Proton NMR bandshape studies of lamellar liquid crystals and gel phases containing lecithins and cholesterol.

Proton NMR spectra for gel and liquid crystalline samples, composed of dimyristoyl and/or dipalmitoyl lecithin, cholesterol and water, can be consistently interpreted in terms of mesophase symmetry and molecular diffusion according to a model proposed by Wennerstrom (Wennerstrom, H. (1973) Chem. Phys. Lett. 18, 41-44). It is shown by computer simulation that the characteristic "super-lorentzian" bandshape of the lamellar mesophase can be described by the superposition of three gaussian curves. The NMR signal of the gel phase can be simulated by the superposition of two gaussian curves with widths at half height of 2.5 kHz and 19 kHz. An upper limit of the lateral diffusion coefficient of the lecithin molecules in the gel phase is calculated to be about 5-10(-15) m-2/s. It is therefore concluded that the static intermolecular dipolar couplings average to zero in the lamellar mesophase. An estimation of the order parameter of the liquid crystalline phase is made from experimental data and a calculated "rigid lattice" linewidth. A two phase system is shown to exist in the temperature range 28-34 degrees C for a mesophase of a mixture of dimyristoyl and dipalmitoyl lecithin. The presence of cholesterol results in enhanced lateral diffusion of the lecithin molecules at temperatures below the Chapman transition point.     

Proton and deuteron relaxation of muscle water over wide ranges of resonance frequencies.

The spin-lattice relaxation time (T1) of water protons in mouse muscle was studied from 10(4) to 10(8) Hz at several temperatures, and the deuteron T1 of muscle water was studied from 2.0 X 10(3) to 1.54 X 10(7) Hz at several temperatures. Proton T1's of muscle and brain water with different D2O contents were measured at 25 degrees C and 35 MHz. From the results of variable frequency and temperature measurements and the data of isotope substitution, it is concluded that the major relaxation mechanism for the protons in muscle water is the intermolecular dipolar interaction between the protons of the macromolecules and the protons of the water molecules in the hydration layer. It is also suggested that the relaxation of deuterons can be accounted for a very small fraction of water molecules directly hydrogen-bonded to the macromolecules.     

Study of water permeability through phospholipid vesicle membranes by 17O NMR.

Vesicle suspensions of up to 5% egg lecithin and 2.5% cholesterol have been found to have no effect on the NMR relaxation times of 17O from water. Addition of 1-5 mM Mn2+ to an equimolar vesicle suspension of egg lecithin and cholesterol permitted resolution of the free induction decay into two exponential components, a fast one arising from the external water and a slow one arising from the intravesicular fluid. From the rates of relaxation the mean life time of the water molecules within the vesicles was calculated to be 1+/- 0.1 ms at 22 degrees C. The size of the vesicle was estimated from electron micrographs to be about 500 A in diameter. These data yield an equilibrium water permeability, Pw, of about 8 mus-1 for the vesicle membranes. From the temperature dependence of Pw an activation energy of 12+/-2 kcal/mol was obtained. The longitudinal relaxation time (T1) of water within vesicles remained the same as in pure water.     

Equilibrium studies of lecithin-cholesterol interactions I. Stoichiometry of lecithin-cholesterol complexes in bulk systems

