X-ray diffraction and spectroscopic studies of oleic acid-sodium oleate

The sodium oleate-oleic acid (1:1) complex (NaHOl(2)) is characterized using X-ray diffraction, FT-IR photoacoustic spectroscopy, FT-Raman spectroscopy, and DSC. The special arrangement of hydrogen-bonded pairs of carboxylic acid and carboxylate groups into unique "head-group" is supported by frequency shifts and partial or total disappearance of characteristic vibrations of carboxylic acid dimer and of carboxylate groups. The well-ordered state of hydrocarbon chains is demonstrated by the existence of sharp Raman bands in the C-C stretching region (1000-1150 cm-1) and other conformationally sensitive modes. The FT-Raman results suggest that the transition at about 32 degrees C involves the cooperative melting of methyl- and carboxyl/carboxylate-sided hydrocarbon chains. From the X-ray diffraction data it is clear that this transition is associated with the disintegration of the hydrogen-bonded carboxylate-carboxylic acid complex, followed by the separate formation of oleic acid and sodium oleate. The packing of hydrocarbon chain in the acid-soap complex is different from the parent oleic acid or sodium oleate. The hydrocarbon chains in the NaHOl(2) form more stable packing (O subcell) in comparison to that of oleic acid. A temperature composition phase diagram is presented.     

Temperature and compositional dependence of the structure of hydrated dimyristoyl lecithin.

Differential scanning calorimetry and x-ray diffraction techniques have been used to investigate the structure and phase behavior of hydrated dimyristoyl lecithin (DML) in the hydration range 7.5 to 60 weight % water and the temperature range -10 to +60 degrees C. Four different calorimetric transitions have been observed: T1, a low enthalpy transition (deltaH approximately equal to 1 kcal/mol of DML) at 0 degrees C between lamellar phases (L leads to Lbeta); T2, the low enthalpy "pretransition" at water contents greater than 20 weight % corresponding to the transition Lbeta leads to Pbeta; T3, the hydrocarbon chain order-disorder transition (deltaH = 6 to 7 kcal/mol of DML) representing the transition of the more ordered low temperature phases (Lbeta, Pbeta, or crystal C, depending on the water content) to the lamellar Lalpha phase; T4, a transition occurring at 25--27 degrees C at low water contents representing the transition from the lamellar Lbeta phase to a hydrated crystalline phase C. The structures of the Lbeta, Pbeta, C, and Lalpha phases have been examined as a function of temperature and water content. The Lbeta structure has a lamellar bilayer organization with the hydrocarbon chains fully extended and tilted with respect to the normal to the bilayer plane, but packed in a distorted quasihexagonal lattice. The Pbeta structure consists of lipid bilayer lamellae distorted by a periodic "ripple" in the plane of the lamellae; the hydrocarbon chains are tilted but appear to be packed in a regular hexagonal lattice. The diffraction pattern from the crystalline phase C indexes according to an orthorhombic cell with a = 53.8 A, b = 9.33 A, c = 8.82 A. In the lamellae bilayer Lalpha strucure, the hydrocarbon chains adopt a liquid-like conformation. Analysis of the hydration characteristics and bilayer parameters (lipid thickness, surface area/molecule) of synthetic lecithins permits an evaluation of the generalized hydration and structural behavior of this class of lipids.     

Fluid phase lipid areas and bilayer thicknesses of commonly used phosphatidylcholines as a function of temperature

The structural parameters of fluid phase bilayers composed of phosphatidylcholines with fully saturated, mixed, and branched fatty acid chains, at several temperatures, have been determined by simultaneously analyzing small-angle neutron and X-ray scattering data. Bilayer parameters, such as area per lipid and overall bilayer thickness have been obtained in conjunction with intrabilayer structural parameters (e.g. hydrocarbon region thickness). The results have allowed us to assess the effect of temperature and hydrocarbon chain composition on bilayer structure. For example, we found that for all lipids there is, not surprisingly, an increase in fatty acid chain trans-gauche isomerization with increasing temperature. Moreover, this increase in trans-gauche isomerization scales with fatty acid chain length in mixed chain lipids. However, in the case of lipids with saturated fatty acid chains, trans-gauche isomerization is increasingly tempered by attractive chain-chain van der Waals interactions with increasing chain length. Finally, our results confirm a strong dependence of lipid chain dynamics as a function of double bond position along fatty acid chains.     

