Bilayer polarity and its thermal dependency in the ?o and ?d phases of binary phosphatidylcholine/cholesterol mixtures

Diverse variations in membrane properties are observed in binary phosphatidylcholine/cholesterol mixtures. These mixtures are nonideal, displaying single or phase coexistence, depending on chemical composition and other thermodynamic parameters. When compared with pure phospholipid bilayers, there are changes in water permeability, bilayer thickness and thermomechanical properties, molecular packing and conformational freedom of phospholipid acyl chains, in internal dipolar potential and in lipid lateral diffusion. Based on the phase diagrams for DMPC/cholesterol and DPPC/cholesterol, we compare the equivalent polarity of pure bilayers with specific compositions of these mixtures, by using the Py empirical scale of polarity. Besides the contrast between pure and mixed lipid bilayers, we find that liquid-ordered (?_o) and liquid-disordered (?_d) phases display significantly different polarities. Moreover, in the ?_o phase, the polarities of bilayers and their thermal dependences vary with the chemical composition, showing noteworthy differences for cholesterol proportions at 35, 40, and 45 mol%. At 20 °C, for DMPC/cholesterol at 35 and 45 mol%, the equivalent dielectric constants are 21.8 and 23.8, respectively. Additionally, we illustrate potential implications of polarity in various membrane-based processes and reactions, proposing that for cholesterol containing bilayers, it may also go along with the occurrence of lateral heterogeneity in biological membranes.     

Molecular dynamics study of changes in physico-chemical properties of DMPC lipid bilayers by addition of nonionic surfactant C12E10

Changes in physico-chemical properties of dimyristoyl phosphatidylcholine (DMPC) lipid bilayers caused by the addition of 9.4 mol% nonionic surfactant decaoxyethylene monododecyl ethers (C₁₂E₁₀) have been investigated by molecular dynamics calculations. In spite of addition of single chain C₁₂E₁₀, the lipid bilayers showed an increase of membrane area. Isothermal area compressibility, which is a measure of membrane softness in lateral direction, also increased by 50% for DMPC/C₁₂E₁₀ mixed bilayers. Furthermore, the order parameter of C–H vector for DMPC acyl tails decreased. We found that these changes are caused by the hydrophilic head groups of C₁₂E₁₀ which are located near the glycerol backbone of the DMPC molecules and have bulky random coil conformation without any preferential ordered structures.     

Vibrational raman spectra of lipid systems containing amphotericin B

Resonance-enhanced and normal vibrational Raman spectra were observed for both multilamellar and single-wall vesicle assemblies of dimyristoyl phosphatidylcholine containing amphotericin B, a channel-forming polyene antibiotic, and cholesterol. The decrease in the frequency of the polyene antibiotic C  C stretching mode at 1556 cm^?1 and the increase in intensity of the CCH in-plane deformation mode at 1002 cm^?1 indicate that amphotericin B is ordered in a lipid-cholesterol medium similarly to the solid, but is surrounded by a slightly more polar environment. The intensity of the C  C stretching mode I₁₅₅₆ decreases 4-fold during the broadened gel to liquid crystalline phase transition (16–32°C) of dimyristoyl lecithin-cholesterol (4 : 1) multilayers. Other resonance-enhanced vibrations of amphotericin B exhibit similar behavior. For amphotericin B in pure dimyristoyl lecithin multilayer or vesicle systems, however, the vibrational intensity associated with the C  C stretching mode remains constant during the melting of lipid hydrocarbon chains. In addition, a third effect occurs in liquid crystalline egg lecithin-cholesterol (4 : 1, mol ratio) multilayers in which I₁₅₅₆ first increases by 25% between 3 and 25°C, in parallel with the loss of active channels, and then remains constant as the temperature increases from 25 to 42°C. This latter intensity pattern is masked in the dimyristoyl lecithin-cholesterol system by the overwhelming effect upon the C  C mode from changes in the lipid chain packing characteristics which occur during the phase transition.The broadened phase transition in 4 : 1 dimyristoyl lecithin-cholesterol multilayers (16–32°C), as followed by the ratio of intensities at 2880 and 2850 cm^?1 (asymmetric and symmetric methylene C-H stretching modes, respectively) is slightly narrowed by the addition of amphotericin B, and effect from which a binding stoichiometry at 24° of 1 : 1 amphotericin B : cholesterol is estimated. This stoichiometry was confirmed by differential calorimetric scans, which also show the presence of a peak proportional to cholesterol content.Raman I_2880/2850 peak height ratios in pure dimyristoyl lecithin bilayers were increased over the 14–38°C range by amphotericin B, a spectral effect which suggests an ordering of the lipid matrix perhaps as a consequence of the polyene binding to the bilayer surface. For bilayers containing cholesterol, the ratios of intensities of the 2935 cm^?1 feature, composed mainly of acyl chain terminal methyl and underlying methylene C-H stretching modes, to the 2850 cm^?1 feature are significantly increased by amphotericin B. This effect indicates that the antibiotic penetrates the bilayer in the lipid-sterol system.     