The maximum molar ratio of lecithin:cholesterol in aqueous dispersions has been reported to be 2:1, 1:1, or 1:2. The source of the desparate results has been examined in this study by analyzing (a) the phase relations in anhydrous mixtures (from which most dispersions are prepared) and (b) various methods of preparing aqueous dispersions, with the purpose of avoiding the formation of metastable states that may be responsible for the variability of the lecithin-cholesterol stoichiometry. Temperature-composition phase diagrams for anhydrous mixtures of cholesterol (CHOL) with dimyristoyl (DML) and with dipalmitoyl (DPL) lecithin were obtained by differential scanning calorimetry (DSC). Complexes form with molar ratios for lecithin:CHOL of 2:1 and 1:2; they are stable up to 70°C. When x(CHOL) < 0.33, two phases coexist: complex (2:1) plus pure lecithin; when 0.33 < x(CHOL) < 0.67 complexes (2:1) and (1:2) coexist as separate phases. The corresponding phase diagram in water for these mixtures was determined by DSC and isopycnic centrifugation in D₂O-H₂O gradients. Aqueous dispersions were prepared by various methods (vortexing, dialysis, sonication) yielding identical results except as noted below. The data presented supports the following phase relations. When x(CHOL) < 0.33, two lipid phases coexist: pure lecithin plus complex (2:1) where the properties of the lecithin phase are determined by whether the temperature is below or above T_c, the gel-liquid crystal transition temperature. Therefore, complex (2:1) will coexist with gel state below T_c and with liquid crystal above T_c. The densities follow in the order gel > complex (2:1) > liquid crystal. The density of complex (2:1) is less sensitive to temperature in the range 5°-45°C compared to the temperature dependence for DML and DPL where large changes in density occur at T_c. When x(CHOL) > 0.33, CHOL phase coexists with complex (2:1); anhydrous complex (1:2) is apparently not stable in H₂O. The results are independent of the method and temperature used for preparing the lipid dispersions. However, when dispersions are prepared by sonication or with solvents at T > T_c, an apparent 1:1 complex is formed. Evidence suggests the 1:1 complex is metastable.     

The diffusion-concentration product of oxygen in lipid bilayers using the spin-label T1 method

A method is described to measure the oxygen diffusion-concentration product, DO[O2], at any locus that can be probed or labeled using nitroxide radicals. The method is based on the dependence of the spin-lattice relaxation time T1 of the spin label on the bimolecular collision rate with oxygen. Strong Heisenberg exchange between spin label and oxygen contributes directly to T1 of the spin label, while dipolar interactions are negligible. Both time-domain and continuous wave saturation methods for studying T1 are considered. The method has been applied to phospholipid liposomes using fatty acid spin labels. A discontinuity in DO[O2] at the main phase transition was observed.     

Lateral diffusion of the phospholipid molecule in dipalmitoylphosphatidylcholine bilayers. An investigation using nuclear spin--lattice relaxation in the rotating frame

Measurements of the proton NMR spin--lattice relaxation time in the rotating frame (T 1rho) have permitted the explicit determination of the lateral diffusion coefficient of phospholipid molecules in the lamellar mesophase of dipalmitoylphosphatidylcholine at temperatures above the phase-transition temperature. The experimentally observed temperature and frequency dependence of T 1rho for the dipalmitoylphosphatidylcholine protons suggest that intermolecular dipole--dipole relaxation contributions are important. Proton T 1rho experiments involving dilution with deuterated dipalmitoylphosphatidylcholine support the premise that intermolecular dipolar interactions are significant and, concomitantly, that lateral diffusion is the motion modulating that interaction. The lateral diffusion coefficient is determined directly from the dependence of the rotating frame spin--lattice relaxation rate (1/T 1rho) on the strength of the applied radiofrequency field in the spin-locking experiment. A series of experiments with varying concentrations of dipalmitoylphosphatidylcholine in the lamellar mesophase indicates that the lateral diffusion coefficient varies as a function of phospholipid concentration.     

Alcohol interactions with lipids: a carbon-13 nuclear magnetic resonance study using butanol labeled at C-1

The interactions of carbon-13 enriched butanol with dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were studied using C-13 nuclear magnetic resonance. It was found that above the gel to liquid crystal phase transition the resonance from the butanol could be resolved into two signals with similar chemical shifts but different T1 values and line widths. In contrast, only one narrow resonance was observed for ethanol, which has considerably less solubility in the lipids than butanol. Thermodynamic analyses of the effects of butanol on the phase transition temperature predict much greater solubility or butanol when the lipid is above the phase transition temperature than when it is below. It was concluded that the two butanol resonances represent two slowly exchanging populations, the free butanol in the aqueous phase and butanol dissolved in the liquid crystalline region of the lipid. No bound butanol was detected below the gel to liquid crystal phase transition. Relaxation studies were performed on the resonance of the bound butanol in DPPC and DMPC, including measurements of T1, line width, and nuclear Overhauser enhancement. Theoretical analysis of the relaxation parameters indicates that the lipid-bound alcohol has very high mobility within the fluid lipid bilayer. The data are consistent with butanol being present at the aqueous boundary or head group region of the lipid.     

