The outer membrane of Proteus mirabilis. II. The extractable lipid fraction and electron-paramagnetic resonance analysis of the outer and cytoplasmic membranes.

1. The lipid fraction extracted from the outer and cytoplasmic membranes of Proteus mirabilis with chloroform/methanol consisted almost entirely of phosphatidylethanolamine, diphosphatidylglycerol and phosphatidylglycerol. 2. The phospholipid content of the cytoplasmic membrane was more than twice that of the outer membrane (38% as against 18% of the total dry weight) and the proportions of the three phospholipids differed somewhat in the two membranes. Yet, the fatty acid composition of the extractable lipids was essentially the same in both membranes. 3. The freedom of motion of spin-labeled fatty acids in the outer membrane of P. mirabilis depended markedly on temperature and on the position of the nitroxide group on the hydrocarbon chain of the probe, suggesting that the local environment of the probe is an associate lipid structure with the properties of a bilayer. Nevertheless, the mobility of the probe was more restricted in the outer membrane than in the cytoplasmic membrane, indicating a higher viscosity of the outer membrane. 4. Chloroform/methanol completely removed the phospholipids from the outer membrane, leaving the lipopolysaccharide moiety intact. The motion of spin-labeled fatty acids in the extracted membranes was, however, highly restricted, suggesting that, in the native outer membrane, the local environment of the probe is composed of phospholipids rather than lipopolysaccharide. Aqueous acetone extraction removed only 75-80% of the phospholipids of the outer membrane. Nevertheless, the mobility of the spin-labeled fatty acid remained highly restricted, suggesting the existence of two phospholipid environments in the outer membrane differing in the nature of their association with the lipopolysaccharide and protein moieties.     

Nuclear magnetic resonance studies of lipid-protein interactions. A model of the dynamics and energetics of phosphatidylcholine bilayers that contain cytochrome c oxidase.

Reconstituted membrane systems of synthetic phosphatidylcholines and the integral membrane enzyme cytochrome c oxidase were prepared in order to conduct nuclear magnetic resonance studies of lipid-protein interactions. These lipids, labeled with a geminate difluoro group on the 1-position hydrocarbon chain, were combined with the enzyme to give active lipid-protein particles with a well-defined ratio of lipid to protein. The fluorine magnetic resonance spectra of a series of preparations with different lipid/protein ratios suggest that the hydrocarbon chain mobility of the lipid is substantially reduced with increasing amounts of protein. The fluorine spectra of a single lipid-protein preparation show a dramatic increase in the number of the more mobile lipid chains with increasing temperature. The results suggest that the enzyme orders the lipid bilayer well beyond those lipids in direct contact with the protein surface, and that the amount of the lipid restricted by the enzyme is dependent upon temperature. The exchange of lipid between the restricted and the more mobile lipid environments most probably does not occur over the time scale measurable by the magnetic resonance techniques, about 10(-3) s.     

Outer membrane of Salmonella typhimurium. Electron spin resonance studies

The supramolecular structure of the outer membrane of Salmonella typhimurium that produces an Rc-type lipopolysaccharide was studied by adding spin-labeled fatty acid probes to membranes as well as model bilayers. Lipopolysaccharide of this organism apparently formed a bilayer structure in 0.2 M NaCl/0.01 M MgCl₂, and the electron spin resonance spectra suggested that the motion of the segments of hydrocarbon chains near the carboxyl end was quite restricted even at high temperature; this is presumably due to the anchoring of more than a dozen fatty acid residues to a single backbone structure. In the presence of Mg²⁺, we could produce lipopolysaccharide-phospholipid mixed bilayers containing up to 50% (by weight) lipopolysaccharide. Their spectra showed no sign of major heterogeneity, and the maximum hypertine splitting values were considerably larger than in phospholipid-only liposomes; these results suggest that the two components are finely interspersed and that the mobility of phospholipid hydrocarbons in severely restricted by the hydrocarbon chains of lipopolysaccharide. In spite of the presence of lipopolysaccharide in an amount equal to or exceeding that of phospholipids, the outer membrane produced spectra remarkably similar to those of the inner membrane, which does not contain lipopolysaccharide, and there was little sign of immobilization by lipopolysaccharides. Signals corresponding to the pure lipopolysaccharide phase were not detected, either. These results suggest that the phospholipids and lipopolysaccharides are segregated into separate domains in the outer membrane, and the fatty acid probes enter almost exclusively into the phospholipid domains. This conclusion was fully corroborated by determining, through the exchange broadening of line width, the total area of the domains that accommodated the spin label probes.     

