Quantitative determination of conformational disorder in the acyl chains of phospholipid bilayers by infrared spectroscopy

On the relationship between the order parameter and the shape of orientation distributions

The molecular orientation is generally expressed by an "order parameter," [Formula: see text] which depends on both the angular position and the shape of the orientation distribution. This parameter is an average made over all orientations of the structural units studied in a sample and, consequently, a given [Formula: see text] value can correspond to different orientation distributions. In this article, model distributions are used to show the relationship between the shape, width, and angular position of the center of the orientation distribution on the [Formula: see text] coefficient, for the case where the distribution of the molecular chains exhibits cylindrical symmetry with respect to the reference direction. A significant difference is observed between the order parameters calculated for distributions of Gaussian and Lorentzian shapes with similar width at half-height. The variation of the [Formula: see text] coefficient as a function of the width at half-height, W1/2, and of the position of the center of the distribution, θC, is analyzed. Figures showing the range of W1/2–θC coordinates that can correspond to a given [Formula: see text] value are presented. As an example, the influence on the order parameter of the disorder between the different domains of phospholipid samples (mosaic spread) and of the conformational disorder in the acyl chains of these molecules is also studied. This example permits the evaluation of the magnitude of the errors that can be introduced in calculations of the tilt angle of the molecular chains in the case of distributions of finite widths or of bimodal character. Keywords: orientation, orientation function, phospholipid bilayers, conformational disorder, mosaic spread.

Hydration dependence of chain dynamics and local diffusion in L-alpha-dipalmitoylphosphtidylcholine multilayers studied by incoherent quasi-elastic neutron scattering

Incoherent quasi-elastic neutron scattering is applied to study the local diffusion and chain dynamics of L-alpha-diplamiotylphosphatidylcholine molecules in oriented model membranes. Different motions are distinguished by changing the hydration of the multilayers as well as by measuring below and above the gel-to-liquid crystalline phase transition. The time range of the utilized time-of-flight spectrometer permits to observe two types of motion to be observed more closely: chain defect motions and the local diffusion of the whole molecule in its solvation cage. Oriented lipid membranes are a useful system for the observation of chain defects, as they can be macroscopically oriented, in contrast to most polymers. As a representative model for a chain defect a kink is chosen and the corresponding scattering functions are derived. The kink motion can explain the entire dynamics seen in the gel phase, and the lifetime of such a defect was found to be 10-15 ps, in good agreement with theoretical predictions. On the other hand the dynamics in the liquid crystalline phase cannot be explained even by a superposition of several kinks and thus requires the consideration of an additional motion: the local diffusion of the molecule in its solvation cage. The size of the solvation cage is increasing with multilayer hydration and reduced temperature. Particularly interesting in view of recent discussions about the origin of the short-range repulsive forces between membranes is the experimental finding of an out-of-plane motion with an amplitude of 1-1.5 A, which cannot be explained by the undulation of the whole membrane.

Calorimetric and theoretical studies of the effects of lindane on lipid bilayers of different acyl chain length

The effects of the insecticide lindane on the phase transition in multilamellar bilayers of saturated diacylphosphatidylcholines of different acyl chain length (DC14PC, DC16PC, and DC18PC) have been studied by means of differential scanning calorimetry (DSC), as well as computer-simulation calculations on a molecular interaction model. The calorimetric data show that increasing concentrations of lindane lower the transition temperature and lead to a broadening of the specific heat in a systematic way depending on the lipid acyl chain length. Kinetic effects in the observed calorimetric traces indicate that the incorporation of lindane into multilamellar lipid bilayers is slow, but faster for the shorter lipid species. Large unilamellar vesicles do not show such kinetic effects. The transition enthalpy is for all three lipid species found to be independent of the lindane concentration which implies that the entropy of mixing is vanishingly small. This lends support to a microscopic molecular interaction model which assigns the absorbed lindane molecules to interstitial sites in the bilayer. Computer-simulation calculations on this model, which assumes a specific interaction between lindane and certain excited acyl chain configurations, lead to predictions of the lipid-water partition coefficient in qualitative agreement with experimental measurements (Antunes-Madeira and Madeira (1985) Biochim. Biophys. Acta 820, 165-172). The partition coefficient has a peak near the phase transition which is a consequence of enhanced interfacial adsorption of lindane at lipid-domain interfaces.