Dielectric relaxation in lecithin/cholesterol/water-mixtures

The dielectric spectrum of aqueous solutions of dimyristoyl-l-3-phosphatidylcholine and dipalmitoyl-l-3-phosphatidylcholine with admixed cholesterol has been determined by means of a pulse reflection method which was used to measure the complex permittivity of the solutions as a function of frequency between 100 kHz and 50 MHz. Measurements have been performed at various concentrations of cholesterol in dependence of temperature around the crystal-line/liquid-crystalline phase transition temperature of the solutions.The measured dielectric spectra are treated in terms of a Debye-function. The dielectric relaxation strength and the relaxation time decrease distinctly with increasing cholesterol concentration. In addition, the data are treated on the basis of a theoretical solution model in order to allow for conclusions concerning the lecithin head group motion in the lipid bilayer surface. One important result is that increasing cholesterol concentration affects the interaction of the lecithin head groups and increases their mobility. These effects already occur at small concentrations of cholesterol.     

Mixed monolayers of straight-chain/branched-chain phospholipids. I. Mixed monolayers of distearoyl phosphatidylcholine and diisoeicosanoyl phosphatidylcholine

The temperature dependence of the force/area isotherms of monolayer of distearoyl phosphatidylcholine (DSPC), diisoeicosanoyl phosphatidylcholine (DIEPC) and a complete mixed compositional range of these two lecithins are reported. The isotherms for DSPC closely resemble those previously reported for dipalmitoyl phosphatidylcholine but are shifted to higher temperatures by 16 degrees C. The isotherms of DIEPC, an iso-branched lecithin, show differences from these obtained for similar straight-chain lecithins in that the full condensed isotherms are more expanded, the fully expanded isotherms are more condensed and therefore the liquid expanded (LE)/liquid condensed (LC) intermediate region is significantly reduced. This means that the condensed state is more disordered and the expanded state is less disordered than the corresponding states in straight-chain lecithins. Data for the mixed films are interpreted in terms of surface pressure/mole fraction phase diagrams and both energies and entropies of compression associated with the LE/LC transition. The phase diagrams at 34.1 degrees C, 35.8 degrees C and 38.5 degrees C are all of the negative azeotropic type with the surface pressure minimum point shifting with temperature. The thermodynamic analysis indicates that from 34.1 degrees C to 38.5 degrees C the driving force for mixing changes from the entropy to the energy of the transition. It would seem that at the lower temperature the packing of the distearoyl lecithin is perturbed by the diisoeicosanoyl lecithin, while at higher temperatures the very high entropy of pure or nearly pure diisoeicosanoyl lecithin results in other mixtures having less entropy than would be expected on an ideal mixing basis.     

A 31P-NMR spin-lattice relaxation and 31P[1H] nuclear Overhauser effect study of sonicated small unilamellar phosphatidylcholine vesicles.

