Raman spectroscopic study of an interdigitated lipid bilayer. Dipalmitoylphosphatidylcholine dispersed in glycerol

Effect of polyols on membrane structures of liposomes: A study using small-angle X-ray scattering data and generalized indirect Fourier transformation

This study aimed to evaluate the membrane structure of distearoylphosphatidylcholine (DSPC) liposomes dispersed in water containing various types of polyols with low molecular weight such as glycerin (Gly), 1,3-butandiol (BG), and propylene glycol (PG). To clarify the detailed membrane structure, generalized indirect Fourier transformation (GIFT) analysis, which provides information about the bilayer spacing, bilayer thickness, number of lamellar layers, and membrane flexibility, was applied to small-angle X-ray scattering (SAXS) data of the present system. The GIFT results showed that the bilayer thickness of the DSPC liposomes followed the order Gly>>BG>PG. In addition, the membrane flexibility estimated by the Caille parameter was in the order Gly>>BG>PG; this result was supported by the gel-liquid crystal phase transition temperature (T_c) obtained by differential scanning calorimetry (DSC). These results, together with the Raman spectra, suggest that BG and PG incorporated into the bilayers of DSPC liposomes result in the formation of an interdigitated lamellar structure.

Structure and Interaction of Ceramide-Containing Liposomes with Gold Nanoparticles as Characterized by SERS and Cryo-EM

Due to the great potential of surface-enhanced Raman scattering (SERS) as local vibrational probe of lipid-nanostructure interaction in lipid bilayers, it is important to characterize these interactions in detail. The interpretation of SERS data of lipids in living cells requires an understanding of how the molecules interact with gold nanostructures and how intermolecular interactions influence the proximity and contact between lipids and nanoparticles. Ceramide, a sphingolipid that acts as important structural component and regulator of biological function, therefore of interest to probing, lacks a phosphocholine head group that is common to many lipids used in liposome models. SERS spectra of liposomes of a mixture of ceramide, phosphatidic acid, and phosphatidylcholine, as well as of pure ceramide and of the phospholipid mixture are reported. Distinct groups of SERS spectra represent varied contributions of the choline, sphingosine, and phosphate head groups and the structures of the acyl chains. Spectral bands related to the state of order of the membrane and moreover to the amide function of the sphingosine head groups indicate that the gold nanoparticles interact with molecules involved in different intermolecular relations. While cryogenic electron microscopy shows the formation of bilayer liposomes in all preparations, pure ceramide was found to also form supramolecular, concentric stacked and densely packed lamellar, nonliposomal structures. That the formation of such supramolecular assemblies supports the intermolecular interactions of ceramide is indicated by the SERS data. The unique spectral features that are assigned to the ceramide-containing lipid model systems here enable an identification of these molecules in biological systems and allow us to obtain information on their structure and interaction by SERS.

Two-Dimensional Titanium Dioxide-Surfactant Photoactive Supramolecular Networks: Synthesis, Properties, and Applications for the Conversion of Light Energy

The desire to harness solar energy to address current global environmental problems led us to investigate two-dimensional (2D) core-shell hybrid photocatalysts in the form of a 2D-TiO2-surfactant, mainly composed of fatty acids. The bulk products, prepared by two slightly different methods, consist of stacked host-guest hybrid sheets held together by van der Waals forces between alkyl carboxylate moieties, favoring the synergistic conjugation of the photophysical properties of the core and the hydrophobicity of the self-assembled surfactant monolayer of the shell. X-ray diffraction and the vibrational characteristics of the products revealed the influence of synthesis strategies on two types of supramolecular aggregates that differ in the core chemical structure, guest conformers of alkyl surfactant tails and type, and the bilayer and monolayer of the structure of nanocomposites. The singular ability of the TiO2 core to anchor carboxylate leads to commensurate hybrids, in contrast to both layered clay and layered double-hydroxide-based ion exchangers which have been previously reported, making them potentially interesting for modeling the role of fatty acids and lipids in bio-systems. The optical properties and photocatalytic activity of the products, mainly in composites with smaller bandgap semiconductors, are qualitatively similar to those of nanostructured TiO2 but improve their photoresponse due to bandgap shifts and the extreme aspect-ratio characteristics of two-dimensional TiO2 confinement. These results could be seen as a proof-of-concept of the potential of these materials to create custom-designed 2D-TiO2-surfactant supramolecular photocatalysts.