Head group and chain behavior in biological membranes: a molecular dynamics computer simulation

A computer-modeled hydrated bilayer model of the lipid 2,3-dimyristoyl-D-glycero-1-phosphorylcholine in the L alpha phase was built. Particular care was taken in building the starting structure with the inclusion of structural detail reported in experiments on the L alpha phase. Molecular dynamics simulations using the molecular dynamics and energy refinement program AMBER 3.1 force field with an optimized parameters for liquid simulation parameter set were run to study the motions and conformations of the lipid molecules and characterize the behavior and structure of the head groups and the hydrocarbon lipid chains. Although the head groups were observed to show great flexibility, certain head-group torsion combinations appeared favored. The observed tilt of the lipid chains is discussed and is consistent with previous experimental findings. Motion of the lipid chains is shown to be correlated with those chains immediately surrounding, but correlation with chains more distant varies with time.

Thermotropic phase properties of 1,2-di-O-tetradecyl-3-O-(3-O-methyl- beta-D-glucopyranosyl)-sn-glycerol

The hydration properties and the phase structure of 1,2-di-O-tetradecyl-3-O(3-O-methyl-beta-D-glucopyranosyl)-sn-glycerol (3-O-Me-beta-D-GlcDAIG) in water have been studied via differential scanning calorimetry, 1H-NMR and 2H-NMR spectroscopy, and x-ray diffraction. Results indicate that this lipid forms a crystalline (Lc) phase up to temperatures of 60-70 degrees C, where a transition through a metastable reversed hexagonal (Hll) phase to a reversed micellar solution (L2) phase occurs. Experiments were carried out at water concentrations in a range from 0 to 35 wt%, which indicate that all phases are poorly hydrated, taking up < 5 mol water/mol lipid. The absence of a lamellar liquid crystalline (L alpha) phase and the low levels of hydration measured in the discernible phases suggest that the methylation of the saccharide moiety alters the hydrogen bonding properties of the headgroup in such a way that the 3-O-Me-beta-D-GlcDAIG headgroup cannot achieve the same level of hydration as the unmethylated form. Thus, in spite of the small increase in steric bulk resulting from methylation, there is an increase in the tendency of 3-O-Me-beta-D-GlcDAIG to form nonlamellar structures. A similar phase behavior has previously been observed for the Acholeplasma laidlawii A membrane lipid 1,2-diacyl-3-O-(6-O-acyl-alpha-D-glucopyranosyl)-sn-glycerol in water (Lindblom et al. 1993. J. Biol. Chem. 268:16198-16207). The phase behavior of the two lipids suggests that hydrophobic substitution of a hydroxyl group in the sugar ring of the glucopyranosylglycerols has a very strong effect on their physicochemical properties, i.e., headgroup hydration and the formation of different lipid aggregate structures.

Water transport across biological membranes

The rate of the lateral diffusion of straight-chain phospholipids predicts the rate of water diffusion through bilayers. A new model of lipid dynamics integrates these processes. Substances such as cholesterol that reduce water diffusion proportionally reduce lateral diffusion. The model yields a number of predictions about the dynamics of the lipids at the Tm and suggests different mechanisms for how water diffuses across bilayers of other-than-straight-chain lipids, and how proteins bind to membranes. A second recent development in water transport across biological membranes is the discovery of a ubiquitous family of water transport proteins that facilitate large-volume water translocation. Like water diffusion through lipid bilayers, water transport by these proteins is directed by osmosis and is therefore under the control of ATP and ion pumps. The presence of water transport proteins in membranes is often regulated by hormones.