The influence of protein-lipid interactions on the order-disorder conformational transitions of the hydrocarbon chain.

The phases of simple systems involving one type of protein (lysozyme or cytochrome c) and one type of lipid (phosphatidic acid) have been characterized by X-ray crystallography, chemical analysis and spin-labeling technique as a function of temperature. They are of the lamellar type with alternative protein monolayers and lipid bilayers. According to the pH, two types of lamellar phases are obtained, one where the lipid-protein interactions are mainly hydrophobic, the other where they are electrostatic. In both cases, a phase transition occurs as temperature is lowered, between a high temperature phase, where all the lipids are in the liquid-like state, and another phase where some lipid chains are rigid. In the case of the phases with electrostatic interaction, it is shown that the onset of the order-disorder transition is shifted towards low temperature as compared with the homologous lipid-water phase and that the protein content of the phase decreases as the ratio of the liquid to rigid hydrocarbon chains decreases. This leads us to suggest that in the systems studied in this work the proteins interact only with lipid in the liquid-like state. In the case of the phases with hydrophobic interaction, it is shown that the extent of hydrophobic interaction between protein and lipid increases as the unsaturation of the hydrocarbon chains increases. The onset of the order-disorder transition shows a greater shift towards low temperature than the one observed in the case of the phase with electrostatic interaction.     

High-pressure small-angle neutron scattering (SANS) study of 1,2-dielaidoyl-sn-glycero-3-phosphocholine bilayers

Small-angle neutron scattering of the trans-unsaturated DEPC has been investigated as a function of pressure at 12, 18.6 and 35 degrees C. A pressure-induced structural phase transition from a liquid-crystalline state to a gel state is observed at the temperatures studied. The critical pressure of this transition increases with increasing temperature with a delta P/delta T value of 51 bar/C degrees. The small-angle neutron scattering results indicate that the effect of the trans double bonds in DEPC is to enhance the conformational disorder in the hydrocarbon chains. In DEPC bilayers, a pressure-induced conformational ordering process is observed not only in the liquid-crystalline phase but also in the gel phase, which indicates that conformational disorder exists in the liquid-crystalline phase as well as in the gel phase.     

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)     

The influence of protein-lipid interactions on the order-disorder conformational transitions of the hydrocarbon chain

The phases of simple systems involving one type of protein (lysozyme or cytochrome $\text{c}$) and one type of lipid (phosphatidic acid) have been characterized by X-ray crystallography, chemical analysis and spin-labeling technique as a function of temperature. They are of the lamellar type with alternative protein monolayers and lipid bilayers. According to the pH, two types of lamellar phases are obtained, one where the lipid-protein interactions are mainly hydrophobic, the other where they are electrostatic. In both cases, a phase transition occurs as temperature is lowered, between a high temperature phase, where all the lipids are in the liquid-like state, and another phase where some lipid chains are rigid. In the case of the phases with electrostatic interaction, it is shown that the onset of the order-disorder transition is shifted towards low temperature as compared with the homologous lipid-water phase and that the protein content of the phase decreases as the ratio of the liquid to rigid hydrocarbon chains decreases. This leads us to suggest that in the systems studied in this work the proteins interact only with lipid in the liquid-like state. In the case of the phases with hydrophobic interaction, it is shown that the extent of hydrophobic interaction between protein and lipid increases as the unsaturation of the hydrocarbon chains increases. The onset of the order-disorder transition shows a greater shift towards low temperture than the one observed in the case of the phase with electrostatic interaction.     

Lecithin bilayers. Density measurement and molecular interactions.

