On the localization of ubiquinone in phosphatidylcholine bilayers

The location of ubiquinone-10 in phospholipid bilayers was analyzed using a variety of physical techniques. Specifically, we examined the hypothesis that ubiquinone localizes at the geometric center of phospholipid bilayers. Light microscopy of dipalmitoylphosphatidylcholine at room temperature in the presence of 0.05-0.5 mol fraction ubiquinone showed two separate phases, one birefringent lamellar phase and one phase that consisted of isotropic liquid droplets. The isotropic phase had a distinct yellow color, characteristic of melted ubiquinone. [13C]NMR spectroscopy of phosphatidylcholine liposomes containing added ubiquinone indicated a marked effect on the 13C-spin lattice relaxation times of the lipid hydrocarbon chain atoms near the polar head region of the bilayer, but almost no effect on those atoms nearest the center of the bilayer. X-ray diffraction experiments showed that for phosphatidylcholine bilayers, both in the gel and liquid-crystal-line phases, the presence of ubiquinone did not change either the lamellar repeat period or the wide-angle reflections from the lipid hydrocarbon chains. In electron micrographs, the hydrophobic freeze-fracture surfaces of bilayers in the rippled (P beta') phase were also unmodified by the presence of ubiquinone. These results indicate that the ubiquinone which does partition into the bilayer is not localized preferentially between the monolayers, and that an appreciable fraction of the ubiquinone forms a separate phase located outside the lipid bilayer.     

On the localization of ubiquinone in phosphatidylcholine bilayers

The location of ubiquinone-10 in phospholipid bilayers was analyzed using a variety of physical techniques. Specifically, we examined the hypothesis that ubiquinone localizes at the geometric center of phospholipid bilayers. Light microscopy of dipalmitoylphosphatidylcholine at room temperature in the presence of 0.05–0.5 mol fraction ubiquinone showed two separate phases, one birefringent lamellar phase and one phase that consisted of isotropic liquid droplets. The isotropic phase had a distinct yellow color, characteristic of melted ubiquinone. [¹³C]NMR spectroscopy of phosphatidylcholine liposomes containing added ubiquinone indicated a marked effect on the ¹³C-spin lattice relaxation times of the lipid hydrocarbon chain atoms near the polar head region of the bilayer, but almost no effect on those atoms nearest the center of the bilayer. X-ray diffraction experiments showed that for phosphatidylcholine bilayers, both in the gel and liquid-crystal-line phases, the presence of ubiquinone did not change either the lamellar repeat period or the wide-angle reflections from the lipid hydrocarbon chains. In electron micrographs, the hydrophobic freeze-fracture surfaces of bilayers in the rippled (Pβ′) phase were also unmodified by the presence of ubiquinone. These results indicate that the ubiquinone which does partition into the bilayer is not localized preferentially between the monolayers, and that an appreciable fraction of the ubiquinone forms a separate phase located outside the lipid bilayer.     

An X-ray diffraction study of the effect of alpha-tocopherol on the structure and phase behaviour of bilayers of dimyristoylphosphatidylethanolamine.

The effect of alpha-tocopherol on the thermotropic phase transition behaviour of aqueous dispersions of dimyristoylphosphatidylethanolamine was examined using synchrotron X-ray diffraction methods. The temperature of gel to liquid-crystalline (Lbeta-->Lalpha) phase transition decreases from 49.5 to 44.5 degrees C and temperature range where gel and liquid-crystalline phases coexist increases from 4 to 8 degrees C with increasing concentration of alpha-tocopherol up to 20 mol%. Codispersion of dimyristoylphosphatidylethanolamine containing 2.5 mol% alpha-tocopherol gives similar lamellar diffraction patterns as those of the pure phospholipid both in heating and cooling scans. With 5 mol% alpha-tocopherol in the phospholipid, however, an inverted hexagonal phase is induced which coexists with the lamellar gel phase at temperatures just before transition to liquid-crystalline lamellar phase. The presence of 10 mol% alpha-tocopherol shows a more pronounced inverted hexagonal phase in the lamellar gel phase but, in addition, another non-lamellar phase appears with the lamellar liquid-crystalline phase at higher temperature. This non-lamellar phase coexists with the lamellar liquid-crystalline phase of the pure phospholipid and can be indexed by six diffraction orders to a cubic phase of Pn3m or Pn3 space groups and with a lattice constant of 12.52+/-0.01 nm at 84 degrees C. In mixed aqueous dispersions containing 20 mol% alpha-tocopherol, only inverted hexagonal phase and lamellar phase were observed. The only change seen in the wide-angle scattering region was a transition from sharp symmetrical diffraction peak at 0.43 nm, typical of gel phases, to broad peaks centred at 0.47 nm signifying disordered hydrocarbon chains in all the mixtures examined. Electron density calculations through the lamellar repeat of the gel phase using six orders of reflection indicated no difference in bilayer thickness due to the presence of 10 mol% alpha-tocopherol. The results were interpreted to indicate that alpha-tocopherol is not randomly distributed throughout the phospholipid molecules oriented in bilayer configuration, but it exists either as domains coexisting with gel phase bilayers of pure phospholipid at temperatures lower than Tm or, at higher temperatures, as inverted hexagonal phase consisting of a defined stoichiometry of phospholipid and alpha-tocopherol molecules.     