Molecular dynamics simulation of a hydrated phospholipid bilayer

A hydrated bilayer of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) has been studied in the course of a molecular dynamics simulation. Comparison of the simulation results with experiment indicates that generally the two agree well. Data are presented concerning all the major system regions, including the hydrocarbon chains, the glycerol region, the lipid headgroups and the hydrating water molecules. The simulations suggest that this model can be extended to the study of more complex systems of greater biochemical interest, such as membrane bound proteins.

An electron spin resonance study of interactions between gramicidin A' and phosphatidylcholine bilayers

The model of microscopic order and macroscopic disorder was used to stimulate electron spin resonance spectra of spin-labeled lipids, 5-PC, 10-PC, and 16-PC in multilamellar vesicles of dipalmitoylphosphatidylcholine (DPPC) containing gramicidin A' (GA) at temperatures above the gel-to-liquid crystal transition of DPPC. The simulations show that at a lower concentration of GA (i.e., molar ratios of DPPC/GA greater than 3), GA has only a slight effect on the acyl chain dynamics. The rotational diffusion rate around the axis parallel to the long hydrocarbon chain remains unchanged or increases slightly, while the rate around the perpendicular axes decreases slightly. These spectra from DPPC/GA mixtures could only be fit successfully with two or more components consistent with the well-known concept of "boundary lipids," that is, the peptide induces structural inhomogeneity in lipid bilayers. However, the spectra were significantly better fit with additional components that exhibit increased local ordering, implying decreased amplitude of rotational motion, rather than immobilized components with sharply a reduced rotational rate. The largest relative effects occur at the end of the acyl chains, where the average local order parameter St of 16-PC increases from 0.06 for pure lipid to 0.66 for 1:1 DPPC/GA. The inhomogeneity in ordering in DPPC bilayers due to GA decreases with increasing temperature. The hyperfine tensor component Azz increases for 10-PC and 16-PC when GA is incorporated into DPPC bilayers, indicating that water has deeply penetrated into the DPPC bilayers. Simulations of published electron spin resonance spectra of 14-PC in dimyristoylphosphatidylcholine/cytochrome oxidase complexes were also better fit by additional components that were more ordered, rather than immobilized. The average local order parameter in this case is found to increase from 0.11 for pure dimyristoylphosphatidylcholine to 0.61 for a lipid/protein ratio of 50. These spectra and their simulations are similar to the results obtained with 16-PC in the DPPC/GA mixtures. The relevance to studies of lipid-protein interactions for other proteins is briefly discussed.

Fourier transform infrared spectroscopy characterization of the lamellar and nonlamellar structures of free lipid A and Re lipopolysaccharides from Salmonella minnesota and Escherichia coli

The structural polymorphism of free lipid A and deep rough mutant lipopolysaccharide (LPS Re) from Salmonella minnesota strain R595 and Escherichia coli strain F515 was characterized by Fourier transform infrared (IR) spectroscopy. For this, the beta <--> alpha phase states and the three-dimensional supramolecular structures, the latter deduced from small-angle synchrotron radiation x-ray diffraction, were investigated at different water contents, Mg2+ concentrations, and temperatures. The analysis of the IR data for vibrations originating from the hydrophobic moiety shows that the beta <--> alpha acyl chain melting is strongly expressed only for the stretching and scissoring modes of the methylene groups. Vibrational groups originating from the interface region sense the acyl chain melting well (ester carbonyl bands) or only weakly (amide bands), and those resulting from the pure polar moiety not at all. From the x-ray data, the existence of lamellar (L), different cubic, and, for lipid A and LPS R595, also inverted hexagonal (HII) structures could be proven in the temperature range 20-80 degrees C with cubic <--> cubic and cubic <--> HII transitions for the Mg(2+)-free and L <--> HII transitions for the Mg(2+)-containing samples. These structural transitions can be characterized most readily by specific changes of the vibrational bands resulting from the interface region: the ester carbonyl and the amide bands. The magnitude of the changes corresponds to that of the structural rearrangement, i.e., is highest for the L <--> HII, lower for the cubic <--> HII, and lowest for the cubic <--> cubic transitions. The structural transitions are only marginally expressed for vibrational bands of the hydrophobic moiety. Similarly, the band contours of vibrations from the hydrophilic region are no indicators of the structural reorientations except for the carboxylate bands of LPS Re. Particularly the stretching vibrations of the phosphate groups are nearly completely invariant; the absolute values of their half bandwidths, however, differ significantly for lipid A and LPS Re, which seems to be of biological relevance. The ability of IR spectroscopy to detect supramolecular changes also beyond the measurability by x-ray diffraction, i.e., at water contents > 95 to 99.5%, is demonstrated.