The motional properties of the inner and outer monolayer headgroups of egg phosphatidylcholine (PC) small unilamellar vesicles (SUV) were investigated by 31P-NMR temperature-dependent spin-lattice relaxation time constant (T1) and 31P[1H] nuclear Overhauser effect (NOE) analyses. Three different aspects of the dynamics of PC headgroups were investigated using the T1 analysis. First, differences in the dynamics of the headgroup region of both surfaces of the SUV were measured after application of a chemical shift reagent, PrCl3, to either the extra- or intravesicular volumes. Second, the ability of the T1 experiment to resolve the different motional states was evaluated in the absence of shift reagent. Third, comparison between correlation times obtained from a resonance frequency dependent 31P[1H] NOE analysis allowed a determination of the applicability of a simplified motional model to describe phosphorus dipolar relaxation. Temperature-dependent 31P-NMR T1 values obtained for the individual monolayers at 81.0 and 162.0 MHz were modelled assuming that phosphorus undergoes both a dipolar and an anisotropic chemical shielding relaxation mechanism, each being described by the same correlation time, tau. At 162.0 MHz, the position of the T1 minimum for the inner monolayer was 9 degrees higher than that of the outer region, indicating a higher level of motional restriction for the inner leaflet, in agreement with 31P[1H] NOE measurements. The 162.0 MHz T1 profile of the combined SUV monolayers exhibited a smooth minimum located at the midpoint of the monolayer minima positions, effectively masking the presence of the individual surfaces. 31P[1H] NOE results obtained at 32.3, 81.0 and 162.0 MHz did not agree with those predicted from a simple dipolar relaxation model. These results suggest a T1-temperature method can neither discriminate two or more closely related motional time scales in a heterogeneous environment (such as incorporation of protein into lipid bilayers) nor allow accurate determination of the correlation time at the position of the minimum when the dipolar relaxation rate makes a significant contribution to the overall rate.     

Transverse nuclear spin relaxation in phosphatidylcholine bilayers containing gramicidin.

Deuterium nuclear magnetic resonance has been used to study transverse relaxation in samples of 1,2-dimyristoyl-sn-glycero-3-phosphocholine, perdeuterated and specifically deuterated at the alpha position of the chains, containing the polypeptide gramicidin at concentrations of 0, 1, and 4 mol%. For 4 mol% gramicidin, the bilayer is thought to undergo a continuous phase change rather than a phase transition proceeding via two phase coexistence. Information is obtained regarding lipid dynamics in the continuous phase change region of the phase diagram. In the presence of gramicidin, the transverse relaxation time measured by the quadrupole echo technique, T2e, passes through a minimum in the gel phase. The gramicidin concentration dependence of T2e suggests that the polypeptide reduces the temperature sensitivity of the correlation time responsible for the minimum. The polypeptide also increases the sensitivity of the first spectral moment, M1, to the quadrupole echo pulse separation. This behavior is attributed to a polypeptide-induced enhancement of the spread in T2e along the acyl chains. Quadrupole Carr-Purcell-Meiboom-Gill experiments are used to separate contributions to the observed behavior from fast and slow motions.     

Conformational dynamics and intersubunit energy transfer in wild-type and mutant lipoamide dehydrogenase from Azotobacter vinelandii. A multidimensional time-resolved polarized fluorescence study.

Time-resolved fluorescence and fluorescence anisotropy data surfaces of flavin adenine dinucleotide bound to lipoamide dehydrogenase from Azotobacter vinelandii in 80% glycerol have been obtained by variation of excitation energy and temperature between 203 and 303 K. The fluorescence kinetics of a deletion mutant lacking 14 COOH-terminal amino acids were compared with the wild-type enzyme to study a possible interaction of the COOH-terminal tail with the active site of the enzyme. The flavin adenine dinucleotide fluorescence in both proteins exhibits a bimodal lifetime distribution as recovered by the maximum entropy method of data analysis. The difference in standard enthalpy and entropy of associated conformational substates was retrieved from the fractional contributions of the two lifetime classes. Activation energies of thermal quenching were obtained that confirm that the isoalloxazines in the deletion mutant are solvent accessible in contrast to the wild-type enzyme. Red-edge spectroscopy in conjunction with variation of temperature provides the necessary experimental axes to interpret the fluorescence depolarization in terms of intersubunit energy transfer rather than reorientational dynamics of the flavins. The results can be explained by a compartmental model that describes the anisotropy decay of a binary, inhomogeneously broadened, homoenergy transfer system. By using this model in a global analysis of the fluorescence anisotropy decay surface, the distance between and relative orientation of the two isoalloxazine rings are elucidated. For the wild-type enzyme, this geometrical information is in agreement with crystallographic data of the A. vinelandii enzyme, whereas the mutual orientation of the subunits in the deletion mutant is slightly altered. In addition, the ambiguity in the direction of the emission transition moment in the isoalloxazine ring is solved. The anisotropy decay parameters also provide information on electronic and dipolar relaxational properties of the flavin active site. The local environment of the prosthetic groups in the deletion mutant of the A. vinelandii enzyme is highly inhomogeneous, and a transition from slow to rapid dipolar relaxation is observed over the measured temperature range. In the highly homogeneous active site of the wild-type enzyme, dipolar relaxation is slowed down beyond the time scale of fluorescence emission at any temperature studied. Our results are in favor of a COOH-terminal polypeptide interacting with the active site, thereby shielding the isoalloxazines from the solvent. This biological system forms a very appropriate tool to test the validity of photophysical models describing homoenergy transfer.     