Phospholipid/cholesteryl ester microemulsions containing unesterified cholesterol: model systems for low density lipoproteins

As models for the effects of unesterified cholesterol (UC) on the lipid organization of low density lipoprotein (LDL), microemulsions containing either egg yolk phosphatidylcholine (EYPC) or dimyristoyl phosphatidylcholine (DMPC) as the surface component, cholesteryl oleate (CO) as the core component, and varying amounts of unesterified cholesterol were prepared by sonication. Gel filtration chromatography showed coelution of each of the lipid components, demonstrating the formation of well-defined microemulsion populations. Unesterified cholesterol incorporation into the microemulsions was proportional to the composition of the original mixture at low unesterified cholesterol compositions, but reached saturation at compositions of approximately 15 and 10 mol% unesterified cholesterol for EYPC/CO and DMPC/CO microemulsions, respectively. The Stokes' radius of the microemulsions was constant and similar to native LDL for initial compositions less than 15 mol% unesterified cholesterol, but increased at compositions above 15 mol%. In both EYPC/CO/UC and DMPC/CO/UC microemulsions, no significant changes were observed for the calorimetric or Van't Hoff enthalpy for the thermal transition of the core cholesteryl ester; however, increases in the transition temperature as a function of increasing unesterified cholesterol composition suggests that unesterified cholesterol has a stabilizing effect on the core transition. In DMPC/CO/UC microemulsions, the effect of unesterified cholesterol on the surface-located DMPC could be clearly observed as a broadening of the thermal transition of the acyl chains. These results demonstrate that unesterified cholesterol is located primarily in the surface of these protein-free lipid model systems for LDL.     

Raman spectroscopy of the thermal properties of reassembled high-density lipoprotein: apolipoprotein A-I complexes of dimyristoylphosphatidylcholine

Isolated complexes of apolipoprotein A-I (apoA-I), the major apoprotein of human plasma high-density lipoproteins, and dimyristoylphosphatidylcholine (DMPC) have been prepared and studied by differential scanning calorimetry (DSC) and Raman spectroscopy. DSC studies establish that complexes having lipid to protein ratios of 200, 100, and 50 to 1 each exhibit a broad reversible thermal transition at Tc = 27 degrees C. The enthalpy of lipid melting for each of the three complexes is about 3 kcal/mol of DMPC. Raman spectroscopy indicates that the physical state of lipid molecules in the complexes is different from that in DMPC multilamellar liposomes. Analysis of the C-H stretching region (2800-3000 cm-1) of the complexes and of the pure components in water suggests that below 24 degrees C (Tc for DMPC) there is considerably less lateral order among lipid acyl chains in the complexes than in DMPC liposomes. Above 24 degrees C, these types of interactions appear to contribute equally or slightly less to the complex structure than in pure DMPC. The temperature dependence of peaks in the C-C stretching region (1000-1180 cm-1) reveals a continuous increase in the number of lipid acyl chain C-C gauche isomers over a broad range with increasing temperature. Compared to liposomes, DMPC in the complexes has more acyl chain trans isomers at temperatures above 24 degrees C; at temperatures above ca. 30 degrees C, trans isomer content is about the same for complexes and liposomes. A large change was observed in a protein vibrational band at 1340 cm-1 for pure vs. complexed apoA-I, indicating that protein hydrocarbon side chains are immobilized by lipid binding. The Raman data indicate that the reduction in melting enthalpy for complexes DMPC (approximately 3 kcal/mol) compared to that for free DMPC (approximately 6 kcal/mol) is due to reduced van der Waals interactions in the low-temperature lipid phase.     