Density measurement are reported for bilayer dispersions of a series of saturated lecithins. For chain lengths with, respectively, 14, 15, 16, 17, and 18 carbons per chain, the values for the volume changes at the main transition are 0.027, 0.031, 0.037, 0.040 and 0.045 ml/g. The main transition temperature extrapolates with increasing chain length to the melting temperature of polyethylene. Volume changes at the lower transition are an order of magnitude smaller than the main transition. Single phase thermal expansion coefficients are also reported. The combination of X-ray data and density data indicated that the volume changes are predominantly due to the hydrocarbon chains, thus enabling the volume vCH2 of the methylene groups to be computed as a function of temperature. From this and knowledge of intermolecular interactions in hydrocarbon chains, the change in the interchain van der Waals energy, delta UvdW, at the main transition is computed for the lecithins and also for the alkanes and polyethylene at the melting transition. Using the experimental enthalpies of transition and delta UvdW, the energy equation is consistently balanced for all three systems. This yields estimates of the change in the number of gauche rotamers in the lecithins at the main transition. The consistency of these calculations supports the conclusion that the most important molecular energies for the main transition in lecithin bilayers are the hydrocarbon chain interactions and the rotational isomeric energies, and the conclusion that the main phase transition is analogous to the melting transition in the alkanes from the hexagonal phase to the liquid phase, but with some modifications.     

Interdigitated hydrocarbon chain packing causes the biphasic transition behavior in lipid/alcohol suspensions

It has been shown recently by Rowe ((1983) Biochemistry 22, 3299-3305) that ethanol has a 'biphasic' effect on the transition temperature (Tm) of phosphatidylcholine bilayers, reducing Tm at low concentrations but increasing Tm at high concentrations. Our X-ray diffraction data show that this reversal of Tm is a consequence of the induction of an unusual gel phase, where the lipid hydrocarbon chains from apposing monolayers fully interpenetrate or interdigitate. The properties of this interdigitated phase also explain the lipid chain length dependence of the reversal in the Tm versus ethanol concentration curves and the narrow width of the transition at high ethanol concentrations, as well as spectroscopic and calorimetric data from lipid suspensions containing other drugs such as methanol, benzyl alcohol, phenyl ethanol, and chlorpromazine.     

The effect of cholesterol on the structure of phosphatidylcholine bilayers

The effect of cholesterol on the structure of phosphatidylcholine bilayers was investigated by X-ray diffraction methods. Electron density profiles at 5 Å resolution along with chain tilt and chain packing parameters were obtained and compared for phosphatidylcholine/cholesterol bilayers and for pure phosphatidylcholine bilayers in both the gel and liquid crystalline states. The cholesterol in the bilayer was localized by noting the position of discrete elevations in the electron density profiles. Cholesterol can either increase or decrease the width of the bilayer depending on the physical state and chain length of the lipid before the introduction of cholesterol. For saturated phosphatidylcholines containing 12–16 carbons per chain, cholesterol increases the width of the bilayer as it removes the chain tilt from gel state lipids or increases the trans conformations of the chains for liquid crystalline lipids. However, cholesterol reduces the width of 18 carbon chain bilayers below the phase transition temperature as the long phospholipid chains must deform or kink to accomodate the significantly shorter cholesterol molecule. Although cholesterol has a marked effect on hydrocarbon chain organization, it was found that, within the resolution limits of the data, the phosphatidylcholine head group conformation is unchanged by the addition of cholesterol to the bilayer. The head group is oriented parallel to the plane of the bilayer for phosphatidylcholine in the gel and liquid crystalline states and this orientation is not changed by the addition of cholesterol.     