大肠杆菌细胞外膜渗透性与脂多糖结构的关系

细胞外膜是大肠杆菌的半透膜屏障, 其主要成分是脂多糖。选取并构造共9种具有不同脂多糖结构的大肠杆菌, 用于考察脂多糖结构对细胞外膜渗透性的影响。从9种菌株中提取出脂多糖和类脂A, 并且用薄层层析色谱和离子源质谱来鉴定其结构。用N-苯基-1-萘胺作为荧光探针来测定细胞外膜渗透性大小。野生型大肠杆菌表现出最小的渗透性, 因敲除或表达某些基因而导致脂多糖结构改变的突变株均表现出较高的渗透性。脂多糖上的磷酸基团、脂肪酸链和多糖链的改变都影响了大肠杆菌的渗透性, 其中多糖链长度的改变对渗透性影响最大, 其次是脂肪酸链的数目变化。实验结果表明渗透性和脂多糖的结构具有较强的相关性。     

Spin label saturation transfer ESR studies of protein-lipid interactions in Photosystem II-enriched membranes

Saturation transfer ESR has been used to study the dynamic behaviour of lipids in the appressed regions of thylakoid membranes from pea seedlings. Four different phospho- and galacto-lipid spin labels (phosphatidylcholine labelled at the 12 or 14 C-atom positions of the sn-2 chain, phosphatidylglycerol labelled at the 14-position of the sn-2 chain, and monogalactosyldiacylglycerol labelled at the 12-position of the sn-2 chain) were used to probe the lipid environment in photosystem II-enriched membranes prepared by detergent extraction. The ESR spectra show that the majority of the lipid in these preparations is strongly motionally restricted. Values for the effective rotational correlation times of the labelled chains were deduced from the lineheight ratios and integrals of thhe saturation transfer ESR spectra. The effective rotational correlation times were found to be in the 10⁵ range, indicating a very low lipid chain mobility which correlates with the low lipid content of these preparations. Comparison of the effective rotational correlation times deduced from the different diagnostic regions of the spectrum revealed little anisotropy in the chain mobility, indicating that the dominant motional mode was trans-gauche isomerization. The effective rotational correlation times deduced from the spectral integrals were similar to those deduced from the lineheight ratios, consistent with the absence of any appreciable fluid lipid component in these preparations. The results also indicate some selectivity of interaction between the lipid species, with phosphatidylcholine exhibiting appreciably slower motion than either phosphatidylglycerol or monogalactosyldiacylglycerol.     

Outer membrane of Salmonella typhimurium. Electron spin resonance studies.

The supramolecular structure of the outer membrane of Salmonella typhimurium that produces an Rc-type lipopolysaccharide was studied by adding spin-labeled fatty acid probes to membranes as well as model bilayers. Lipopolysaccharide of this organism apparently formed a bilayer structure in 0.2 M NaCl/0.01 M MgCl2, and the electron spin resonance spectra suggested that the motion of the segments of hydrocarbon chains near the carboxyl end was quite restricted even at high temperature; this is presumably due to the anchoring of more than a dozen fatty acid residues to a single backbone structure. In the presence of Mg2+, we could produce lipoplysaccharide-phospholipid mixed bilayers contining up to 50% (by weight) lipoplysaccharide. Their spectra showed no sign of major heterogeneity, and the maximum hyperfine splitting values were considerably larger than in phospholipid-only liposomes; these results suggest that the two components are finely interspersed and that the mobility of phospholipid hydrocarbons is severely restricted by the hydrocarbon chains of lipopolysaccharide. In spite of the presence of lipoplysaccharide in an amount equal to or exceeding that of phospholipids, the outer membrane produced spectra remarkably similar to those of the inner membrane, which does not contain lipoplysaccharide, and there was little sign of immobilization by lipopolysaccharides. Signals corresponding to the pure lipoplysaccharide phase were not detected, either. These results suggest that the phospholipids and lipopolysaccharides are segregated into separate domains in the outer membrane, and the fatty acid probes enter almost exclusively into the phospholipid domains. This conclusion was fully corroborated by determining, through the exchange broadening of line width, the total area of the domains that accommodated the spin label probes.     