Glycerol Solvates DPPC Headgroups and Localizes in the Interfacial Regions of Model Pulmonary Interfaces Altering Bilayer Structure

The inclusion of glycerol in formulations for pulmonary drug delivery may affect the bioavailability of inhaled steroids by retarding their transport across the lung epithelium. The aim of this study was to evaluate whether the molecular interactions of glycerol with model pulmonary interfaces provide a biophysical basis for glycerol modifying inhaled drug transport. Dipalmitoylphosphatidylcholine (DPPC) monolayers and liposomes were used as model pulmonary interfaces, in order to examine the effects of bulk glycerol (0-30% w/w) on their structures and dynamics using complementary biophysical measurements and molecular dynamics (MD) simulations. Glycerol was found to preferentially interact with the carbonyl groups in the interfacial region of DPPC and with phosphate and choline in the headgroup, thus causing an increase in the size of the headgroup solvation shell, as evidenced by an expansion of DPPC monolayers (molecular area increased from 52 to 68 Ų) and bilayers seen in both Langmuir isotherms and MD simulations. Both small angle neutron scattering and MD simulations indicated a reduction in gel phase DPPC bilayer thickness by ∼3 Å in 30% w/w glycerol, a phenomenon consistent with the observation from FTIR data, that glycerol caused the lipid headgroup to remain oriented parallel to the membrane plane in contrast to its more perpendicular conformation adopted in pure water. Furthermore, FTIR measurements suggested that the terminal methyl groups of the DPPC acyl chains were constrained in the presence of glycerol. This observation is supported by MD simulations, which predict bridging between adjacent DPPC headgroups by glycerol as a possible source of its putative membrane stiffening effect. Collectively, these data indicate that glycerol preferentially solvates DPPC headgroups and localizes in specific areas of the interfacial region, resulting in structural changes to DPPC bilayers which may influence cell permeability to drugs.

Dynamic Nuclear Polarization/Solid-State NMR Spectroscopy of Membrane Polypeptides: Free-Radical Optimization for Matrix-Free Lipid Bilayer Samples

Dynamic nuclear polarization (DNP) boosts the sensitivity of NMR spectroscopy by orders of magnitude and makes investigations previously out of scope possible. For magic-angle-spinning (MAS) solid-state NMR spectroscopy studies, the samples are typically mixed with biradicals dissolved in a glass-forming solvent and are investigated at cryotemperatures. Herein, we present new biradical polarizing agents developed for matrix-free samples such as supported lipid bilayers, which are systems widely used for the investigation of membrane polypeptides of high biomedical importance. A series of 11 biradicals with different structures, geometries, and physicochemical properties were comprehensively tested for DNP performance in lipid bilayers, some of them developed specifically for DNP investigations of membranes. The membrane-anchored biradicals PyPol-C16, AMUPOL-cholesterol, and bTurea-C16 were found to exhibit improved g-tensor alignment, inter-radical distance, and dispersion. Consequently, these biradicals show the highest signal enhancement factors so far obtained for matrix-free membranes or other matrix-free samples and may potentially shorten NMR acquisition times by three orders of magnitude. Furthermore, the optimal biradical-to-lipid ratio, sample deuteration, and membrane lipid composition were determined under static and MAS conditions. To rationalize biradical performance better, DNP enhancement was measured by using the ¹³ C and ¹⁵ N signals of lipids and a peptide as a function of the biradical concentration, DNP build-up time, resonance line width, quenching effect, microwave power, and MAS frequency.

Molecular Rationale for Improved Dynamic Nuclear Polarization of Biomembranes

Dynamic nuclear polarization (DNP) enhanced solid-state NMR can provide orders of magnitude in signal enhancement. One of the most important aspects of obtaining efficient DNP enhancements is the optimization of the paramagnetic polarization agents used. To date, the most utilized polarization agents are nitroxide biradicals. However, the efficiency of these polarization agents is diminished when used with samples other than small molecule model compounds. We recently demonstrated the effectiveness of nitroxide labeled lipids as polarization agents for lipids and a membrane embedded peptide. Here, we systematically characterize, via electron paramagnetic (EPR), the dynamics of and the dipolar couplings between nitroxide labeled lipids under conditions relevant to DNP applications. Complemented by DNP enhanced solid-state NMR measurements at 600 MHz/395 GHz, a molecular rationale for the efficiency of nitroxide labeled lipids as DNP polarization agents is developed. Specifically, optimal DNP enhancements are obtained when the nitroxide moiety is attached to the lipid choline headgroup and local nitroxide concentrations yield an average e(-)-e(-) dipolar coupling of 47 MHz. On the basis of these measurements, we propose a framework for development of DNP polarization agents optimal for membrane protein structure determination.