Synthesis and polymorphic phase behaviour of polyunsaturated phosphatidylcholines and phosphatidylethanolamines

A series of phosphatidylcholines and phosphatidylethanolamines was synthesized containing two acyl chains of the following polyunsaturated fatty acids: linoleic acid (18:2), linolenic acid (18:3), arachidonic acid (20:4) and docosahexaenoic acid (22:6). In addition two phospholipids with mixed acid composition were synthesized: 16:0/18:1_c phosphatidylcholine and 16:0/18:1_c phosphatidylethanolamine. The structural properties of these lipids in aqueous dispersions in the absence and in the presence of equimolar cholesterol were studied using ³¹P-NMR, freeze fracturing and differential scanning calorimetry (DSC).The phosphatidylcholines adopt a bilayer configuration above 0°C. Incorporation of 50 mol% of cholesterol in polyunsaturated species induces a transition at elevated temperatures into structures with ³¹P-NMR characteristics typical of non-bilayer organizations. When the acyl chains contain three or more double bonds, this non-bilayer organization is most likely the hexagonal H_II phase, 16:0/15:1_c phosphatidylethanolamine shows a bilayer to hexagonal transition temperature of 75°C. The polyunsaturated phosphatidylethanolamines exhibit a bilayer to hexagonal transition temperature below 0°C which decreases with increasing unsaturation and which is lowered by approximately 10°C upon incorporation of 50 mol% of cholesterol. Finally, it was found that small amounts of polyunsaturated fatty acyl chains in a phosphatidylethanolamine disproportionally lower its bilayer to hexagonal transition temperature.     

Elongation of acyl-CoAs by microsomes from etiolated leek seedlings

Long chain fatty acid synthesis was studied using etiolated leek seedling microsomes. In the presence of ATP, [2-¹⁴C]malonyl-CoA was incorporated into fatty acids of C₁₆C₂₆. The omission of ATP, even in the presence of acetyl-CoA, led to a complete loss of activity, which was restored by addition of exogeneous acyl-CoAs. Comparison of acyl-CoA (C₁₂C₂₄) elongation showed that stearoyl-CoA, in the presence of [2-¹⁴C]malonyl-CoA, was the more efficient precursor leading to the formation of fatty acids having a chain length of C₂₀C₂₆. [1-¹⁴C]C₁₆CoA and [1-¹⁴C]C₁₈CoA were elongated in the presence of malonyl-CoA, without degradation of the acyl chain. The time-course and the malonyl-CoA concentration curves showed that [1-¹⁴C]C₁₈CoA was a better primer than [1-¹⁴C]C₁₆CoA. Acyl-CoA elongation was also studied over the concentration range 4.5–45 μM [1-¹⁴C]C₁₈CoA. Comparison of the radioactivity incorporated into the fatty acids formed using [2-¹⁴C]malonyl-CoA in the presence of C₁₈CoA, on the one hand, and [1-¹⁴C]C₁₈CoA in the presence of malonyl-CoA, on the other, demonstrated clearly that the acyl chain of the acyl-CoA was elongated by malonyl-CoA.     

Exploration of physical principles underlying lipid regular distribution: effects of pressure, temperature, and radius of curvature on E/M dips in pyrene-labeled PC/DMPC binary mixtures.