Synthesis, export, and assembly of Aeromonas salmonicida A-layer analyzed by transposon mutagenesis

Suicide plasmid pJB4JI, containing transposon Tn5 and phage Mu, was introduced into Aeromonas salmonicida 449 which produces a surface protein array known as the A-layer. Kanamycin-resistant exconjugants of 449 with altered ability to produce the A-layer were selected by virtue of their altered colonial morphology and color on medium containing the dye Congo red. Analysis of culture supernatants, periplasmic shock fluid, outer membranes, and whole-cell lysates by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and immunoblotting with a monoclonal antibody to A-protein revealed five classes of single-insertion mutations that affected the ability of cells to produce and export A-protein and to assemble the A-layer. These studies suggest that A-protein is produced from a single chromosomal gene. The subunits subsequently pass through the periplasm and across the outer membrane. At least one gene product is required for this export. Assembly of A-layer on the cell surface then requires the presence of O polysaccharide chains on the lipopolysaccharide. In one case, insertion of Tn5 resulted in loss of ability to produce both A-protein and lipopolysaccharide with O polysaccharide chains, suggesting that synthesis of A-protein and synthesis of lipopolysaccharide may involve coordinate regulation.     

Lipid phase transitions in cytoplasmic and outer membranes of Escherichia coli

The cytoplasmic and outer membranes containing either trans-Δ⁹-octadecenoate, trans-Δ⁹-hexadecenoate or cis-Δ⁹-octadecenoate as predominant unsaturated fatty acid residues in the phospholipids were prepared from a fatty acid auxotroph, Escherichia coli strain K1062. Order-disorder transitions of the phospholipids were revealed in both fractions of the cell envelope by fluorescent probing or wide angle X-ray diffraction. The mid-transition temperatures, $\text{T}\text{t}$, and the range of the transition, $\text{ΔT}$, are similar in the outer and cytoplasmic membrane. Relative to the corresponding extracted lipids, 60–80% of the hydrocarbon chains take part in the transition in the cytoplasmic membrane whereas in the outer membrane only 25–40% of the chains become ordered. The results suggest that in the outer membrane part of the lipids form fluid domains in the form of mono- and/or bilayers.     

The influence of membrane lipid structure on plasma membrane Ca2+ -ATPase activity

Lipid composition and Ca(2+)-ATPase activity both change with age and disease in many tissues. We explored relationships between lipid composition/structure and plasma membrane Ca(2+)-ATPase (PMCA) activity. PMCA was purified from human erythrocytes and was reconstituted into liposomes prepared from human ocular lens membrane lipids and synthetic lipids. Lens lipids were used in this study as a model for naturally ordered lipids, but the influence of lens lipids on PMCA function is especially relevant to the lens since calcium homeostasis is vital to lens clarity. Compared to fiber cell lipids, epithelial lipids exhibited an ordered to disordered phase transition temperature that was 12 degrees C lower. Reconstitution of PMCA into lipids was essential for maximal activity. PMCA activity was two to three times higher when the surrounding phosphatidylcholine molecules contained acyl chains that were ordered (stiff) compared to disordered (fluid) acyl chains. In a completely ordered lipid hydrocarbon chain environment, PMCA associates more strongly with the acidic lipid phosphatidylserine in comparison to phosphatidylcholine. PMCA associates much more strongly with phosphatidylcholine containing disordered hydrocarbon chains than ordered hydrocarbon chains. PMCA activity is influenced by membrane lipid composition and structure. The naturally high degree of lipid order in plasma membranes such as those found in the human lens may serve to support PMCA activity. The absence of PMCA activity in the cortical region of human lenses is apparently not due to a different lipid environment. Changes in lipid composition such as those observed with age or disease could potentially influence PMCA function.     

Cytoplasmic membrane vesicles of Escherichia coli. A simple method for preparing the cytoplasmic and outer membranes.