Order-disorder conformational transitions of the hydrocarbon chains of lipids

In many lipid-containing systems (intact membranes, lipid-water and proteinlipid-water phases) the hydrocarbon chains are known to undergo a reversible temperature-dependent transition between a highly disordered (type α) and a partly ordered (type β) conformation; in the β conformation the chains, stiff and all parallel, are packed with rotational disorder according to a two-dimensional hexagonal lattice. This work describes an X-ray diffraction and freeze-fracturing electron microscope study of the phases involved in this conformational transition. Several lipid-water systems were studied: mitochondrial lipids; phosphatidic acid, synthetic lecithin; hen egg lecithin. The conformational transition is found to be a complex phenomenon dependent upon the chemical composition of the lipids, the amount of water and temperature. When the lipid is a pure chemical species the transition involves two phases; one with all the chains in the α conformation the other with all the chains in the β conformation. If the chains are heterogeneous, then from the onset of the transition from type α, they segregate into regions with different conformation, presumably according to their length and degree of saturation. One of the phases (Lαβ) consists of regularly stacked lipid lamellae, each of which is a disordered mosaic of two types of domains; one with the chains in the α, the other in the β conformation. In another phase (Lγ) each lipid lamella is formed by one monolayer of type α and one of type β, joined by their apolar faces. Two other phases (Pγ and Pαβ) display two-dimensional lattices, and consist of lipid lamellae distorted by wave-like ripples, with an ordered segregation of domains in the α and in the β conformation. The number and the structure of the phases involved in the conformational transition are strongly dependent upon the heterogeneity of the hydrocarbon chains and upon the charge and hydration of the polar groups. The results of this study have a bearing on the conformation of the chains in membranes, and on the possible biological significance of conformational transitions.     

Investigations into the polymorphism of lipid A from lipopolysaccharides of Escherichia coli and Salmonella minnesota by Fourier-transform infrared spectroscopy

The polymorphism of lipid A, the endotoxic principle of the lipopolysaccharides of gram-negative bacteria, has been investigated in the fully hydrated state at temperatures between 5 degrees and 58 degrees C via Fourier-transform infrared spectroscopy. These measurements were supplemented by X-ray diffraction, fluorescence intensity techniques and differential thermal analysis. Up to three distinct phase transitions could be detected, with the main transition temperatures lying at approximately 41 degrees, 46 degrees, 44 degrees and 47 degrees C for Escherichia coli lipid A, Salmonella minnesota lipid A, and the synthetic lipid A compounds 506 and 516, respectively. 4'-Monophosphoryl-lipid A samples exhibited their main transition temperatures at considerably higher temperatures (about 52 degrees C for E. coli lipid A). The analysis of greater than CH2 stretching absorption bands as well as the wide-angle scattering behaviour of the lipid A samples showed that the main transition apparently involved the completion of hydrocarbon chain melting of lipid A, as typically observed for phospholipids. However, the phase transition behaviour was found to be much more complex than that usually observed for model phospholipid systems. Even below the main transition temperature, considerable amounts of the methylene segments of the acyl chains of lipid A were found to assume gauche conformations. These conformational changes might be related to the occurrence of up to two further transitions located at about 22 degrees, 30 degrees, 27 degrees and 25.5 degrees C (first transition) and at about 34 degrees, 42 degrees, 38.5 degrees and 40.5 degrees C (second transition) for E. coli lipid A, S. minnesota lipid A and the synthetic lipid A compounds 506 and 516, respectively. Furthermore, by the analysis of some characteristic infrared absorption bands related to the hydrophilic backbone, it could be demonstrated that the temperature-induced conformational changes occurring within the hydrocarbon chains were constantly and simultaneously accompanied by detectable rearrangements within the interfacial region and the polar head group of lipid A. The following conclusions were drawn: Up to about 30 degrees C the lipid A assemblies were supposed to adopt virtually bilayered, true lamellar arrangements, as revealed by the analysis of greater than CH2 scissoring vibrations and X-ray diffraction pattern. However, as indicated by fluorometric techniques, no stable closed vesicles seemed to be formed even under these conditions.(ABSTRACT TRUNCATED AT 400 WORDS)     

On the use of deuterium nuclear magnetic resonance as a probe of chain packing in lipid bilayers