Inverted hexagonal and cubic phases induced by alpha-tocopherol in fully hydrated dispersions of dilauroylphosphatidylethanolamine

The effect of alpha-tocopherol on the thermotropic phase behaviour and structure of aqueous dispersions of 1,2-di-lauryl-sn-glycero-3-phosphoethanolamine was examined by synchrotron X-ray diffraction. The pure phospholipid exhibited a lamellar gel to liquid-crystal phase transition at 30 degrees C on heating at 3 degrees C min(-1) between 10 degrees C and 90 degrees C. The transition was reversible with a temperature hysteresis of 0.3 degrees C on cooling. At temperatures less than 10 degrees C only lamellar gel phase of the pure phospholipid was seen in co-dispersions of up to 20 mol % alpha-tocopherol. The presence of 2.5 mol % alpha-tocopherol caused the appearance of inverted hexagonal phase at temperatures just below the main phase transition temperature that co-existed with the lamellar gel phase. The intensity of scattering from the hexagonal-II phase increased with increasing proportion of alpha-tocopherol in the mixture and in proportions greater than 10 mol % it persisted at temperatures above the main transition and co-existed with the lamellar liquid-crystal phase of the pure phospholipid. At higher temperatures all co-dispersions containing up to 15 mol % alpha-tocopherol showed the presence of cubic phases. These phases indexed a Pn3m or Pn3 space grouping. When the proportion of alpha-tocopherol was increased to 20 mol % the only non-lamellar phase observed was inverted hexagonal phase. This phase co-existed with lamellar gel and liquid-crystal phases of the pure phospholipid, but was the only phase present at temperatures >60 degrees C. The X-ray diffraction data were used to construct a partial phase diagram of the lipid mixture in excess water between 10 degrees and 90 degrees C and up to 20 mol % alpha-tocopherol in phospholipid.     

The interaction of alpha-tocopherol with bilayers of 1-palmitoyl-2-oleoyl-phosphatidylcholine

The effect of alpha-tocopherol on the structure and phase behaviour of 1-palmitoyl-2-oleoyl-phosphatidylcholine was examined by real-time synchrotron X-ray diffraction and freeze-fracture electron microscopic methods. X-ray scattering intensity was recorded from mixed aqueous dispersions of phospholipid with 2.5, 5, 10 and 20 mol% alpha-tocopherol during temperature scans at 3 degrees /min between -25 and 10 degrees C. A ripple structure is induced by the presence of alpha-tocopherol that coexists with the ripple phase characteristic of the pure phospholipid in mixtures containing 2.5 mol% alpha-tocopherol but completely replaces it in mixtures containing greater proportions of alpha-tocopherol. Freeze-fracture replicas of dispersions containing 5 mol% alpha-tocopherol indicate a ripple phase with a periodicity of about 9 nm. Increasing amounts of alpha-tocopherol result in a progressive reduction in temperature of the gel to liquid-crystal phase transition and broadening of the transition. Two lamellar phases coexist in the liquid-crystal state, one with a spacing of 6.4 nm assigned to an alpha-tocopherol-enriched lamellar structure and the other with a lamellar repeat of 6.1 nm corresponding to bilayers of pure phospholipid.     