A simple preparative method is described for isolation of the cytoplasmic and outer membranes from E. coli. The characteristics of both membrane fractions were studied chemically, biologically, and morphologically. Spheroplasts of E. coli K-12 strain W3092, prepared by treating cells with EDTA-lysozyme [EC 3.2.1.17], were disrupted in a French press. The crude membrane fraction was washed with 3 mM EDTA-10% (w/v) sucrose, pH 7.2, and the cytoplasmic membranes and outer membranes were separated by sucrose isopycnic density gradient centrifugation. The crude membrane fraction contained approximately 10% of the protein of the whole cells, 0.3% of the DNA, 0.7% of the RNA, 0.3% of the peptidoglycan, and about 30% of the lipopolysaccharide. The cytoplasmic membrane fraction was rich in phospholipid, while the outer membrane fraction contained much lipopolysaccharide and carbohydrate; the relative contents of lipopolysaccharide and carbohydrate per mg protein in the cytoplasmic membrane fraction were 12 and 40%, respectively, of the contents in the outer membrane fraction. Cytochrome b1, NADH oxidase, D-lactate dehydrogenase [EC 1.1.1.28], succinate dehydrogenase [EC 1.3.99.1], ATPase [EC 3.5.1.3], and activity for concentrative uptake of proline were found to be localized mainly in the cytoplasmic membranes; their specific activities in the outer membrane fraction were 1.5 to 3% of those in the cytoplasmic membrane fraction. In contrast, a phospholipase A appeared to be localized mainly in the outer membranes and its specific activity in the cytoplasmic membrane fraction was only 5% of that in the outer membrane fraction. The cytoplasmic and outer membrane fractions both appeared homogeneous in size and shape and show vesicular structures by electron microscopy. The advantages of this method for large scale preparation of the cytoplasmic and outer membrane fractions are discussed.     

Lipid mobility and order in bovine rod outer segment disk membranes. A spin-label study of lipid-protein interactions

Lipid-protein interactions in bovine rod outer segment disk membranes have been studied by using a series of eight stearic acid spin-label probes which were labeled at different carbon atom positions in the chain. In randomly oriented membrane dispersions, the electron spin resonance (ESR) spectra of the C-8, C-9, C-10, C-11, C-12, C-13, and C-14 atom positional isomers all apparently consist of two components. One of the components corresponds closely to the spectra obtained from dispersions of the extracted membrane lipids, and the other, which is characterized by a considerably greater degree of motional restriction of the lipid chains, is induced by the presence of the protein. Digital subtraction has been used to separate the two components. The proportion of the motionally restricted lipid component is approximately constant, independent of the position of the spin-label group, and corresponds to 30-40% of the total spin-label spectral intensity. The hyperfine splitting of the outer maxima in the difference spectra of the motionally restricted component decreases, and concomitantly, the line widths increase with increasing temperature but change relatively little with increasing distance of the spin-label group from the polar head-group region. This indicates that the corresponding chain motions of the protein-interacting lipids lie in the slow-motion regime of spin-label ESR spectroscopy (tau R approximately 10(-8) S) and that the mobility of these lipids increases with increasing temperature but does not vary greatly along the length of the chain. The data from the hyperfine splittings also suggest the existence of a polarity gradient immediately adjacent to the protein surface, as observed in the fluid lipid regions of the membrane. The more fluid lipid component is only slightly perturbed relative to the lipids alone (for label positions 5-14, inclusive), indicating the presence of chain motions on the nanosecond time scale, and the spectra also reveal a similar polarity profile in both lipid and membrane environments. ESR spectra have also been obtained as a function of magnetic field orientation with oriented membrane samples. For the C-14 atom positional isomer, the motionally restricted component is observed to have a large hyperfine splitting, with the magnetic field oriented both parallel and perpendicular to the membrane normal. This indicates that the motionally restricted lipid chains have a broad distribution of orientations at this label position.(ABSTRACT TRUNCATED AT 400 WORDS)     

Conformation of liquid N-alkanes.

The conformations of liquid n-alkanes have been studied using neutron scattering techniques to better understand the conformational forces present in membrane lipid interiors. We have studied hydrocarbon chains having lengths comparable to those found for esterified membrane lipid fatty acids, and find that the steric constraints of packing in the liquid state do not change the conformational distributions of hydrocarbon chains from those imposed by the intrachain forces present in the gas phase. It follows that the central region of membranes containing lipids in the disordered state should contain hydrocarbon chain conformations determined primarily by intrachain forces.     