The packing of hydrocarbon chains in the bilayers of lamellar (L alpha) phases of soap/water and phospholipid/water mixtures has been studied by deuterium NMR spectroscopy and X-ray diffraction. A universal correlation is shown to exist between the average C-D bond order parameter SCD of hydrocarbon chains and the average area per chain ach, irrespective of the chemical structure of the surfactant (hydrophilic group, number of chains per molecule, and chain length), composition, and temperature. The practical utility of the correlation is illustrated by its application to the characterization of the distribution of various hydrophobic and amphiphilic solutes in bilayers. The distribution of hydrocarbons within a bilayer is shown to depend upon their molecular structure in a manner which highlights the nature of the molecular interactions involved. For example, benzene is shown to be fairly uniformly distributed across the bilayer with an increasing tendency to distribute into the center at high concentrations. In contrast, the more complex hydrocarbon tetradecane preferentially distributes into the center of the bilayer at low concentrations, while at higher concentrations it intercalates between the surfactant chains. Alcohols such as benzyl alcohol, octanol, and decanol all interact similarly with the bilayer in so far as they are pinned to the polar/apolar interface, presumably by involvement of the hydroxyl group in a hydrogen bond. But the response of the surfactant chains to the void volume created in the center of the bilayer is dependent upon the distance of penetration of the alcohol into the bilayer. For benzyl alcohol, the shortest molecule, this void volume is taken up by the disordering of the chains, while for decanol, the longest molecule, it is absorbed by interdigitation of the chains of apposing monolayers. For octanol, the chain interdigitation mechanism is dominant at low concentrations, but there is a transition to chain disordering at high concentrations. Finally, it is shown that the correlation provides a useful test for statistical mechanical models of chain ordering in lipid bilayers.     

Accommodation of hydroxyl groups and their hydrogen bond system in a hydrocarbon matrix

From data of sisingle crystal analysis of 12-D-hydroxyoctadecanoic acid methyl ester principles for the incorporation of hydroxyl groups into a hydrocarbon chain matrix can be deduced. In the crystalline compound infinite hydrogen bond systems are accommodated in an orthorhombic perpendicular (O) chain arrangement. The O hydrocarbon subcell is expanded towards a hexagonal packing pattern, allowing more space and optimal geometry for the hydrogen bond system. The arrangement of the bond system in the O subcell requires that hydrogen bonded carbon chains carry alternatingly hydroxyl roups with opposite configuration. For the enantiomeric compound this requirement is met by a head to tail packing of molecules in a single layer arrangement. The corresponding racemates on the other hand pack head to head in double layers as confirmed by X-ray powder and IR studies. In monolayers both enantiomers and racemates behave identicalyy. The hydrogen bonding of the hydroxyl groups apparently leads to the formation of lipid clusters, in which the geometric conditions for both a close packing of hydrocarbon chains and the formation of an extensive hydrogen bond system do not exist.     

Thermodynamic and structural characterization of amino acid-linked dialkyl lipids

Using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR), we determined some thermodynamic and structural parameters for a series of amino acid-linked dialkyl lipids containing a glutamic acid-succinate headgroup and di-alkyl chains: C12, C14, C16 and C18 in CHES buffer, pH 10. Upon heating, DSC shows that the C12, C14 and annealed C16 lipids undergo a single transition which XRD shows is from a lamellar, chain ordered subgel phase to a fluid phase. This single transition splits into two transitions for C18, and FTIR shows that the upper main transition is predominantly the melting of the hydrocarbon chains whereas the lower transition involves changes in the headgroup ordering as well as changes in the lateral packing of the chains. For short incubation times at low temperature, the C16 lipid appears to behave like the C18 lipid, but appropriate annealing at low temperatures indicates that its true equilibrium behavior is like the shorter chain lipids. XRD shows that the C12 lipid readily converts into a highly ordered subgel phase upon cooling and suggests a model with untilted, interdigitated chains and an area of 77.2A(2)/4 chains, with a distorted orthorhombic unit subcell, a=9.0A, b=4.3A and beta=92.7 degrees . As the chain length n increases, subgel formation is slowed, but untilted, interdigitated chains prevail.     

The interfacial structure of phospholipid bilayers: differential scanning calorimetry and Fourier transform infrared spectroscopic studies of 1,2-dipalmitoyl-sn-glycero-3-phosphorylcholine and its dialkyl and acyl-alkyl analogs.