Phase behavior of ganglioside-lecithin mixtures. Relation to dispersion of gangliosides in membranes.

Ganglioside GM1 and mixed brain gangliosides were mixed with 1-stearoyl-2-oleoyl lecithin (SOPC) and examined by differential scanning calorimetry as a function of ganglioside content and temperature. Low mole fractions of ganglioside GM1 and of mixed brain gangliosides are shown to be miscible with SOPC in the gel phase up to X = 0.3, with the possible exception of a small region of immiscibility for the mixed brain gangliosides system centered around X = 0.05. Above X = 0.3, the low-temperature phases demix into a (gel) phase of composition X = 0.3 and a (micellar) phase of composition X = 1.0. Above the endothermic phase transition temperature, no phase boundaries are discerned. It is pointed out that phase structures need to be determined in each domain delineated in the phase diagrams, and that cylindrical phases may exist at higher temperatures and intermediate compositions. The effects of addition of wheat germ agglutinin, which binds to ganglioside GM1, on a ganglioside GM1-SOPC mixture (X = 0.5), are described and interpreted in terms of partial demixing of ganglioside and lecithin. Behavior of the ganglioside-SOPC system is discussed with respect to the kinetics of cholera toxin action in lymphocytes, as well as to other physiological roles of gangliosides in membranes.     

A theoretical model of the temperature- and pressure-induced phase transition of phospholipid bilayers

A statistical thermodynamic model of phospholipid bilayers is developed. In the model, a new concept of a closely packed system is applied, i.e., a system of hard cylinders of equal radii, the radius being a function of the average number of gauche rotations in a hydrocarbon chain. Using this concept of a closely packed system, reasonable values are obtained for the change in specific volume at the order-disorder transition of lecithin bilayers. In addition to interactions between the lipid matrix and water molecules, between the head groups themselves and between hydrocarbon chains, as well as the intramolecular energy associated with chain conformation, the Hamiltonian of the membrane also includes the energy of the pressure field. Thus, the phase transition of phospholipid membranes induced not only by temperature hut also by hydrostatic pressure is described by this model simultaneously. In accordance with the experimental results, a linear relationship is obtained between the phase transition temperature and phase transition pressure. The other calculated phase transition properties of lecithin homologues. e.g., changes in enthalpy, surface area. thickness and gauche number per chain are in agreement with the available experimental data. The ratio of kink to interstitial conduction of bilayers is also estimated.     

Tight Coupling of Metabolic Oscillations and Intracellular Water Dynamics in Saccharomyces cerevisiae

We detected very strong coupling between the oscillating concentration of ATP and the dynamics of intracellular water during glycolysis in Saccharomyces cerevisiae. Our results indicate that: i) dipolar relaxation of intracellular water is heterogeneous within the cell and different from dilute conditions, ii) water dipolar relaxation oscillates with glycolysis and in phase with ATP concentration, iii) this phenomenon is scale-invariant from the subcellular to the ensemble of synchronized cells and, iv) the periodicity of both glycolytic oscillations and dipolar relaxation are equally affected by D₂O in a dose-dependent manner. These results offer a new insight into the coupling of an emergent intensive physicochemical property of the cell, i.e. cell-wide water dipolar relaxation, and a central metabolite (ATP) produced by a robustly oscillating metabolic process.     