Physical studies of lipid organization and dynamics in mycoplasma membranes

It should be clear from this summary that we currently know a great deal about the organization and dynamics of the lipids in mycoplasma membranes in general, and in the cell membrane of A. laidlawii in particular. In fact, research on mycoplasma membranes has been important in unambiguously establishing the fundamental lipid bilayer structure of all biological membranes and in elucidating some of the major properties of bilayers in biomembranes, such as their thermotropic phase behavior and interactions with cholesterol and membrane proteins. Although a great deal has been learned, a number of issues have not been fully resolved. In particular, the concept of membrane lipid fluidity must be refined and quantitated, and the relationship between orientational order and rates of motion better understood. This will require that the apparent discrepancies between some of the results obtained, for example, by the various spectroscopic techniques, be resolved. In particular, the nature of the boundary lipid surrounding integral membrane proteins will require further study, as will the question of the specificity of lipid-protein interactions. Also, accurate quantitative measurements for the lateral and rotational mobilities of the various lipid components in the mycoplasma membranes have not yet been made. Although not reviewed in this chapter, the related questions of the in vivo rate of phospholipid, glycolipid, and cholesterol transverse diffusion (flip-flop), and the possible asymmetric transbilayer distribution of these components in mycoplasma membranes, are still not well understood. Although much remains to be done, particularly with respect to our understanding of protein structure and function in mycoplasma membranes, a solid basis for further advances has now been laid. The many natural advantages of mycoplasma for biochemical and biophysical investigations of membrane structure and function should continue to make these organisms very useful for membrane studies for years to come.

Comparison between orientational and conformational orders in fluid lipid bilayers

The orientational order as determined by 2H NMR and the infrared frequencies of the C--H stretching modes of the methylene groups have been measured for several systems (POPC, POPC/cholesterol and POPE), all in the fluid phase, and then were compared; this work reveals an unexpected linear correlation between them. This experimental result shows that both measurements are essentially sensitive to a common motion, most likely trans/gauche isomerisation. This new correlation with those already found in the literature suggest that several measurements related to the hydrophobic core of the fluid bilayer describe different aspects of a universal behavior. The correlation presented here does not extend to the lipid in gel phase where slower motions affect the NMR lineshape.

Acyl chain conformational ordering in 1,2 dipalmitoylphosphatidylethanolamine. Integration of FT-IR and 2H NMR results

The extent of trans-gauche isomerization at the 4 and 4' positions of the acyl chains of fully hydrated 4,4,4',4'-d4 1,2-dipalmitoylphosphatidylethanolamine (4-d4 DPPE) bilayers was quantitatively evaluated from the infrared (IR) intensity of the CD2 rocking modes. About 20% gauche conformers were observed at 72 degrees C (above Tm), while at 23 degrees C, well below Tm, about 4% were noted. The order parameter SC-D was determined from 2H nuclear magnetic resonance (NMR) quadrupolar splittings. SC-D is the product of a segmental order parameter (S gamma), which depends on conformational order, and a chain order parameter (S alpha) which depends on slower motions such as chain wobble. The IR-determined percentage of gauche forms was converted into a segmental order parameter and factored out of the measured value for SC-D to yield an estimate of S alpha = 0.59 for L alpha phase DPPE. A comparison with S alpha for 1,2-dipalmitoylphosphatidylcholine (DPPC) suggests that increased wobble is responsible for enhanced motional averaging of the quadrupolar splittings in the latter at a similar reduced temperature. The extent of conformational disordering [at the 4(4') position] is essentially unchanged between the two molecules. The current study demonstrates the advantage of integrating quantitative IR with 2H NMR data, for elucidation of the contributions of the individual motions that average the NMR quadrupolar splittings.

