8-10 A. These data place severe constraints on how TA can pack between adjacent bilayers and cause vesicles to adhere. We conclude that TA promotes vesicle aggregation by reducing the fluid spacing between bilayers by the formation of transient interbilayer bridges by inserting its digallic acid residues into the interfacial regions of adjacent bilayers and spanning the interbilayer space.

The cryoprotectant trehalose destabilises the bilayer organisation of Escherichia coli-derived membrane systems at elevated temperatures as determined by 2H and 31P-NMR

In this study, 2H and 31P-NMR techniques were used to study the effects of trehalose and glycerol on phase transitions and lipid acyl chain order of membrane systems derived from cells of E. coli unsaturated fatty acid auxotroph strain K1059, which was grown in the presence of [11,11-2H2]-oleic acid or [11,11-2H2]-elaidic acid. From an analysis of the temperature dependence of the quadrupolar splitting it could be concluded that neither 1 M trehalose or glycerol generally had any significant effect on the temperature of the lamellar gel to liquid-crystalline phase transition. In the case of the oleate-containing hydrated total lipid extract, glycerol but not trehalose caused a 5 degrees C increase of this transition temperature. In general, both cryoprotectants induced an ordering of the acyl chains in the liquid-crystalline state. Trehalose and glycerol both decrease the bilayer to non-bilayer transition temperature of the hydrated lipid extract of oleate-grown cells by about 5 degrees C, but only trehalose in addition induces an isotropic to hexagonal (HII) phase transition. In the biological membranes, trehalose and not glycerol destabilised the lipid bilayer, and in the case of the E. coli spheroplasts, part of the induced non-bilayer structures is ascribed to a hexagonal (HII) phase in analogy with the total lipids. Interestingly, 1 mM Mg2+ was a prerequisite for the destabilisation of the lipid bilayer. In the hydrated total lipid extract of E. coli grown on the more ordered elaidic acid, both transition temperatures were shifted about 20 degrees C upwards compared with the oleate-containing lipid, but the effect of trehalose on the lipid phase behaviour was similar. The bilayer destabilising ability of trehalose might have implications for the possible protection of biological systems by (cryo-)protectants during dehydration, in that protection is unlikely to be caused by preventing the occurrence of polymorphic phase transitions.

Phase transitions of phospholipid vesicles under osmotic stress and in the presence of ethylene glycol

The effects of poly(ethylene glycol) (PEG) on the phase transition of phospholipid multilamellar vesicles (MLVs) were investigated by using differential scanning calorimetry (DSC). Main transition temperature (Tm) and the pre-transition temperature (Tp) of neutral phospholipid-, DMPC-1, DPPC- and DSPC-MLVs increased with an increase in PEG concentration. The subtransition temperature of DPPC-MLV also increased with an increase in PEG concentration. These results could be qualitatively explained by enhancement of the lateral packing on the basis of the osmoelastic coupling theory. The pretransition temperature increased faster than the main transition temperature did with an increase in PEG concentration. The increment of Tm depended on the hydrocarbon chain length, the shorter the hydrocarbon chain length was, the larger the increment was. The transition width in the DSC peak was broadened with an increase in PEG concentration. These three above-mentioned effects are the main differences between the effects of the osmotic stress on the phase transition of MLVs and those of hydrostatic pressure. On the other hand, ethylene glycol (EG), which is the monomer of PEG, had a biphasic effect on transition temperature of DPPC-, DSPC-, and DMPC-MLV, reducing Tm and Tp at low concentrations, but increasing Tm and extinguishing pretransition at high concentrations. This is explained by the induction of an interdigitated gel phase at high concentrations of EG, which indicates that EG can easily penetrate into the head group region of the lipid, in contrast with PEG 6K, because EG is small. Temperature-EG concentration phase diagrams for the various PC-MLVs were determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Solution effects on the thermotropic phase transition of unilamellar liposomes

We have investigated the effect of two monosaccharides, glucose and fructose, and two disaccharides, sucrose and trehalose, on the thermotropic phase transition of unilamellar extruded vesicles of DPPC. All the sugars investigated raise the main transition temperature (Tm) of some fraction of the lipid, but there are differences between the effect of glucose and the other three sugars. At low concentrations of glucose, Tm is lowered. At high concentrations of glucose there are two transitions, one with a low Tm and one with a high Tm. The data suggest that at low concentrations, all of the glucose present may bind to the bilayer and increase headgroup spacing by physical intercalation or increased hydration. The appearance of a Tm above that of pure hydrated DPPC suggests the possibility of the dehydration of some other population of phospholipid molecules. The other three sugars increase Tm, but at high concentrations of trehalose, sucrose, and fructose a second peak occurs at a low Tm. The other sugars appear to dehydrate the bilayer at low concentrations, but may show some binding or increased hydration of some portion of the lipid at very high concentrations. The sugar effects on unilamellar vesicles are strikingly different from the effects of these sugars on multilamellar vesicles.

Conformational changes of the phosphatidylcholine headgroup due to membrane dehydration. A 2H-NMR study

The influence of hydration on the orientation of the phosphocholine dipole in bilayer membranes was studied with nuclear magnetic resonance. The phosphocholine headgroup of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) was deuterated at the two methylene segments. Phosphorus-31 and deuterium nuclear magnetic resonance measurements were made as a function of hydration in the range of 10-70 wt.% H2O revealing a distinct change in the alignment of the phosphocholine headgroup. With decreasing hydration the N+ end of the phosphocholine head group dipole moves closer to the hydrocarbon layer. The conformational change induced by the loss of water molecules at the membrane surface is qualitatively similar to that observed upon addition of polyhydroxyl compounds.