In a previous study, we observed a series of dips in the plot of E/M (the ratio of excimer to monomer fluorescence intensity) versus the mole fraction of 1-palmitoyl-2-(10-pyrenyl)decanoyl-sn-glycerol-3-phosphatidylcholine (Pyr-PC) in Pyr-PC/DMPC binary mixtures at 30 degrees C. In the present study, we have characterized the physical nature of E/M dips in Pyr-PC/DMPC binary mixtures by varying pressure, temperature, and vesicle diameter. The E/M dips at 66.7 and at 71.4 mol% PyrPC in DMPC multilamellar vesicles remain discernible at 30-43 degrees C. At higher temperatures (e.g., 53 degrees C), the depth of the dip abruptly becomes smaller. This result agrees with the idea that E/M dips appear as a result of regular distribution of pyrene-labeled acyl chains into hexagonal super-lattices at critical mole fractions. Regular distribution is a self-ordering phenomenon. Usually, in self-ordered systems, the number of structural defects increases with increasing temperature, and thermal fluctuations eventually result in an order-to-disorder transition. The effect of vesicle diameter on the E/M dip at 66.7 mol% Pyr-PC in DMPC has been studied at 37.5 degrees C by using unilamellar vesicles of varying sizes. The E/M dip is observable in large unilamellar vesicles; however, the depth of the E/M dip decreases when the vesicle diameter is reduced. When the vesicle diameter is reduced to about 64 nm, the dip becomes shallow and split. This result suggests that the curvature-induced increase in the separation of lipids in the outer monolayer decreases the tendency of regular distribution for pyrene-labeled acyl chains. Regular distribution is believed to arise from the long-range repulsive interaction between Pyr-PC molecules due to the elastic deformation of the lipid matrix around the bulky pyrene moiety. When the radius of curvature becomes small, outer monolayer lipids are more separated. Therefore, pyrene-containing acyl chains fit better into the membrane matrix, which alleviates the deformation of the lattice and diminishes the long-range repulsive interactions between pyrene-containing acyl chains. Furthermore, we have shown a striking difference in the pressure dependence of E/M at critical Pyr-PC mole fractions and at noncritical mole fractions. In the pressure range between 0.001 and 0.7 kbar at 30 degrees C, E/M decreases steadily with increasing pressure at noncritical mole fractions; in contrast, E/M changes little with pressure at critical mole fractions (e.g., 33.3 and 50.0 mol% Pyr-PC).(ABSTRACT TRUNCATED AT 400 WORDS)     

Carbon-13 nuclear magnetic resonance study of spin labelled cholesteryl ester in model membranes

Carbon-13 NMR longitudinal relaxation times for unilamellar vesicles of egg phosphatidyl-choline (PC) in aqueous dispersion have been measured following the incorporation of spin labelled cholesteryl palmitate. The spin label induced relaxation rates. 1/T_1.5L, for fatty acyl chain carbons show that the C5 segment of the cholesteryl ester acyl chain is located near the C1 and C2 segments of the phospholipid acyl chains. A greater spin label induced enhancement of relaxation rate was observed for the inner vesicle layer than for the outer, and is attributed to a higher ester incorporation and/or tighter lipid packing in the inner layer.     

A time-resolved synchrotron X-ray study of a crystalline phase bilayer transition and packing in a saturated monogalactosyldiacylglycerol-water system

The structural changes associated with a phase transition between the gel-phase bilayer (L_β) in which the acyl chains pack in a hexagonal subcell, and a crystalline bilayer phase (L_C1) where the acyl chains are packed in an orthorhombic subcell in a saturated monogalactosyldiacylglycero-water system are reported. The phase change is cooperative and takes place isothermally after the lamellar-gel phase has been held at 20°C for about 8 min. The transformation of the acyl chain subcell from hexagonal to orthorhombic induces a change in diffraction maxima observed in the region 0.6–0.7 nm which is interpreted as a change in packing of the galactose residues from an orthorhombic to hexagonal subcell. We conclude that the rearrangement of the acyl chains into a more closely packed subcell requires the head groups to reorient to reduce the steric hindrance between the bulky galactose residues.     