A method for dynamic nuclear polarization enhancement of membrane proteins

Dynamic nuclear polarization (DNP) magic-angle spinning (MAS) solid-state NMR (ssNMR) spectroscopy has the potential to enhance NMR signals by orders of magnitude and to enable NMR characterization of proteins which are inherently dilute, such as membrane proteins. In this work spin-labeled lipid molecules (SL-lipids), when used as polarizing agents, lead to large and relatively homogeneous DNP enhancements throughout the lipid bilayer and to an embedded lung surfactant mimetic peptide, KL4 . Specifically, DNP MAS ssNMR experiments at 600 MHz/395 GHz on KL4 reconstituted in liposomes containing SL-lipids reveal DNP enhancement values over two times larger for KL4 compared to liposome suspensions containing the biradical TOTAPOL. These findings suggest an alternative sample preparation strategy for DNP MAS ssNMR studies of lipid membranes and integral membrane proteins.

Iodothyronine-phospholipid interactions in the lipid gel phase probed by Raman spectral markers

A better understanding of the structural effects induced by thyroid hormones in model membranes is attained by Raman spectroscopy. The interactions of T3 and T4 with multilamellar vesicles of dipalmytoylphosphatidylcholine (DPPC) in the gel phase are characterized by analyzing the spectral behavior of the C-H and C-C stretching vibrations of the acyl chains. The spectra evidence an increase in the relative number of gauche conformation, which indicates the hormones are able to penetrate into the hydrophobic region of the bilayer and partially alter the lipid structure. In addition, the density packing of the acyl chains appears increased and the rotational mobility of the terminal methylene groups is slightly reduced in the iodothyronine/DPPC mixtures. These effects are interpreted in terms of the transition to an interdigitated phase due to the hormone incorporation to the membrane. The polar heads of the lipids also interact with the hormone, as evidenced by the PO2(-) symmetric stretching band.

Reorientational and conformational ordering processes at elevated pressures in 1,2-dioleoyl phosphatidylcholine: a Raman and infrared spectroscopic study

Raman and infrared spectra of fully hydrated bilayers of 1,2-dioleoyl phosphatidylcholine (DOPC) were measured at increasing hydrostatic pressures up to -37 kbar. Under ambient conditions aqueous dispersions of DOPC are in the liquid crystalline state. The application of an external hydrostatic pressure induces conformational and dynamic ordering processes in DOPC, which trigger a first-order structural phase transition at 5 kbar from a disordered liquid crystalline state to a highly ordered gel state. In the gel phase the methylene chains of each molecule are fully extended and the two all-trans chain segments on both sides of the rigid cis double bond form a bent structure. The bent oleoyl chains in each molecule, as well as in neighboring molecules are packed parallel to each other. To achieve this parallel interchain packing, the double bonds of the sn-1 and sn-2 chains of each molecule must be aligned at the same position with respect to the bilayer interface which is achieved by a rotation of the C-C bonds in the glycerol moiety in the head group. The extremely strong interchain interactions in the gel phase of DOPC are unique for this lipid with cis dimono-unsaturated acyl chains. Our experimental results suggest that in the pressure-induced gel phase of DOPC the olefinic CH bonds are rotated out of the phase of the bent oleoyl chains and that the oleoyl chains of opposing bilayers bend towards opposite directions.

Development of a CP 31P NMR broadline simulation methodology for studying the interactions of antihypertensive AT1 antagonist losartan with phospholipid bilayers

A cross-polarization (CP) (31)P NMR broadline simulation methodology was developed for studying the effects of drugs in phospholipids bilayers. Based on seven-parameter fittings, this methodology provided information concerning the conformational changes and dynamics effects of losartan in the polar region of the dipalmitoylphosphatidylcholine bilayers. The test molecule for this study was losartan, an antihypertensive drug known to exert its effect on AT(1) transmembrane receptors. The results were complemented and compared with those of differential scanning calorimetry, solid-state (13)C NMR spectroscopy, Raman spectroscopy, and electron spin resonance. More specifically, these physical chemical methodologies indicated that the amphipathic losartan molecule interacts with the hydrophilic-head zone of the lipid bilayers. The CP (31)P NMR broadline simulations showed that the lipid molecules in the bilayers containing losartan displayed greater collective tilt compared to the tilt displayed by the load-free bilayers, indicating improved packing. The Raman results displayed a decrease in the trans/gauche ratio and increased intermolecular interactions of the acyl chains in the liquid crystalline phase. Additional evidence, suggesting that losartan possibly anchors in the realm of the headgroup, was derived from upfield shift of the average chemical shift sigma(iso) of the (31)P signal in the presence of losartan and from shift of the observed peak at 715 cm(-1) attributed to C-N stretching in the Raman spectra.