CD2 rocking modes as quantitative infrared probes of one-, two-, and three-bond conformational disorder in dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine/cholesterol mixtures

The use of CD2 rocking modes in the IR spectrum as quantitative probes of phospholipid conformational disorder has recently been described for aqueous dispersions of 1,2-dipalmitoylphosphatidylcholine (DPPC) and DPPC/cholesterol mixtures [Mendelsohn et al. (1989) Biochemistry 28, 8934-8939; Davies et al. (1990) Biochemistry 29, 4368-4373]. Initial studies focused at the 4, 6, and 10 acyl chain positions of DPPC. In the current work, the method is extended to the 2, 3, 12, and 13 positions. Conformational disorder in the L alpha phase is approximately the same (about 20% gauche) at positions 4, 10, and 13, but an unexpected higher value is observed (about 30%) at the 6 position. Cholesterol (33 mol%) restricts gauche rotamer formation by factors ranging from 6 to 9 at positions 4 and 6, respectively, to 1.5-2 at positions 10, 12, and 13. Quantitative analysis for the DPPC/cholesterol "liquid-ordered" phase indicates the occurrence of 1.2 gauche bonds/chain, a marked reduction from the 3.6-4.2 gauche bonds/chain for DPPC alone. Proximity to the ester moiety at acyl chain position 3 perturbs the vibrational coupling patterns of the CD2 rocking modes and eliminates their sensitivity to conformational change. In addition, the feasibility of a method based on the conformation-dependent coupling between CD2 rocking frequencies of two successive CD2 groups for the quantitative detection of specific, position-dependent king (gtg') and isolated gauche (gtt) conformers is demonstrated. Finally, comparisons between IR measurements and explicit theoretical predictions of acyl chain conformational order are presented.

Physical properties of the fluid lipid-bilayer component of cell membranes: a perspective

The motivation for this review arises from the conviction that, as a result of the mass of experimental data and observations collected in recent years, the study of the physical properties of membranes is now entering a new stage of development. More and more, experiments are being designed to answer specific, detailed questions about membranes which will lead to a quantitative understanding of the way in which the physical properties of membranes are related to and influence their biological function.

Phospholipid phase transitions in model and biological membranes as studied by infrared spectroscopy

Fourier transform infrared (FT-IR) spectroscopy is an extremely powerful yet non-perturbing physical technique for monitoring the conformation and dynamics of all portions of the phospholipid molecule. In this brief review we summarize some recent FT-IR spectroscopic studies of phospholipid phase transitions in model lipid bilayer and in biological membranes which illustrate the great utility of this technique. We show that FT-IR spectroscopy can accurately monitor the gel to liquid-crystalline phase transition and can provide a large amount of detailed information about phospholipid structure and organization in both the gel and liquid-crystalline states of lipid bilayers.

Nonbilayer phases of membrane lipids

Numerous liquid crystalline biomembrane lipids are known to exhibit non-lamellar phases characterized by curvature of their component lipid monolayers. An understanding of the phase stability of these systems begins with analysis of the energy of bending the monolayers, the interactions which lead to the bending energy, and the geometrical constraints which lead to competing energy terms which arise when the monolayers are bent and packed onto lattices with different structures. Diffraction and other techniques suitable for probing lipid phase structure are described. A phenomenological model is reviewed which successfully explains many of the qualitative features of lipid mesomorphic phase behavior. A key result of this model is that lipid bilayer compositions which are close to the non-lamellar phase boundaries of their phase diagrams are characterized by a frustrated elastic stress which may modulate the activity of imbedded membrane proteins and which may provide a rationale for the prevalence of non-lamellar-tending lipid species in biomembrane bilayers. Areas in need of future research are discussed.