Thermodynamics of interaction of octyl glucoside with phosphatidylcholine vesicles: partitioning and solubilization as studied by high sensitivity titration calorimetry

The interaction of the surfactant octyl glucoside (OG) with dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), distearoylphosphatidylcholine (DSPC), and soy bean phosphatidylcholine (soy bean PC) was studied using high-sensitivity titration calorimetry. We determined the partition coefficient of OG between water and lipid bilayers and the transfer enthalpy of the surfactant by addition of lipid vesicles to OG monomers or vice versa. Comparison with the micellization enthalpy of the surfactant gives information on differences in the hydrophobic environment of OG in a liquid-crystalline bilayer or a micelle. The average partition coefficient P in mole fraction units for x_e≈0.12–0.2 decreases slightly from 4152 at 27°C to 3479 at 70°C for DMPC and from 4260 to 3879 for soy bean PC, respectively. The transfer enthalpy ΔH^T of OG into lipid vesicles is positive at 27°C and negative at 70°C. Its temperature dependence is larger for the incorporation of OG into DMPC than into soy bean PC vesicles. It is concluded that OG in DMPC vesicles is better shielded from water than in soy bean PC vesicles or in micelles. Titration calorimetry was also used to determine the phase boundaries of the coexistence region of mixed vesicles and mixed micelles in the systems OG/DMPC, OG/DPPC, OG/DSPC, and OG/soy bean PC vesicles at 70°C in the liquid-crystalline phase. DMPC and soy bean PC solubilization was also studied at 27°C to investigate the effect of temperature. The effective surfactant to lipid ratios at saturation, R_e^sat, for all PCs studied are in the range between 1.33–1.72 and the ratios at complete solubilization, R_e^sol, are between 1.79–3.06. At 70°C, the R_e^sat values decrease with increasing chain length of the saturated PC. The ratios depend also slightly on temperature and the degree of unsaturation of the fatty acyl chains. For the OG/soy bean PC system, the coexistence range for mixed vesicles and mixed micelles is larger than for the corresponding PCs with saturated chains.     

The influence of cholesterol on interactions and dynamics of ibuprofen in a lipid bilayer

In this work, molecular dynamics (MD) simulations with atomistic details were performed to examine the influence of the cholesterol on the interactions and the partitioning of the hydrophobic drug ibuprofen in a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) bilayer. Analysis of MD simulations indicated that ibuprofen molecules prefer to be located in the hydrophobic acyl chain region of DMPC/cholesterol bilayers. This distribution decreases the lateral motion of lipid molecules. The presence of ibuprofen molecules in the bilayers with 0 and 25 mol% cholesterol increases the ordering of hydrocarbon tails of lipids whereas for the bilayers with 50 mol% cholesterol, ibuprofen molecules perturb the flexible chains of DMPC lipids which leads to the reduction of the acyl chain order parameter. The potential of the mean force (PMF) method was used to calculate the free energy profile for the transferring of an ibuprofen molecule from the bulk water into the DMPC/cholesterol membranes. The PMF studies indicated that the presence of 50 mol% cholesterol in the bilayers increases the free energy barrier and slows down the permeation of the ibuprofen drug across the DMPC bilayer. This can be due to the condensing and ordering effects of the cholesterol on the bilayer.     

Effects of polypeptide-phospholipid interactions on bilayer reorganizations Raman spectroscopic study of the binding of polymyxin B to dimyristoylphosphatidic acid and dimyristoylphosphatidylcholine dispersions

The interactions of the antibiotic polymixin B, a polycationic cyclic polypeptide containing a branched acyl side chain, with dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidic acid (DMPA) bilayers were investigated by Raman spectroscopy for a wide range of lipid/polypeptide mole fractions. Temperature profiles, constructed from peak height intensity ratios derived from the lipid methylene C-H stretching and acyl chain C-C stretching mode regions, reflected changes originating from lateral chain packing effects and intrachain trans / gauche rotamer formation, respectively. For DMPC/polymyxin B bilayers the temperature dependent curves indicate a broadening of the gel-liquid crystalline phase transition accompanied by an approx. 3 C deg. increase in the phase transition temperature from 22.8°C for the pure bilayer to 26°C for the polypeptide complex. For a 10:1 lipid/polypeptide mole ratio the temperature profile derived from the C-C mode spectral parameters displays a second order/disorder transition, at approx. 35.5°C, associated with the melting behavior of approximately three bilayer lipids immobilized by the antibiotic's charged cyclic headgroup and hydrophobic side chain. For the 10:1 mole ratio DMPA/polypeptide liposomes, the temperature profiles indicate three order/disorder transitions at 46, 36 and 24°C. Pure DMPA bilayers display a sharp lamellar-micellar phase transition at 51°C.     