Effect of high surface curvature on the main phase transition of supported phospholipid bilayers on SiO2 nanoparticles

Investigation of the physical properties of highly curved membranes is important in biology, for example, in fusion intermediates, and in pharmaceutical or chromatographic applications, where nanoscale features may affect substrate binding. However, vesicle fusion below 40 nm precludes study of this size regime. In this investigation, the effect of high surface curvature on the adsorption and morphology of phosphotidylcholine lipids with alkyl chain lengths of 14 (DMPC), 16 (DPPC), and 18 (DSPC) onto silica (SiO2) nanobeads was investigated by thermogravimetric analysis (TGA), high sensitivity nanocalorimetry, and vibrational spectroscopy. The SiO2 beads ranged in size from 5 to 100 nm. Stable supported bilayers were formed on all bead sizes by vesicle fusion of the parent MLVs at temperatures above the main phase transition temperature (T(m)) of the lipids. A downward shift in T(m), and a broadening (deltaT1/2) of the transition with respect to the parent MLVs, was observed for the 100 nm beads. With decreasing bead size, T(m) first decreased, but then increased. On the smallest bead size, whose dimensions were comparable to those of the adsorbed lipids, T(m)'s were higher than those of the parent MLVs. The increase in T(m) indicated a stiffening of the supported bilayer, which was confirmed by Raman spectroscopic data. Narrowing of the phase transition or the appearance of peak doublets occurred at the smaller bead sizes. The results were consistent with a model in which the high free volume and increased outer headgroup spacing of lipids on highly curved surfaces induced interdigitation in the supported lipids.

The phase transition behavior of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) model membrane influenced by 2,4-dichlorophenol—an FT-Raman Spectroscopy Study

The effect of 2,4-dichlorophenol (DCP) on the structures and phase transitions of fully hydrated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) liposomes was studied using FT-Raman spectroscopy. Whereas the Raman frequency shifts of the most frequently investigated bands of C-C and C-H stretching regions only indicate the main phase transition (P(beta')-L(alpha)) of the pure DPPC/water system, the Raman shift of C-H scissoring vibration at 1440 cm(-1) was found to be able to reveal the pretransition (L(beta')-P(beta')) as well. Analyzing the spectral parameters of the trans band at 1128 cm(-1), which does not overlap with DCP vibrational modes, a continuous decrease of trans conformations was found with increasing DCP concentration at 26 degrees C accompanying the phase transitions L(beta')-P(beta') and P(beta')-L(alpha). The intensity ratio of the symmetrical and asymmetrical methylene stretching bands (at 2850 cm(-1) and 2880 cm(-1)), defined as the disorder parameter by Levin [Levin, I.W., 1985. Two types of hydrocarbon chain interdigitation in sphingomielin bilayers. Biochemistry 24, 6282-6286], indicated that in the interdigitated phase (L(I)) the order is markedly high and comparable with that of L(beta). Both the phase transition P(beta')-L(alpha) in the DCP/DPPC molar ratio range of 10/100-50/100 and the phase transition L(I)-L(alpha) led to a significant increase of disordered chains and the presence of DCP molecules induced a more disordered chain region than that observed in the L(alpha) phase of DPPC. Nevertheless, it was found that the L(alpha) phase with DCP contains approximately the same amount of trans conformers than that without DCP.

Packing characteristics of two-component bilayers composed of ester- and ether-linked phospholipids

The miscibility properties of ether- and ester-linked phospholipids in two-component, fully hydrated bilayers have been studied by differential scanning calorimetry (DSC) and Raman spectroscopy. Mixtures of 1,2-di-O-hexadecyl-rac-glycero-3-phosphocholine (DHPC) with 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DHPE) and of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) with 1,2-di-O-hexadecyl-sn-glycero-3-phosphoethanolamine (DHPE) have been investigated. The phase diagram for the DPPC/DHPE mixtures indicates that these two phospholipids are miscible in all proportions in the nonrippled bilayer gel phase. In contrast, the DHPC/DPPE mixtures display two regions of gel phase immiscibility between 10 and 30 mol% DPPE. Raman spectroscopic measurements of DHPC/DPPE mixtures in the C-H stretching mode region suggest that this immiscibility arises from the formation of DHPC-rich interdigitated gel phase domains with strong lateral chain packing interactions at temperatures below 27 degrees C. However, in the absence of interdigitation, our findings, and those of others, lead to the conclusion that the miscibility properties of mixtures of ether- and ester-linked phospholipids are determined by the nature of the phospholipid headgroups and are independent of the character of the hydrocarbon chain linkages. Thus it seems unlikely that the ether linkage has any significant effect on the miscibility properties of phospholipids in biological membranes.