Effect of fatty acyl chain length and structure on the lamellar gel to liquid-crystalline and lamellar to reversed hexagonal phase transitions of aqueous phosphatidylethanolamine dispersions

The lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transitions of aqueous dispersions of a number of synthetic phosphatidylethanolamines containing linear saturated, branched chain, and alicyclic fatty acyl chains of varying length were studied by differential scanning calorimetry, 31P nuclear magnetic resonance spectroscopy, and X-ray diffraction. For any given homologous series of phosphatidylethanolamines containing a single chemical class of fatty acids, the lamellar gel/liquid-crystalline phase transition temperature increases and the lamellar liquid-crystalline/reversed hexagonal phase transition temperature decreases with increases in hydrocarbon chain length. For a series of phosphatidylethanolamines of the same hydrocarbon chain length but with different chemical structures, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary markedly and in the same direction. In particular, at comparable effective hydrocarbon chain lengths, both the lamellar gel/liquid-crystalline and the lamellar liquid-crystalline/reversed hexagonal phase transition temperatures vary in parallel, such that the temperature difference between these two phase transitions is nearly constant. Moreover, at comparable effective acyl chain lengths, the d spacings of the lamellar liquid-crystalline phases and of the inverted hexagonal phases are all similar, implying that the thickness of the phosphatidylethanolamine bilayers at the onset of the lamellar liquid-crystalline/reversed hexagonal phase transition and the diameter of the water-filled cylinders formed at the completion of this phase transition are comparable and independent of the chemical structure of the acyl chain.(ABSTRACT TRUNCATED AT 250 WORDS)     

The kinetics and mechanism of the formation of crystalline phase of dipalmitoylphosphatidylethanolamine dispersed in aqueous dimethyl sulfoxide solutions

The phase transition kinetics and mechanism of formation of a lamellar-crystalline phase of dipalmitoylphosphatidylethanolamine (DPPE) dispersed in different concentrations of aqueous dimethyl sulfoxide (DMSO) during cooling have been examined by differential scanning calorimetry and synchrotron X-ray diffraction techniques. In dispersions containing mole fractions of DMSO (x<0.22), the phase transition sequence of the phospholipid is from lamellar liquid-crystal phase to lamellar-gel phase. Increasing the mole fraction of DMSO to 0.220.5 resulted in a direct transition from liquid-crystal phase to lamellar crystal phase with no detectable intermediate gel phase. A temperature versus DMSO concentration phase diagram was constructed based on calorimetric data with phase assignments made using synchrotron X-ray diffraction measurements. The non-isothermal formation kinetics of the lamellar crystal phase, which is expressed as the half time of the transformation process, was found to depend on DMSO concentration. The inducement of lamellar crystal phase in DPPE by DMSO is discussed in terms of the dehydration effect of DMSO and competitive molecular interactions between DMSO, water, and the phospholipid.     

The effect of dolichol on the structure and phase behaviour of phospholipid model membranes

The effect of dolichol C(95) on the structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine and stearoyloleoylphosphatidylethanolamine has been examined by synchrotron X-ray diffraction and differential scanning calorimetry. The presence of dolichol C(95) had no detectable effects on the temperature of either the gel to ripple or the ripple to liquid-crystal phase transition of dipalmitoylphosphatidylcholine. A proportionate increase of a few degrees in the temperature of the gel to lamellar liquid-crystal phase transition is observed in dispersions of dipalmitoylphosphatidylethanolamine and significantly there is a decrease in the temperature of the lamellar to non-lamellar phase transition of stearoyloleoylphosphatidylethanolamine. There was no significant change in the bilayer repeat spacing of all three mixed dispersions in gel phase in the presence of up to 20 mol% dolichol C(95). Electron density calculations showed that there was no change of bilayer thickness of dipalmitoylphosphatidylcholine with incorporation of up to 7.5 mol% dolichol C(95). These data suggest that effect of dolichol on the phospholipid model membranes depend on both the head group and the hydrocarbon chains of the phospholipid molecules. The presence of dolichol in phosphatidylcholine bilayers conforms to a model in which the polyisoprene compound is phase separated into a central domain sandwiched between the two monolayers in gel phase. In bilayers of phosphatidylethanolamines dolichol tends to stabilize the bilayers in gel phase at low temperatures and destabilize the bilayers in lamellar disordered structure at high temperatures. Non-lamellar structures coexist with lamellar disordered phase over a wide temperature range suggesting that dolichol is enriched in domains of non-lamellar structure and depleted from lamellar phase. These findings are useful to understand the function of dolichol in cell membranes.     