The effects of bilirubin on the thermal properties of phosphatidylcholine bilayers.

The thermotropic properties of multilamellar vesicles of dimyristoylphosphatidylcholine (DMPC), dipalmitoylphosphatidylcholine (DPPC), and distearoylphosphatidylcholine (DSPC), as a function of the concentration of bilirubin in the range of 0.1 to 1 mol%, were measured. The exact effects of bilirubin depended on the chain length of the polymethylene chains. But the general effects of bilirubin were the same in all systems. At the lowest concentrations tested (0.1 mol bilirubin/100 mol phospholipid (0.1 mol%)), bilirubin broadened and shifted to higher temperatures the main phase transitions of all bilayers. For DPPC and DSPC, but not DMPC, this concentration of bilirubin was associated with a new transition at 25 degrees C (DPPC) or 34 degrees C (DSPC). Bilirubin at 0.2 mol% was required for the detection of a similar transition (at 13.7 degrees C) in DMPC. Higher concentrations of bilirubin (> 0.2 mol%) suppressed completely the main phase transitions in all bilayers but increased the enthalpy of the new transition. Maximal values of delta H for these transitions were reached at 0.5, 0.25, and 0.2 mol% bilirubin in DMPC, DPPC, and DSPC, respectively. Values of delta H and delta S for these transitions were far larger than for the corresponding gel-to-liquid crystal transitions in pure lipid bilayers but were equal to those expected for a transition between crystalline and liquid crystalline phases.     

Deuterium magnetic resonance of triacylglycerols in phospholipid bilayers

Aqueous dispersions of egg-phosphatidylcholine and egg-phosphatidylcholine/30 mol% cholesterol containing deuterated tripalmitin or triolein were studied at approx. 25°C by ²H-NMR. Incorporation of tripalmitin into egg-phosphatidylcholine bilayers was found to be less than 0.1 mol%, while the incorporation of trolein is approx. 2.5 mol% in the absence and approx. 0.7 mol% in the presence of cholesterol. The profile of order parameter versus chain position for deuterated triolein suggests that the oleoyl chains of the triacylglycerol have an average orientation such that the C2 chain segments and the segments in the vicinity of the C9–C10 double bond are tilted with respect to the bilayer normal, while all other segments are parallel to the bilayer normal. Longitudinal relaxation times were also determined and indicate that the acyl chains of triolein have a motional behaviour similar to that of phospholipid acyl chains in the bilayer.     

E/M dips. Evidence for lipids regularly distributed into hexagonal super-lattices in pyrene-PC/DMPC binary mixtures at specific concentrations.

We have examined the effect of 1-palmitoyl-2-(10-pyrenyl)decanoyl-sn-glycerol-3-phosphatidylcholine (Pyr-PC) concentration on the ratio of excimer fluorescence to monomer fluorescence (E/M) in L-alpha-dimyristoylphosphatidylcholine (DMPC) multilamellar vesicles at 30 degrees C, with special attention focussed on the smoothness of the curve. We observed a series of dips, in addition to kinks, in the plot of E/M versus the mole fraction of Pyr-PC (XPyrPC). The observation of dips is a new finding, perhaps unique for Pyr-PC in DMPC since only kinks were observed for Pyr-PC in L-alpha-dipalmitoylphosphatidylcholine (DPPC) and in egg yolk phosphatidylcholine (egg-PC) (Somerharju et al., 1985. Biochemistry. 24: 2773-2781). The dips/kinks observed here are distributed according to a well defined pattern reflecting a lateral order in the membrane, and distributed symmetrically with respect to 50 mol% Pyr-PC. Some of the dips appear at specific concentrations (YPyrPC) according to the hexagonal super-lattice model proposed by Virtanen et al. (1988. J. Mol. Electr. 4: 233-236). However, the observations of dips at XPyrPC > 66.7 mol% and the kink at 33.3 mol% cannot be interpreted by the model of Virtanen et al. (1988). These surprising results can be understood by virtue of an extended hexagonal super-lattice model, in which we have proposed that if the pyrene-containing acyl chains are regularly distributed as a hexagonal super-lattice in the DMPC matrix at a specific concentration YPyrPC, then the acyl chains of DMPC can form a regularly distributed hexagonal super-lattice in the membrane at a critical concentration (1-YPyrPC).(ABSTRACT TRUNCATED AT 250 WORDS)     

Inhibition of nuclease activity in Bacillus subtilis following infection with bacteriophage SP82G.