Electrical capacitance of lipid bilayer membranes of hydrogenated egg lecithin at the temperature phase transition

Electrical capacitance of the planar bilayer lipid membrane (BLM) formed from hydrogenated egg lecithin (HEL) has been studied during many passages through the phase transition temperature. In contrast to the BLM from individual synthetic phospholipids, membranes from HEL did not demonstrate any capacitance change at the phase transition temperature maximum, as measured by differential scanning calorimeter at 52 degrees C. Instead, two temperatures have been discerned by capacitance records: thickening at 42-43 degrees C and thinning at 57-59 degrees C. The first temperature region is close to the transition temperature of dipalmitoyllecithin, whereas the second is close to that of distearoyllecithin, two main components of the HEL. It was suggested that capacitance measurements were able to reveal a phase separation in the BLM from HEL which was not detected by differential scanning calorimetry. The phase transition of the BLM from the liquid crystal state to the gel state is followed by thickening of the bilayer structure, partly due to a gauche- trans transition of lipid molecules but mainly due to redistribution of the solvent n-decane.     

Dipolar NMR relaxation of nonprotonated aromatic carbons in proteins. Structural and dynamical effects.

The crystal structure and a 96-ps molecular dynamics simulation used to analyze structural and motional contributions to spin-lattice (T1) relaxation times of phenylalanine and tyrosine C gamma carbons of the pancreatic trypsin inhibitor. The H beta and H delta protons geminal to C gamma are calculated to account for approximately 80% of the dipolar relaxation for each residue. Experimental T1 values for the phenylalanine residues obtained at 25 MHz are observed to be 15-25% longer than estimates based on the rigid crystal structure. It is shown how an increase in T1 can be related to order parameters for the picosecond motional averaging of the important C,H dipolar interactions, and how these order parameters can be calculated from a protein molecular dynamics trajectory.     

The binary phase diagram of lecithin and cholesteryl linolenate

The condensed binary phase diagram of cholesteryl linolenate-egg yolk lecithin has been determined by polarizing light microscopy, differential scanning calorimetry and X-ray diffraction. On increasing the temperature lecithin forms rectangular, cubic and hexagonal liquid-crystalline structures into which varying amounts of cholesteryl linolenate are incorporated. As more cholesteryl linolenate is incorporated, the transition temperatures between different phases are lowered. The rectangular and cubic structures incorporate only small amounts of cholesteryl linolenate; the molar ratios, lecithin to cholesteryl linolenate, being 11:1 and 16:1, respectively. However, the hexagonal phase, in which the phosphorylcholine groups of the lecithin molecules form the core of the rod-like assembly of molecules, incorporates up to approximately 25% cholesteryl linolenate by weight, corresponding to a molar ratio 3:1. At higher concentrations, cholesteryl linolenate forms an excess phase and may be present as crystals, smectic or cholesteric liquid crystals, or as liquid oil, depending on the temperature. At higher temperatures, a large zone of a single isotropic liquid phase exists in which large amounts of lecithin are solubilized by the cholesterol ester. Up to 40% cholesteryl linolenate by weight, the transition temperatures between different phases are influenced by approximately 1% water (by weight) associated with egg lecithin.It is probable that the incorporated apolar cholesterol ester molecules are associated primarily with the apolar hydrocarbon chain region of the different lecithin structures. The resultant decrease in the observed transition temperatures would suggest an overall chain-disordering role for the incorporated cholesteryl linolenate molecules. The influence of cholesteryl linolenate on the thermodynamic stability of the different lecithin structures, together with the models suggested for the molecular orientations of cholesterol esters in the different liquid crystalline structures, may be relevant to the role of these lipids in more complex biological systems, particularly serum lipoproteins.     

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.