Interaction of electric dipoles with phospholipid head groups. A 2H and 31P NMR study of phloretin and phloretin analogues in phosphatidylcholine membranes

Phloretin, 4-hydroxyvalerophenone, and 2-hydroxy-omega-phenylpropiophenone are lipophilic dipolar substances that modify ionic conductances of bilayer membranes. The structural changes at the level of the head groups and the hydrocarbon chains as induced by the incorporation of phloretin and its analogues were investigated with deuterium and phosphorus nuclear magnetic resonance. Membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) were selectively deuterated at the choline head group and at the hydrocarbon chains, and 2H and 31P NMR spectra were recorded with varying concentrations of dipolar agents. Incorporation of phloretin leaves the bilayer structure intact, induces only a small disordering of the hydrocarbon chains, and has no significant effect on the head-group dynamics. On the other hand, quite distinct structural changes are observed for the phosphocholine head group. While the -P-N+ dipole is oriented approximately parallel to the membrane surface for pure POPC bilayers, addition of phloretin, and to a lesser extent 4-hydroxyvalerophenone and 2-hydroxy-omega-phenylpropiophenone, rotates the N+ end of the -P-N+ dipole closer to the hydrocarbon layer. The resulting normal component of the -P-N+ dipole partly compensates the electric field of the dipolar agents. In addition to this structural change, phloretin also modifies the hydration layer at the lipid-water interface. Much less 2H2O is adsorbed to the membrane surface when the bilayer contains phloretin, 4-hydroxyvalerophenone, or 2-hydroxy-omega-phenylpropiophenone. Moreover, a rather large change in the residual phosphorus chemical shielding anisotropy argues in favor of hydrogen-bond formation between the phosphate segment and the phloretin hydroxyl groups.

Protection by sugars against phase transition-induced leak in hydrated dimyristoylphosphatidylcholine liposomes

The disaccharides trehalose and sucrose have small effects on temperature and enthalpy of the pre- and main phase transition in hydrated DMPC bilayers. In contrast, these sugars cause a considerable retention of carboxyfluorescein when large unilamellar vesicles of DMPC are heated through the main transition. This effect is sugar specific, as the monosaccharides glucose and fructose are less effective and ethyleneglycol has no effect at all.

Magnitude of the solvation pressure depends on dipole potential

As polar surfaces in solvent are brought together, they experience a large repulsive interaction, termed the solvation pressure. The solvation pressure between rough surfaces, such as lipid bilayers, has been shown previously to decay exponentially with distance between surfaces. In this paper, we compare measured values of the solvation pressure between bilayers and the dipole potential for monolayers in equilibrium with bilayers. For a variety of polar solvents and lipid phases, we find a correlation between the measured solvation pressures and dipole potentials. Analysis of the data indicates that the magnitude of the solvation pressure is proportional to the square of the dipole potential. Our experiments also show that the oriented dipoles in the lipid head-group region, including those of both the lipid and solvent molecules, contribute to the dipole potential. We argue that (i) the field produced by these interfacial dipoles polarizes the interbilayer solvent molecules giving rise to the solvation pressure and (ii) both the solvation pressure and the dipole potential decay exponentially with distance from the bilayer surface, with a decay constant that depends on the packing density of the interbilayer solvent molecules (1-2 A in water). These results may have importance in cell adhesion, adsorption of proteins to membranes, characteristics of channel permeability, and the interpretation of electrokinetic experiments.

Electric charge effects on phospholipid headgroups. Phosphatidylcholine in mixtures with cationic and anionic amphiphiles

The influence of electric surface charges on the polar headgroups and the hydrocarbon region of phospholipid membranes was studied by mixing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) with charged amphiphiles. A positive surface charge was generated with dialkyldimethylammonium salts and a negative surface charge with dialkyl phosphates. The POPC:amphiphile ratio and hence the surface charge density could be varied over a large range since stable liquid-crystalline bilayers were obtained even for the pure amphiphiles in water. POPC was selectively deuterated at both methylene segments of the choline moiety and at the cis double bond of the oleic acyl chain. Additional experiments were carried out with 1,2-dipalmitoyl-rac-glycero-3-phosphocholine labeled at the C-2 position of the glycerol backbone. Deuterium, phosphorus, and nitrogen-14 nuclear magnetic resonance (NMR) spectra were recorded for liquid-crystalline bilayers with varying concentrations of amphiphiles. Although the hydrocarbon region and the glycerol backbone were not significantly influenced by the addition of amphiphiles, very large perturbations of the phosphocholine headgroup were observed. Qualitatively, these results were similar to those observed previously with other cationic and anionic molecules and suggest that the electric surface charge is the essential driving force in changing the phospholipid headgroup orientation and conformation. While the P-N dipole is approximately parallel to the membrane surface in the pure phospholipid membrane, the addition of a positively charged amphiphile or the binding of cationic molecules moves the N+ end of the dipole toward the water phase, changing the orientation of the phosphate segment by more than 30 degrees at the highest amphiphile concentration.(ABSTRACT TRUNCATED AT 250 WORDS)