Mean field stochastic boundary molecular dynamics simulation of a phospholipid in a membrane

Computer simulations of phospholipid membranes have been carried out by using a combined approach of molecular and stochastic dynamics and a mean field based on the Marcelja model. First, the single-chain mean field simulations of Pastor et al. [(1988) J. Chem. Phys. 89, 1112-1127] were extended to a complete dipalmitoylphosphatidylcholine molecule; a 102-ns Langevin dynamics simulation is presented and compared with experiment. Subsequently, a hexagonally packed seven-lipid array was simulated with Langevin dynamics and a mean field at the boundary and with molecular dynamics (and no mean field) in the center. This hybrid method, mean field stochastic boundary molecular dynamics, reduces bias introduced by the mean field and eliminates the need for periodic boundary conditions. As a result, simulations extending to tens of nanoseconds may be carried out by using a relatively small number of molecules to model the membrane environment. Preliminary results of a 20-ns simulation are reported here. A wide range of motions, including overall reorientation with a nanosecond decay time, is observed in both simulations, and good agreement with NMR, IR, and neutron diffraction data is found.

Quantitative determination of hydrocarbon chain conformational order in bilayers of saturated phosphatidylcholines of various chain lengths by Fourier transform infrared spectroscopy

The infrared spectra of aqueous dispersions of a homologous series of symmetric-chain, disaturated phosphatidylcholines, with fatty acyl chain lengths ranging from 12 to 19 carbons, have been measured at comparable reduced temperatures in their liquid-crystalline phases. The infrared spectra of these compounds contain bands that are dependent on the conformation of the fatty acyl chains. In particular, in the 1400-1300-cm-1 spectral region, there are bands due to CH2 wagging which are specific for the different types of gauche conformers. Thus, gauche-trans-gauché sequences (or kinks) give a band at 1367 cm-1, end-gauche conformers a band at 1341 cm-1, and double-gauche conformers a band at 1355 cm-1. The intensities of these bands were determined and normalized to the intensity of the conformation-insensitive band due to symmetric methyl bending at 1378 cm-1. The intensities of the different "gauche" bands yield a "per chain" intensity, which is directly related to the concentration of the different types of conformational defects. We find that, within experimental error, the concentration of end-gauche and double-gauche conformers is relatively low and practically invariant with chain length when a series of homologous phosphatidylcholines are compared at the same reduced temperature. In contrast, the concentration of gauche-trans-gauché sequences (kink defects) is much higher and increases as the chain length increases. For dipalmitoylphosphatidylcholine we find that there are about 1.2 kink, 0.5-0.6 end-gauche, and 0.4 double-gauche conformers per hydrocarbon chain.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative infrared determination of acyl chain conformation in gramicidin/dipalmitoylphosphatidylcholine mixtures

A quantitative infrared characterization of phospholipid acyl chain disordering in 6,6,6'6'-d4 dipalmitoylphosphatidylcholine/ Gramicidin D bilayers has been made. Three CD2 rocking modes, at 622 cm-1, 646-649 cm-1, and 651-653 cm-1, assigned to particular conformers, were used to determine disorder in the presence of peptide, as well as percentages of particular classes of conformer within the total gauche population. At 44C, the gauche percentages in 10:1 and 30:1 lipid/peptide mixtures were 15% and 17%, respectively. At 34C, the corresponding values were 9.8% and 2.6%. The percentage of (single gauche bend + kink) conformers, relative to multiple gauche forms, decreases dramatically from 78% in the 30:1 mixture to 15% in the 10:1 mixture at 44C. These data provide the first quantitative measure of the extent to which a membrane-spanning peptide disorders phospholipid gel phases and orders liquid crystal phases.