Phase behaviour of distearoylphosphatidylethanolamine in glycerol--a thermal and X-ray diffraction study

The phase behaviour of 1,2-distearoylphosphatidylethanolamine in glycerol has been examined using differential scanning calorimetry and real-time synchrotron X-ray diffraction methods. Dry phospholipid and phospholipid dispersed in glycerol over the concentration range 2.4%-90% (w/w) was equilibrated for 30 min at 20 degrees C and thermal and structural parameters on the temperature range 60 degrees C to 110 degrees C recorded during an initial heating and subsequent reheating. The characteristic feature of the initial heating scan was a direct lamellar crystalline to inverted hexagonal phase transition. In the subsequent cooling scan a lamellar gel structure was formed from the non-lamellar phase which transformed, on reheating, to a lamellar crystalline phase in which the acyl chain packing was titled with respect to the bilayer plane. The mechanism of the formation of the two crystalline phases was examined in the context of a relaxation model, where the liquid-crystal phase below the transition temperature from lamellar crystalline phase is metastable. A binary phase diagram over the temperature range 60 degrees C to 110 degrees C has been constructed.

Effect of ethanol-induced lipid interdigitation on the membrane solubility of Prodan, Acdan, and Laurdan

The effect of ethanol-induced lipid interdigitation on the partition coefficient (Kp) of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) and its two derivatives, 6-acetyl-2-(dimethylamino)naphthalene (Acdan) and 6-lauroyl-2-(dimethylamino)naphthalene (Laurdan), in L-alpha-dipalmitoylphosphatidylcholine (DPPC) vesicles has been examined by a precipitation method over the ethanol concentration range of 0-1.8 M. At 20 degrees C and in the absence of ethanol, the Kp values for Acdan, Prodan, and Laurdan are 2.0 x 10(3), 2.8 x 10(4), and 4.7 x 10(6), respectively. This result suggests that the Kp of Prodan and its derivatives is not simply a linear function of the polymethylene units. As DPPC undergoes the ethanol-induced phase transition from the noninterdigitated to the fully interdigitated gel state, Kp for Prodan and Acdan decreases by a factor of 5 and 2, respectively, whereas Kp for Laurdan exhibits no detectable changes with ethanol. The differences in Kp are in parallel with the differences in the fluorescence emission spectra of these probes over the ethanol concentration range examined. Previous fluorescence and infrared data indicated that membrane perturbation caused by the probes increases in the order: Laurdan > Prodan > Acdan. Thus, the degree of membrane perturbation also seems to be in parallel with Kp. Among these three probes, Prodan fluorescence reflects most correctly the ethanol-induced lipid interdigitation. In conclusion, the partitioning of small solutes in lipid membranes is significantly reduced by ethanol-induced lipid interdigitation, probably as a result of an increased membrane surface density due to the increased intramolecular lipid acyl chain ordering and a tighter overall intermolecular packing.

Real-time X-ray diffraction study at different scan rates of phase transitions for dipalmitoylphosphatidylcholine in KSCN

Multibilayer arrays of dipalmitoylphosphatidylcholine (DPPC) in 1 M KSCN were characterized using real-time X-ray diffraction and differential scanning calorimetry. A phase transition sequence was observed as a function of increasing temperature which involved changes from the interdigitated subgel (Lc(inter)) to interdigitated gel (L beta(inter)) to disordered (L alpha) bilayer states. The phase transition mechanisms were unambiguously determined by comparison of results from fast and slow scans. The Lc(inter)-->L beta(inter) phase transition was shown to involve a continuous change in acyl chain spacing between the rectangular subgel acyl chain unit cell into an hexagonal gel acyl chain unit cell. The mechanism is similar to that for subgel to gel state transitions involving non-interdigitated DPPC bilayers.

Bilayer packing characteristics of mixed chain phospholipid derivatives: Raman spectroscopic and differential scanning calorimetric studies of 1-stearoyl-2-capryl-sn-glycero-3-phosphocholine (C(18):C(10)PC) and 1-stearoyl-2-capryl-sn-glycero-3-phospho-N-trimethylpropanolamine (C(18):C(10)TMPC)