Relay model of lipid peroxidation in biological membranes

The present study examines the evidence for the important role of free radicals, localized on carbon atoms of the hydrocarbon chains, in lipid peroxidation. These radicals show a great inter- and intramolecular mobility in membranes by the way of relay-transfer (isomerisation). The sequence of intermediate steps of shift of free radicals in membranes with correction for molecular organization of the hydrocarbon zone of membranes, the intramembrane localization of unsaturated links and the gradient of mobility of the hydrocarbon chains are described. The effect of inhibitors in lipid peroxidation are interpreted in terms of decay of hydrocarbon free radicals as a result of its interaction with the antioxidant molecules. The natural antioxidants having a side chain (such as tocopherols) may be regarded as a some kind of "channel" through which free radicals leave the hydrocarbon moiety of the membrane. The processes of lipid peroxidation in membranes are subjected to a great extent to the requirements of the theory of oxidation of solid polymers and hydrocarbons.     

Interactions between lipid-anchored and transmembrane proteins. Spin-label ESR studies on avidin-biotinyl phosphatidylethanolamine in membrane recombinants with myelin proteolipid proteins

Interactions between lipid-anchored and transmembrane proteins are relevant to the intracellular membrane sorting of glycosyl phosphatidylinositol-linked proteins. We have studied the interaction of a spin-labeled biotinyl diacyl phospholipid, with and without specifically bound avidin, with the myelin proteolipid protein (or the DM-20 isoform) reconstituted in dimyristoylphosphatidylcholine. Tetrameric avidin bound to the N-biotinyl lipid headgroup is a surface-anchored protein, and the myelin proteolipid is an integral protein containing four transmembrane helices. The electron spin resonance (ESR) spectrum of N-biotinyl phosphatidylethanolamine spin-labeled at the C-14 position of the sn-2 chain consists of two components in fluid-phase membranes of dimyristoylphosphatidylcholine containing the proteolipid. In the absence of avidin, this is characteristic of lipid-protein interactions with integral transmembrane proteins. The more motionally restricted component represents the lipid population in direct contact with the intramembranous surface of the integral protein, and the more mobile component corresponds to the bulk fluid lipid environment of the bilayer. In the presence of avidin, the biotin-lipid chains have reduced mobility because of the binding to avidin, even in the absence of the proteolipid [Swamy, M. J., and Marsh, D. (1997) Biochemistry 36, 7403-7407]. In the presence of the proteolipid, the major fraction of the avidin-anchored chains is further restricted in its mobility by interaction with the transmembrane protein. At a biotin-lipid concentration of 1 mol %, approximately 80% of the avidin-linked chains are restricted in membranes with a phosphatidylcholine:proteolipid molar ratio of 37:1. This relatively high stoichiometry of interaction can be explained when allowance is made for the closest interaction distance between the lipid-anchored avidin tetramer and the transmembrane proteolipid hexamer, without any specific interaction between the two types of membrane-associated proteins. The interaction is essentially one of steric exclusion, but the lipid chains are rendered more sensitive to interaction with the integral protein by being linked to avidin, even though they are removed from the immediate intramembrane protein-lipid interface. This could have implications for the tendency of lipid-anchored chains to associate with membrane domains with reduced lipid mobility.     

Lipid-protein interaction in the photolysis of octopus rhodopsin

Microvillar membranes of octopus photoreceptor cells were treated with phospholipase A₂, phospholipase C, hexane, or their combinations. By these means, various membrane preparations containing qualitatively and quantitatively different lipids were obtained. The lipid composition and phospholipid content of the membrane preparations obtained by the above methods were determined.Photochemical processes in the digitonin extract of the native and treated membranes have been studied by flash photometry. The results suggest that several different variations in the lipids can affect the rates of the photochemical transformations; these are: the content of phospholipid, the amount of unsaturated hydrocarbon chains and free fatty acids.     

Spin-label electron paramagnetic resonance studies on the interaction of avidin with dimyristoyl-phosphatidylglycerol membranes

The interaction of avidin--a basic protein from hen egg-white--with dimyristoyl-phosphatidylglycerol membranes was investigated by spin-label electron paramagnetic resonance spectroscopy. Phosphatidylcholines, bearing the nitroxide spin label at different positions along the sn-2 acyl chain of the lipid were used to investigate the effect of protein binding on the lipid chain-melting phase transition and acyl chain dynamics. Binding of the protein at saturating levels results in abolition of the chain-melting phase transition of the lipid and accompanying perturbation of the lipid acyl chain mobility. In the fluid phase region, the outer hyperfine splitting increases for all phosphatidylcholine spin-label positional isomers, indicating that the chain mobility is decreased by binding avidin. However, there was no evidence for direct interaction of the protein with the lipid acyl chains, clearly indicating that the protein does not penetrate the hydrophobic interior of the membrane. Selectivity experiments with different spin-labelled lipid probes indicate that avidin exhibits a preference for negatively charged lipid species, although all spin-labelled lipid species indirectly sense the protein binding. The interaction with negatively charged lipids is relevant to the use of avidin in applications such as the ultrastructural localization of biotinylated lipids in histochemical studies.     