The Raman Spectra of Blodgett-Langmuir multilayer assemblies made from behenic acid, barium behenate and barium cis-13 erucate are reported. In particular, the peak height intensity ratio of the hydrocarbon chain methylene CH stretch Raman bands, $\text{I}{}_{2890}\text{I}{}_{2850}$, for each multilayer assembly is compared to that of phosphatidylcholine in powders and water dispersions as well as to samples of crystalline hydrocarbon chains. It is found that the fatty acid multilayers are more ordered than the phospholipid samples but less ordered than the crystalline samples. It is suggested that Blodgett-Langmuir multilayer assemblies of lipid might be a useful reference in quantitative studies of packing order in lipid phases.     

Motion and surface accessibility of spin-labeled lipids in a model lipoprotein containing cholesteryl oleate, dimyristoylphosphatidylcholine, and apolipoprotein E

A series of spin-labeled phosphatidylcholines (PCs) and cholesteryl esters (CEs) bearing the paramagnetic 2,2-dimethyloxazolidinyl-1-oxy (doxyl) group at fatty acyl carbon C5', C12', or C16' were used to study acyl chain motions in the polar surface shell and hydrophobic core domains of microemulsion (ME) particles containing cholesteryl oleate and dimyristoylphosphatidylcholine (DMPC), and of particles with apolipoprotein E (apoE) bound to their surfaces. Electron paramagnetic resonance data obtained with the doxyl-labeled PCs indicated a gradient of motion in the ME surface monolayer similar to that observed with the same probes in a bilayer. The 5- and 12-doxyl-CEs clearly demonstrated a higher degree of order for the cholesteryl ester rich core than the corresponding doxyl-PCs showed for the phospholipid-rich surface over the entire range 10-60 degrees C. The temperature dependencies of spectra of the 16-doxyl-CE in the core and PC in the surface of the ME were almost identical, suggesting that there was no sharp boundary between core and surface domains. None of the probes detected either the surface phospholipid transition (31 degrees C) or the cholesteryl ester core transition (46 degrees C) measured previously by differential scanning calorimetry and 13C nuclear magnetic resonance. Binding of apoE to spin-labeled DMPC vesicles increased the order of the 5'-position of the sn-2 acyl chain over the range 15-33 degrees C; the thermal transition was broadened and its midpoint elevated. The effect of protein binding was not as striking for the ME particles.(ABSTRACT TRUNCATED AT 250 WORDS)     

C-A base pairs in transfer ribonucleic acids and codon-anticodon complexes

Two hundred and thirty three nucleotide sequences of tRNAs were investigated to elucidate the frequency of appearance of C-A (cytosine-adenine) pairs in their main two-stranded regions, in the positions 26–44 and 15–48². It was supposed that in the formation of C-A pairs on antiparallel polynucleotide chains the atomic groups -N₄H and -N₃ of cytosine make up Hbonds with the groups N₇- and HN₆- of adenine. On parallel chains, Hbonds, probably, form -N₆H and -N₁ groups of adenine with N₃- and HN₄- of cytosine. The calculation results predicted a significant energy of interaction between cytosine and adenine. By the investigation of the molecular models it was shown that the formation of Hbonded C-A pairs requires considerable changes of conformation in ribose-phosphate chains. In addition, a theoretical analysis revealed the possibility of formation of C-A pairs at the wobble-position of the codon-anticodon complex. The significance of this nucleotide pair in the processes of genetic coding proved to depend on the stability of the codon-anticodon complex, the modification of cytosine 34 and structural features of the distant regions of the tRNA.