The effect of dimethyl sulphoxide on the structure and phase behaviour of palmitoleoylphosphatidylethanolamine

The thermotropic phase behaviour and structure of a nonbilayer-forming lipid, 1-palmitoyl-2-oleoyl-phosphatidylethanolamine, dispersed in water and in aqueous solutions of up to 50 wt% dimethyl sulphoxide (DMSO) have been characterised using synchrotron X-ray diffraction methods. It was found that the presence of DMSO in the solvent induced an increase in the temperature of lamellar-gel to lamellar-liquid-crystal phase transition and a decrease in the temperature of the lamellar-liquid-crystal to inverted-hexagonal phase transition of the phospholipid. The presence of DMSO also caused a decrease in the X-ray repeat spacings of all the phases studied. Electron density profiles of the phospholipid dispersed in water and 50 wt% DMSO in the bilayer gel state were calculated. The presence of 50 wt% DMSO caused the apparent disappearance of the solvent layer separating phospholipid bilayers in the gel state. The results suggest that DMSO contributes to the bilayer electron density profile and that the amphiphilic solvent molecules partition into the interfacial region.     

Phase properties of senescing plant membranes. Role of the neutral lipids

Wide-angle X-ray diffraction studies have indicated that rough and smooth microsomal membranes from bean cotyledons acquire increasing proportions of gel phase lipid at physiological temperature as the tissue senesces. In addition, for both types of membrane the lipid phase transition temperature, defined as the highest temperature at which gel phase lipid can be detected, progressively rises with advancing senescence. Liposomes prepared from total lipid extracts of the membranes show a similar increase in transition temperature with age, indicating that separation of the polar lipids into distinct gel and liquid-crystalline domains is not attributable to peculiar protein-lipid interactions. Liposomes prepared from purified phospholipid fractions of the membranes show little change in transition temperature with age, indicating that the altered phase properties of the lipid do not reflect an increase in fatty acid saturation. However, the formation of gel phase lipid that occurs naturally during senescence can be stimulated by preparing liposomes from a mixture of the phospholipid fraction from young membrane and the neutral lipid fraction from old membrane. By adding the separated components of the neutral lipid fraction to purified phospholipid it was found that sterol esters and several unidentified lipids are able to raise the transition temperature of the polar lipids. Sterols have no effect on the phospholipid transition temperature. The data have been interpreted as indicating that several neutral lipids, which presumably increase in abundance with advancing senescence, induce a lateral phase separation of the polar lipids resulting in distinct gel and liquid-crystalline domains of lipid in the senescent membranes.     

Organotin compounds alter the physical organization of phosphatidylcholine membranes

Organotin compounds have a broad range of biological activities and are ubiquitous contaminants in the environment. Their toxicity mainly lies in their action on the membrane. In this contribution we study the interaction of tributyltin and triphenyltin with model membranes composed of phosphatidylcholines of different acyl chain lengths using differential scanning calorimetry, (31)P-nuclear magnetic resonance, X-ray diffraction and infrared spectroscopy. Organotin compounds broaden the main gel to liquid-crystalline phase transition, shift the transition temperature to lower values and induce the appearance of a new peak below the main transition peak. These effects are more pronounced in the case of tributyltin and are quantitatively larger as the phosphatidylcholine acyl chain length decreases. Both tributyltin and triphenyltin increase the enthalpy change of the transition in all the phosphatidylcholine systems studied except in dilauroylphosphatidylcholine. Organotin compounds do not affect the macroscopic bilayer organization of the phospholipid but do affect the degree of hydration of its carbonyl moiety. The above evidence supports the idea that organotin compounds are located in the upper part of the phospholipid palisade near the lipid/water interface.     