Raman spectroscopy and high-sensitivity differential scanning calorimetry (DSC) were used to compare the effects of headgroup conformation on the acyl chain packing arrangements in two highly asymmetric phosphatidylcholine (PC) analogues, 1-stearoyl-2-capryl-sn-glycero-3-phosphocholine (C(18):C(10)PC) and a polar headgroup derivative of C(18):C(10)PC, 1-stearoyl-2-capryl-sn-glycero-3-phospho-N-trimethylpropanolami ne (C(18):C(10)TMPC), which contains an additional methylene group within the choline moiety; namely, -P-O-(CH2)3-N(CH3)3. The C(18):C(10)TMPC headgroup exhibits an extended trans conformation which is independent of bilayer phase. A comparison of gel phase spectral order parameters of the two lipid species indicates a mixed interdigitated state characteristic of three chains per headgroup for C(18): C(10)TMPC. A more intermolecularly ordered liquid crystalline phase is observed, however, for the C(18):C(10)TMPC bilayers. The phase transition cooperative unit size estimated for the C(18):C(10)PC bilayers (approximately 140 molecules per unit) is about 7-fold greater than that for the C(18):C(10)TMPC dispersions (approximately 20 molecules per unit). We suggest that the extended headgroup for C(18):C(10)TMPC induces a slight tilt in the gel phase packing arrangements for the acyl chains, which may persist in the partially interdigitated liquid crystalline phase bilayer. Macroscopically, tighter packed multilamellar dispersions of C(18):C(10)TMPC occur for systems prepared first in the presence of a higher ionic strength medium. The stacked bilayers may then be transferred to a lower ionic strength environment without loss of their more closely packed adjacent lamellae.

Effects of alcohol-induced lipid interdigitation on proton permeability in L-alpha-dipalmitoylphosphatidylcholine vesicles

6-Carboxyfluorescein was employed to examine the effect of alcohol-induced lipid interdigitation on proton permeability in L-alpha-dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles. Proton permeability was measured by monitoring the decrease of 6-carboxyfluorescein fluorescence after a pH gradient from 3.5 (outside the vesicle) to 8.0 (inside the vesicle) was established. At 20 degrees C and below 1.2 M ethanol, the fluorescence decrease is best described by a single exponential function. Above 1.2 M ethanol, the intensity decrease is better described by a two-exponential decay law. Using the fitted rate constants and the vesicle radii determined from light-scattering measurements, the proton permeability coefficient, P, in DPPC vesicles was calculated as a function of ethanol concentration. At 20 degrees C, P increases monotonically with increasing ethanol content up to 1.0 M, followed by an abrupt increase at 1.2 M. The vesicle size also exhibits a sudden increase at around 1.2 M ethanol, which has been shown to result from vesicle aggregation rather than vesicle fusion. The abrupt increases in P and in vesicle size occur at the concentration region close to the critical ethanol concentration for the formation of the fully interdigitated gel state of DPPC. At 14 degrees C, the abrupt change in P shifts to 1.9-2.0 M ethanol, completely in accordance with the ethanol-temperature phase diagram of interdigitated DPPC. Effects of methanol and benzyl alcohol on lipid interdigitation have also been examined. At 20 degrees C, DPPC large unilamellar vesicles exhibit a dramatic change in P at 3 M methanol and at 40 mM benzyl alcohol. These concentrations come close to the critical methanol and benzyl alcohol concentrations for the formation of fully interdigitated DPPC structures determined previously by others. It can be concluded that proton permeability increases dramatically as DPPC is transformed from the noninterdigitated gel to the fully interdigitated gel state by high concentrations of alcohol. This marked increase in proton permeability can be attributed to the combined effect of the changes in membrane thickness and surface charge density, due to the ethanol-induced lipid interdigitation. The possible effects of the increased proton permeability caused by ingested ethanol on gastric mucosal membranes are discussed.

Interactions of model human pulmonary surfactants with a mixed phospholipid bilayer assembly: Raman spectroscopic studies

The temperature dependence and acyl chain packing properties of the binary lipid mixtures of dipalmitoylphosphatidylcholine-d62 (DPPC-d62)/dipalmitoylphosphatidylglycerol (DPPG) multilayers, reconstituted with two synthetic peptides for modeling the membrane behavior of the SP-B protein associated with human pulmonary surfactant, were investigated by vibrational Raman spectroscopy. The synthetic peptides consisted of 21 amino acid residues representing repeating charged units of either lysine or aspartic acid separated by hydrophobic domains consisting of four leucines (KL4 or DL4, respectively). These peptides were designed to mimic the alternating hydrophobic and hydrophilic sequences defining the low molecular weight SP-B protein. Raman spectroscopic parameters consisting of integrated band intensities, line widths, and relative peak height intensity ratios were used to probe the bilayer order/disorder characteristics of the liposomal perturbations reflected by the reconstituted membrane assemblies. Temperature profiles derived from the various Raman intensity parameters for the 3100-2800-cm-1 carbon-hydrogen (C-H) and the 2000-2300-cm-1 carbon-deuterium (C-D) stretching mode regions, spectral intervals representative of acyl chain vibrations, reflected lipid reorganizations specific to peptide interactions with either the DPPC-d62 or DPPG component of the liposome. For the multilamellar surfactant systems composed of either KL4 or DL4 reconstituted with the binary DPPG/DPPC-d62 lipid mixture, the breadth of the gel to liquid crystalline phase transition temperatures TM, defined by acyl chain C-H and C-D stretching mode order/disorder parameters, increased from about 1 degree C in the peptide-free systems to over 10 degrees C. This breadth in TM indicates an increased lipid disorder and a distinct noncooperative chain melting process for the model liposomes. In comparing the interactions of the synthetic peptides with DPPG/DPPC mixtures and with DPPC liposomes alone, the negatively charged DL4 peptide perturbs the DPPG component of the lipid mixture more strongly than the DPPC-d62 component; moreover, the DL4 peptide disrupts the structure of the DPPG lipid domains in the binary mixture to a greater extent than the KL4 peptide. The microdomain heterogeneity of the binary lipid mixture arising from lipid-peptide interactions is discussed in terms of the Raman spectral properties of the multilayers. The Raman data in conjunction with previous bubble surfactometer and animal studies (Cochrane & Revak, 1991) suggest that lipid domain structures are present in functional surfactants and that the dynamic bilayer microheterogeneity induced by the surfactant peptide or protein is essential for pulmonary mechanics.