The thermotropic phase behavior of aqueous dispersions of dipalmitoylphosphatidylcholine (DPPC) and its 1,2-dialkyl, 1-acyl 2-alkyl and 1-alkyl 2-acyl analogs was examined by differential scanning calorimetry, and the organization of these molecules in those hydrated bilayers was studied by Fourier transform infrared spectroscopy. The calorimetric data indicate that substitution of either or both of the acyl chains of DPPC with the corresponding ether-linked hydrocarbon chain results in relatively small increases in the temperature (< 4 degrees C) and enthalpy (< 1 kcal/mol) of the lipid chain-melting phase transition. The spectroscopic data reveal that replacement of one or both of the ester-linked hydrocarbon chains of DPPC with its ether-linked analog causes structural changes in the bilayer assembly, which result in an increase in the polarity of the local environments of the phosphate headgroups and of the ester carbonyl groups at the bilayer polar/apolar interface. The latter observation is unexpected, given that ester linkages are considered to be intrinsically more polar that ether linkages. This finding cannot be satisfactorily rationalized unless the conformation of the glycerol backbones of the analogs containing ether-linked hydrocarbon chains differs significantly from that of diacyl glycerolipids such as DPPC. A comparison of the alpha-methylene scissoring bands and the methylene wagging band progressions of these lipids with the corresponding absorption bands of specifically chain-perdeuterated analogs of DPPC also supports the conclusion that replacement of the ester-linked hydrocarbon chains of DPPC with the corresponding ether-linked analog induces conformational changes in the lipid glycerol backbone. The suggestion that the conformation of glycerol backbones in the alkyl-acyl and dialkyl derivatives of DPPC differs from that of the naturally occurring 1,2-diacyl glycerolipid suggests that mono- and di-alkyl glycerolipids may not be good models of their diacyl analogs. These results, and previously published evidence that DPPC analogs with ether-linked hydrocarbon chains spontaneously form chain-interdigitated gel phases at low temperatures, clearly indicate that the properties of lipid bilayers can be substantially altered by small changes in the chemical structures of their polar/polar interfaces, and highlight the critical role of the interfacial region as a determinant of the structure and organization of lipid assemblies.     

Physicochemical characterization of silicon-containing glycolipids by DSC,FT-Raman spectroscopy and X-ray diffraction

Derivatives of dimethylalkylchlorosilanes are novel substances which may be used in formulations for drug targeting. In order to design their properties it is essential to perform physicochemical characterization. For this purpose, a combination of differential scanning calorimetry (DSC), FT-Raman spectroscopy and X-ray diffraction is well suited. For the starting material dimethyloctadecylchlorosilane (DMOC), the assignment of Raman bands is discussed. The influence of sugar-containing head groups on the structures of the hydrocarbon chains of 1-O-(dimethyldodecylsilyl)-[2,3,4,6-tetra-O-acetyl-beta-D-glucopyranoside] and 1-O-(dimethyloctadecylsilyl)-[2,3,4,6-tetra-O-acetyl-beta-D-glucopyranoside] was investigated using the band position of the symmetric methylene mode. The temperature dependence of conformationally sensitive bands in the CH(2)-stretching region (2800-2900 cm(-1)), C-C-stretching region (1000-1150 cm(-1)) and CH(3)-rocking region (830-900 cm(-1)) was studied to characterize the state of order of the alkyl chains. Using X-ray diffraction, the repeating distances of layered structures was determined. The phase transitions occurring were found to be completely reversible. The subcell of DMOC shows an orthorhombic perpendicular packing structure in the crystalline state.     

The physical properties of glycosyl diacylglycerols. Calorimetric, X-ray diffraction and Fourier transform spectroscopic studies of a homologous series of 1,2-di-O-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols.