Structure and interactions of ether- and ester-linked phosphatidylethanolamines

The ether-linked phospholipid 1,2-dihexadecylphosphatidylethanolamine (DHPE) was studied as a function of hydration and in fully hydrated mixed phospholipid systems with its ester-linked analogue 1,2-dipalmitoylphosphatidylethanolamine (DPPE). A combination of differential scanning calorimetry (DSC) and X-ray diffraction was used to examine the phase behavior of these lipids. By DSC, from 0 to 10 wt % H2O, DHPE displayed a single reversible transition that decreased from 95.2 to 78.8 degrees C and which was shown by X-ray diffraction data to be a direct bilayer gel to inverted hexagonal conversion, L beta----HII. Above 15% H2O, two reversible transitions were observed which stabilized at 67.1 and 92.3 degrees C above 19% H2O. X-ray diffraction data of fully hydrated DHPE confirmed the lower temperature transition to be a bilayer gel to bilayer liquid-crystalline (L beta----L alpha) phase transition and the higher temperature transition to be a bilayer liquid-crystalline to inverted hexagonal (L alpha----HII) phase transition. The lamellar repeat distance of gel-state DHPE increased as a function of hydration to a limiting value of 62.5 A at 19% H2O (8.6 mol of water/mol of DHPE), which corresponds to the hydration at which the transition temperatures are seen to stabilize by DSC. Electron density profiles of DHPE, in addition to calculations of the lipid layer thickness, confirmed that DHPE in the gel state forms a noninterdigitated bilayer at all hydrations. Fully hydrated mixed phospholipid systems of DHPE and DPPE exhibited two reversible transitions by DSC.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lamellar gel-lamellar liquid crystal phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from aqueous trehalose solutions. A real-time X-ray diffraction study

The mechanism of the phase transition of dipalmitoylphosphatidylcholine multilayers freeze-dried from fully hydrated gel phase (L beta') in the presence of trehalose has been investigated by real-time X-ray diffraction methods. Sequential diffraction patterns were recorded with an accumulation time of 3 s during heating and 1.2 s during cooling between about 20 and 80 degrees C. A transition is observed in the range 47-53 degrees C that involves structural events typical of a lamellar gel-lamellar liquid-crystal (L beta--L alpha) transformation. This transition is completely reversible with a temperature hysteresis of 2-3 degrees C and thereby resembles the main phase transition of fully hydrated dipalmitoylphosphatidylcholine multilayers. The mechanism of the transition from L beta to L alpha as seen in the wide-angle scattering profiles show that the sharp peak at about 0.41 nm, characteristic of the gel phase, broadens and shifts progressively to about 0.44 nm towards the end of the transition. A temperature jump of 6C degrees/s through the phase transition region of a freeze-dried dipalmitoylphosphatidylcholine: trehalose mixture (molar ratio 1:1) showed that the phase transition had a relaxation time of about 2 s which is similar to that of the main transition in the fully hydrated lipid. X-ray diffraction studies of the melting of dipalmitoylphosphatidylcholine freeze-dried from the lamellar-gel phase in the absence of trehalose showed a transition at above 70 degrees C. The low-angle diffraction data of phospholipid/trehalose mixtures are consistent with an arrangement of trehalose molecules in a loosely packed 'monolayer' separating bilayers of phospholipid. Trehalose appears to reduce the direct interbilayer hydrogen bond coupling thereby modifying the thermal stability and the phase transition mechanism of the bilayers.     

The thermotropic properties of coenzyme Q10 and its lower homologues

The thermotropic properties of coenzymes Q₁₀, Q₉, Q₈, and Q₇ have been examined by differential scanning calorimetry and wide-angle X-ray diffraction. Typical scanning calorimetry cooling curves of coenzyme Q from the liquid state exhibit a single exothermic phase transition into a crystalline state at a temperature that decreases as the length of the polyisoprenoid side-chain substituent decreases. Upon subsequent heating, the molecules undergo a series of thermal events which precede the main crystalline-to-liquid endothermic phase transition. The temperature of these transitions increases with increasing chain length. The crystallization phase transition temperature depends markedly on the rate at which the sample is cooled and increases with decreasing scan rate; the temperature of the melting endotherm is not markedly affected by the scan rate. Detailed calorimetric studies of coenzyme Q₁₀ indicate that two crystalline states are formed, one at relatively high cooling rates to low temperatures and the other when preparations are cooled slowly from the liquid state to relatively high temperatures. Heating the crystalline phase formed by rapid cooling causes its transformation into the phase observed by cooling slowly. X-ray diffraction analysis confirmed the existence of these two crystal phases in coenzymes Q₉ and Q₁₀ and the transformation from the rapidly crystallized form to the more ordered form associated with slower cooling rates. At body temperature (310 K) under equilibrium conditions coenzyme Q₁₀ exists in an ordered crystalline phase; the implications of the thermotropic behavior of coenzyme Q₁₀ on mitochondrial functionin vitro andin vivo are discussed.     