Cryopreservation of poultry sperm: the enigma of glycerol

This review summarizes recent data for cryopreservation of poultry sperm and data establishing the contraceptive effect of glycerol. Successful cryopreservation protocols for bovine sperm are compared to the requirements for rooster sperm, with emphasis on glycerol-induced alterations in avian reproductive systems. It has been shown that molar concentrations of glycerol can affect (a) physical features of the cytoplasm (cytoplasmic organization and viscosity), (b) permeability and stability of the membrane bilayer(s), and (c) noncovalent attachment of proteins to the sperm surface. Perturbing effects of glycerol on sperm metabolism and the essentiality of maintaining bioenergetic balance during the temperature changes associated with any cryopreservation protocol are discussed. Emphasis is placed on the processes in avian reproduction that may be altered by interactions with glycerol. Finally, we discuss the potential value of using available genetic models (lines of roosters differing in the capacity of their sperm to survive a freeze-thaw cycle) to clarify and overcome damage to poultry sperm induced by cryopreservation.

Effect of cholesterol on the ethanol-induced interdigitated gel phase in phosphatidylcholine: use of fluorophore pyrene-labeled phosphatidylcholine

It is now recognized that many amphiphilic molecules such as ethanol can induce the formation of the fully interdigitated gel phase (L beta I) in phosphatidylcholines (PC's). In the present study, we have developed a simple detection method for the L beta I phase using pyrene-labeled PC (PyrPC), which is a PC analogue with covalently coupled pyrene moiety at the end of one of its acyl chains. The intensity ratio of its fluorescence vibrational bands is a reflection of the polarity of the environment of the fluorophore. We have tested this fluorophore in several established interdigitated lipid systems, including 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (1,2-DPPC) in the presence of high concentrations of ethanol and 1,2-di-O-hexadecyl-sn-glycero-3-phosphocholine (DHPC) and 1,3-dipalmitoyl-sn-glycero-2-phosphocholine (1,3-DPPC) in the absence of any additives. We have found in each of these systems that the ratio of the intensities of band III (387.5 nm) to band I (376.5 nm) is sensitive to the lipid phase change from the noninterdigitated L beta' phase to the interdigitated L beta I phase. By comparison of the III/I ratios for PyrPC in the lipid systems with the III/I ratios for methylpyrene in organic solvents, it was shown that the polarity of the PyrPC environment in the L beta I phase is similar to that of pentanol or ethanol. Using this method, we investigated the effect of cholesterol on the ethanol induction of the interdigitated gel phase in 1,2-DPPC. We found that the ethanol induction of the interdigitated gel phase is prevented by the presence of 20 mol % cholesterol.

Coupled changes between lipid order and polypeptide conformation at the membrane surface. A 2H NMR and Raman study of polylysine-phosphatidic acid systems

Thermotropism and segmental chain order parameters of sn-2-perdeuteriated dimyristoyl-phosphatidic acid (DMPA)-water dispersions, with and without poly(L-lysine) (PLL) of different molecular weights, have been investigated by solid-state deuterium NMR spectroscopy. The segmental chain order parameter profile of this negatively charged lipid is similar to that already found for other lipids. Addition of long PLL (MW = 200,000) increases the temperature, Tc, of the lipid gel-to-fluid phase transition, whereas short PLL (MW = 4000) has practically no effect on Tc. In the fluid phase both varieties of PLL increase the "plateau" character of segmental order parameters up to carbon position 10. At the same reduced temperature, long PLL more significantly increases the segmental ordering, especially at the methyl terminal position. This leads to the conclusion that polar head-group capping and charge neutralization by PLL induce severe changes in lipid chain ordering, even down to the bilayer core. The structure of PLL bound to the lipid bilayer surface was monitored by Raman spectroscopy, following the amide I bands. Results show that the lipid gel-to-fluid phase transition triggers a conformational transition from ordered beta-sheet to random structure of short PLL, while it does not affect the strongly stabilized beta-sheet structure of long PLL. It is concluded that both short and long PLL can efficiently cap and neutralize lipid head groups, whatever their structure, and that peptide length is a key parameter in whether lipids or peptides are the driving force in conformationally coupled changes of both partners in the membrane.