We have synthesized a homologous series of saturated 1,2-di-O-n-acyl-3-O-(beta-D-galactopyranosyl)-sn-glycerols with odd- and even-numbered hydrocarbon chains ranging in length from 10 to 20 carbon atoms, and have investigated their physical properties using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. The DSC results show a complex pattern of phase behaviour, which in a typical preheated sample consists of a lower temperature, moderately energetic lamellar gel/lamellar liquid-crystalline (L(beta)/L(alpha)) phase transition and a higher temperature, weakly energetic lamellar/nonlamellar phase transition. On annealing at a suitable temperature below the L(beta)/L(alpha) phase transition, the L(beta) phase converts to a lamellar crystalline (L(c1)) phase which may undergo a highly energetic L(c1)/L(alpha) or L(c1)/inverted hexagonal (H(II)) phase transition at very high temperatures on subsequent heating or convert to a second L(c2) phase in certain long chain compounds on storage at or below 4 degrees C. The transition temperatures and phase assignments for these galactolipids are supported by our XRD and FTIR spectroscopic measurements. The phase transition temperatures of all of these events are higher than those of the comparable phase transitions exhibited by the corresponding diacyl alpha- and beta-D-glucosyl glycerols. In contrast, the L(beta)/L(alpha) and lamellar/nonlamellar phase transition temperatures of the beta-D-galactosyl glycerols are lower than those of the corresponding diacyl phosphatidylethanolamines (PEs) and these glycolipids form inverted cubic phases at temperatures between the lamellar and H(II) phase regions. Our FTIR measurements indicate that in the L(beta) phase, the hydrocarbon chains form a hexagonally packed structure in which the headgroup and interfacial region are undergoing rapid motion, whereas the L(c) phase consists of a more highly ordered, hydrogen-bonded phase, in which the chains are packed in an orthorhombic subcell similar to that reported for the diacyl-beta-D-glucosyl-sn-glycerols. A comparison of the DSC data presented here with our earlier studies of other diacyl glycolipids shows that the rate of conversion from the L(beta) to the L(c) phase in the beta-D-galactosyl glycerols is slightly faster than that seen in the alpha-D-glucosyl glycerols and much faster than that seen in the corresponding beta-D-glucosyl glycerols. The similarities between the FTIR spectra and the first-order spacings for the lamellar phases in both the beta-D-glucosyl and galactosyl glycerols suggest that the headgroup orientations may be similar in both beta-anomers in all of their lamellar phases. Thus, the differences in their L(beta)/L(c) conversion kinetics and the lamellar/nonlamellar phase properties of these lipids probably arise from subtly different hydration and H-bonding interactions in the headgroup and interfacial regions of these phases. In the latter case, such differences would be expected to alter the ability of the polar headgroup to counterbalance the volume of the hydrocarbon chains. This perspective is discussed in the context of the mechanism for the L(alpha)/H(II) phase transition which we recently proposed, based on our X-ray diffraction measurements of a series of PEs.     

Structural and dynamical studies of mixed chlorophyll/phosphatidylcholine bilayers via x-ray diffraction, absorption polarization spectroscopy and nuclear magnetic resonance.

The structure and dynamics of phosphatidylcholine bilayers containing chlorophyll were studied by X-ray diffraction and absorption polarization spectroscopy in the form of hydrated orientated multilayers below the thermal phase transition of the lipid chains and by nuclear magnetic resonance in the form of single-wall vesicles above the thermal transition. Our results show that (a) chlorophyll is incorporated into the phosphatidylcholine bilayers with its porphyrin ring located anisotropically in the polar headgroup layer of the membrane and with its phytol chain penetrating in a relatively extended form between the phosphatidylcholine fatty acid chains in the hydrocarbon core of the mixed bilayer membrane and (b) the intramolecular anisotropic rotational dynamics of the host phosphatidylcholine molecules are significantly perturbed upon chlorophyll incorporation into the bilayer at all levels of the phosphatidylcholine structure. These dynamics for the host phosphatidylcholine fatty acids chains are qualitatively different from that of the incorporated chlorophyll phytol chains on a 10(-9)-10(-10)s time scale in the ideally mixed two-component bilayer.