The effect of dolichol on the structure and phase behaviour of phospholipid model membranes

The effect of dolichol C₉₅ on the structure and thermotropic phase behaviour of dipalmitoylphosphatidylcholine, dipalmitoylphosphatidylethanolamine and stearoyloleoylphosphatidylethanolamine has been examined by synchrotron X-ray diffraction and differential scanning calorimetry. The presence of dolichol C₉₅ had no detectible effects on the temperature of either the gel to ripple or the ripple to liquid-crystal phase transition of dipalmitoylphosphatidylcholine. A proportionate increase of a few degrees in the temperature of the gel to lamellar liquid-crystal phase transition is observed in dispersions of dipalmitoylphosphatidylethanolamine and significantly there is a decrease in the temperature of the lamellar to non-lamellar phase transition of stearoyloleoylphosphatidylethanolamine. There was no significant change in the bilayer repeat spacing of all three mixed dispersions in gel phase in the presence of up to 20 mol% dolichol C₉₅. Electron density calculations showed that there was no change of bilayer thickness of dipalmitoylphosphatidylcholine with incorporation of up to 7.5 mol% dolichol C₉₅. These data suggest that effect of dolichol on the phospholipid model membranes depend on both the head group and the hydrocarbon chains of the phospholipid molecules. The presence of dolichol in phosphatidylcholine bilayers conforms to a model in which the polyisoprene compound is phase separated into a central domain sandwiched between the two monolayers in gel phase. In bilayers of phosphatidylethanolamines dolichol tends to stabilize the bilayers in gel phase at low temperatures and destabilize the bilayers in lamellar disordered structure at high temperatures. Non-lamellar structures coexist with lamellar disordered phase over a wide temperature range suggesting that dolichol is enriched in domains of non-lamellar structure and depleted from lamellar phase. These findings are useful to understand the function of dolichol in cell membranes.     

Study of the cytoplasmic and outer membranes of Escherichia coli by deuterium magnetic resonance.

The cytoplasmic and outer membranes of Escherichia coli were studied between 0 and 40 degrees C by deuterium magnetic resonance quadrupolar echo spectroscopy. The L51 strain of E. coli was used to incorporate perdeuterated palmitic acid into the membrane phospholipids. The cytoplasmic and outer membranes were separated using standard techniques. The spectrum of each membrane preparation was dominated at high temperatures (greater than or equal to 37 degrees C) by the characteristic liquid-crystalline plateau previously observed for perdeuterated palmitate chains in model phospholipid membranes. At low temperatures, the shape and width of the spectrum were characteristic of the gel phase. The relative intensities of the liquid-crystalline and gel features varied systematically with temperature. A quantitative analysis of the acyl chain orientational order was carried out by using the method of moments. The orientational order at each temperature was greater in the outer membrane sample than in that of the cytoplasmic membrane, indicating that the liquid-crystalline-gel transition region in the outer membrane is shifted to higher temperatures than that of the cytoplasmic membrane by about 7 degrees C. It is clear from the results that most of the phospholipid molecules participate in the phase transition.     

Proton magnetic resonance spectroscopic studies of the interaction of ubiquinone-10 with phospholipid model membranes

Proton magnetic resonance spectra of ubiquinone-10 and ubiquinone-10 dispersed with dipalmitoylglycerophosphocholine or egg phosphatidylcholine in aqueous medium have been obtained. The dispersions are in the form of multilamellar liposomes as judged by 31P-NMR spectra and the thermal history of the samples have ensured that ubiquinone not incorporated into the phospholipid structure only gives rise to a broad-line NMR proton spectrum. A high-resolution proton spectrum of ubiquinone is observed with upfield shifts of the O-methyl protons of the benzoquinone rings, indicating close proximity of the molecules but with an arrangement different from the pure liquid ubiquinone. Spectra obtained in the presence of the lanthanide shift reagents, dysprosium fluorooctanedionate and Dy(NO3)3, which have a preferred location in the hydrophobic and hydrophilic domains, respectively, of ubiquinone/phospholipid codispersions, are consistent with the partitioning of ubiquinone into a hydrophobic phospholipid environment remote from the aqueous phase. The type of arrangements of ubiquinone that could be accommodated within bilayers of phospholipid are discussed.     