Alterations in the Outer Membrane of the Cell Envelope of Heptose-Deficient Mutants of Escherichia coli

The composition of the cell envelope of a heptose-deficient lipopolysaccharide mutant of Escherichia coli, GR467, was studied after fractionation into its outer and cytoplasmic membrane components by means of sucrose density gradient centrifugation. The outer membrane of GR467 had a lower density than that of its parent strain, CR34. Analysis of the fractionated membranes of GR467 indicated that the phospholipid-to-protein ratio had increased 2.4-fold in the outer membrane. The ratio in the mutant cytoplasmic membrane was also increased, although to a lesser extent. By employing a third parameter, the lipid A content of the outer membrane, it was found that the observed phospholipid-to-protein change in the outer membrane was due predominantly to a decrease in the relative amount of protein. This decrease in protein was particularly significant, since it was concomitant with a 68% decrease in the lipid A recovered in the outer membrane of GR467 relative to the lipid A recovered in the outer membrane of CR34. Similar findings were observed in a second heptose-deficient mutant of E. coli, RC-59. The apparent protein deficiency in GR467 was further studied by subjecting solubilized envelope proteins to sodium dodecyl sulfate-polyacrylamide gel electrophoresis. It was found that major envelope proteins which were localized in the outer membrane were greatly diminished in GR467. Two revertants of GR467 with the wild-type amounts of heptose had wild-type relative levels of protein in their outer membranes. A partial heptose revertant had a relative level of protein in its outer membrane between those of the mutant and wild type.     

Investigation into the fluidity of lipopolysaccharide and free lipid A membrane systems by Fourier-transform infrared spectroscopy and differential scanning calorimetry

The phase behaviour, particularly the fluidity within each phase state and the transitions between them, of lipopolysaccharides and of their lipid moiety, free lipid A, of various species of Gram-negative bacteria, especially of Salmonella minnesota and Escherichia coli, has been investigated by applying mainly Fourier-transform infrared spectroscopy and differential scanning calorimetry. For enterobacterial strains, the transition temperatures of the gel----liquid crystalline (beta----alpha) phase transition of the hydrocarbon chains in dependence on the length of the sugar moiety are highest for free lipids A (around 45 degrees C) and lowest for deep rough mutant lipopolysaccharides (around 30 degrees C). Evaluating certain infrared active vibration bands of the hydrocarbon moiety, mainly the symmetric stretching vibration of the methylene groups around 2850 cm-1, it was found that, in the gel state, the acyl chains of lipopolysaccharides and free lipid A have a higher fluidity as compared with saturated and the same fluidity as compared with unsaturated phospholipids. This 'partial fluidization' of lipopolysaccharide below the transition temperature correlates with its reduced enthalpy change at that temperature compared to phospholipids with the same chain length. The fluidity depends strongly on ambient conditions, i.e. on the Mg2+ and H+ content: higher Mg2+ concentrations and low pH values make the acyl chains of free lipid A and lipopolysaccharide preparations significantly more rigid and also partially increase the transition temperature. The influence of Mg2+ is highest for free lipid A and decreases with increasing length of the sugar side chain within the lipopolysaccharide molecules, whereas the effect of a low pH is similar for all preparations. At basic pH, a fluidization of the lipopolysaccharide and lipid A acyl chains and a decrease in transition temperature take place. Free lipid A and all investigated rough mutant lipopolysaccharides exhibit an extremely strong lyotropic behaviour in the beta----alpha melting enthalpy but not in the value of the transition temperature. The phase transition is distinctly expressed only at water concentrations higher than 50-60%. A further increase of the water content still leads to an increase in the phase-transition enthalpy, particularly for lipopolysaccharides with a more complete sugar moiety. The fluidity of the hydrocarbon chains is shown to be an important parameter with respect to the expression of biological activities.(ABSTRACT TRUNCATED AT 400 WORDS)