The role of the phospholipid phase transition in the regulation of glucagon binding to lecithin

Glucagon can interact rapidly with multilamellar vesicles of dimyristoyl glycerophosphocholine over a narrow temperature range around or above the phase transition temperature of the pure phospholipid. The temperature dependence of the rates arises, in large part, from glucagon-induced alterations in the phase transition properties of the phospholipid. Similar effects are observed with dilaury glycerophosphocholine but the rate of reaction of glucagon with multilamellar dipalmitoyl glycerophosphocholine is too slow to measure.The rate of reaction of glucagon with equimolar mixtures of two phospholipid molecules has also been studied. Mixtures of dilauryl glycerophosphocholine and distearoyl glycerophosphocholine are known to exhibit lateral phase separation in the gel state. The presence of distearoyl glycerophosphocholine has no effect on the rate of reaction with glucagon, despite the increased number of phase boundaries present. In the case of mixtures of dilauryl glycerophosphocholine and dimyristoyl glycerophosphocholine, glucagon appears to induce some lateral phase separation. This is demonstrated by the ability of glucagon to react rapidly with this lipid mixture, even at temperatures well below the phase transition temperature of the mixture and by differential scanning calorimetry.The thermodynamics of the binding of glucagon to dimyristoyl glycerophosphocholine and dilauryl glycerophosphocholine were analyzed with Scatchard plots calculated from measurements of the fluorescence enhancement caused by lipids. Equilibrium binding constants of glucagon to dimyristoyl glycerophosphocholine and dilauryl glycerophosphocholine are 1·10⁵ and 5·10⁴ M^?1, respectively. These values are relatively insensitive to temperature, indicating that the equilibrium being measured is between lipid-bound glucagon and free lipid which has had its phase transition properties altered. The number of moles of lipid bound per mole of glucagon decreases markedly above the phase transition temperature. In the water-soluble complex formed between glucagon and dimyristoyl glycerophosphocholine, the peptide binds directly to only 40% of the lipid molecules but, nevertheless, is able to modify the phase transition properties of all of the lipid in the particle.     

Structural aspects of pressure effects on infrared spectra of mixed-chain phosphatidylcholine assemblies in D2O

The barotropic behavior of D2O dispersions of 1-stearoyl-2-caproyl-sn-glycero-3-phosphocholine, C(18):C(10)PC, a highly asymmetric phospholipid in which the length of the fully extended acyl chain at the sn-1 position of the glycerol backbone is twice as long as that at the sn-2 position, has been investigated by high-pressure Fourier transform infrared spectroscopy. This asymmetric phosphatidylcholine bilayer at room temperature displays a pressure-induced phase transition corresponding to the liquid-crystalline----gel phase transition at 1.4 kbar. A conformational ordering of the lipid acyl chains is observed to take place abruptly at the transition pressure of 1.4 kbar. However, the lamellar lipid molecules and their acyl chains remain to be orientationally disordered in the gel phase until the applied pressure reaches 5.5 kbar. In the gel phase of fully hydrated C(18):C(10)PC, the asymmetric lipid molecules assemble into mixed interdigitated bilayers with perpendicular orientation of the zigzag planes among neighboring acyl chains. The role of excess water played in the interchain structure and the behavior of excess water and bound water under high pressure are also discussed.     

A comparative study of the phase transitions of phospholipid bilayers and monolayers

Phase transitions in bilayers and monolayers of various synthetic phospholipids with different chain lengths as well as different polar head groups were studied by differential scanning calorimetry or with the film balance technique, respectively. With the film balance, area versus temperature curves (isobars) were recorded at different surface pressures. The monolayer phase transition from the fluid-condensed to the fluid-expanded phase is shifted towards higher temperature when the lateral pressure in the monolayer is increased. The temperature dependence of the equilibrium pressure as well as the magnitude of the area change at the transition depends only on the nature of the phospholipid head group and not on the chain length of the hydrocarbon chains of the lipid. Phospholipids with strong intermolecular attractive interactions between the head groups show low values for dpi/dTm and for the area change, deltaf, whereas phospholipids with negatively charged head groups without intermolecular attractive forces exhibit higher values for dpi/dTm and deltaf. The shift of the monolayer phase transition temperature when increasing the chain length of the lipid is almost identical to the shift in Tm observed for the bilayer system of the same phospholipids. A comparison of monolayer and bilayer systems on the basis of the absolute value of the molecular area of the phospholipid in the bilayer gel phase and the change in area at the bilayer and monolayer transition leads to the following conclusions. The behaviour of the bilayer system is very similar to that of the respective monolayer system at a lateral pressure of approx. 30 dyne/cm, because at this pressure the absolute area and the area change in both systems are the same. Further support for this conclusion comes from the experimental finding that a lateral pressure of 30 dyne/cm the shift in Tm due to the increase in charge when the methyl ester of phosphatidic acid is investigated is the same for the bilayer and the monolayer system.