Behavior of spin labels in a variety of interdigitated lipid bilayers

The behavior of a number of spin labels in several lipid bilayers, shown by X-ray diffraction to be interdigitated, has been compared in order to evaluate the ability of the spin label technique to detect and diagnose the structure of lipid bilayers. The main difference between interdigitated and non-interdigitated gel phase bilayers which can be exploited for determination of their structure using spin labels, is that the former have a much less steep fluidity gradient. Thus long chain spin labels with the nitroxide group near the terminal methyl of the chain, such as 16-doxylstearic acid, its methyl ester, or a phosphatidylglycerol spin label containing 16-doxylstearic acid (PG-SL), are more motionally restricted and/or ordered in the interdigitated bilayer than in the non-interdigitated bilayer. This difference is large enough to be of diagnostic value for all three spin labels in the interdigitated bilayers of dihexadecylphosphatidylcholine, dipalmitoylphosphatidylcholine/ethanol, and 1,3-dipalmitoylphosphatidylcholine. However, it is not large enough to be of diagnostic value at low temperatures. Use of probes with the nitroxide group closer to the apolar/polar interface reveals that these latter interdigitated bilayers are more disordered or less closely packed. As the temperature is increased, however, the motion of the PG-SL does not increase as much in these interdigitated bilayers as in non-interdigitated bilayers. The difference in the motion and/or order of PG-SL between interdigitated and non-interdigitated bilayers is large enough at higher temperatures to be of value in diagnosing the structure of the bilayers. Thus by choice of a suitable spin label and a suitable temperature, this technique should prove useful for detection and diagnosis of lipid bilayer structure with a good degree of reliability. Caution must, of course be exercised, as with any spectroscopic technique. Spin labels will also be invaluable for more detailed studies of known interdigitated bilayers, which would be time- and material-consuming, if carried out using X-ray diffraction solely.

Studies of the ethanol-induced interdigitated gel phase in phosphatidylcholines using the fluorophore 1,6-diphenyl-1,3,5-hexatriene

It is now well established that a number of amphiphilic molecules such as ethanol can induce the formation of the fully interdigitated gel phase in phosphatidylcholines. We have shown earlier that alcohols such as ethanol induce biphasic melting behavior in phosphatidylcholines [Rowe, E. S. (1983) Biochemistry 22, 3299-3305] but not in phosphatidylethanolamines [Rowe, E. S. (1985) Biochim. Biophys. Acta 813, 321-330]. Simon and McIntosh [(1984) Biochim. Biophys. Acta 773, 169-172] showed that the alcohol-induced biphasic melting behavior in phosphatidylcholines is a consequence of acyl chain interdigitation. In the present study we demonstrate the detection of the transition of DPPC and DSPC to the interdigitated phase in the presence of ethanol using the fluorescence properties of the commonly used fluorophore 1,6-diphenyl-1,3,5-hexatriene (DPH). By correlating fluorescence and X-ray diffraction results, we have demonstrated the use of fluorescence to study the phase transition from the noninterdigitated to the interdigitated phase. Using this method, we have investigated the temperature and ethanol concentration dependence of the induction of the interdigitated phase in DSPC and DPPC and shown that the induction of interdigitation by ethanol is temperature dependent, with higher temperature favoring interdigitation. The temperature-ethanol phase diagrams have been determined for DPPC and DSPC.

Effect of ethanol on dimyristoylphosphatidylcholine large unilamellar vesicles investigated by quasi-elastic light scattering and vibrational spectroscopy

The gel-like liquid phase transition of dimyristoylphosphatidylcholine (DMPC) large unilamellar vesicles prepared by reverse phase evaporation has been investigated in buffers containing ethanol by quasi-elastic light scattering (QLS) and vibrational (infrared and Raman) spectroscopy. With the QLS technique, the relative change in the vesicles area (which is related to the molecular cross-sectional area of lipid molecules) was followed versus both temperature and ethanol concentration. When the latter was low, the depression of the transition point was a linear function of the alcohol concentration, c, but the vesicles area was practically unmodified. At alcohol concentration 10% v/v, an abrupt change of the vesicles area was observed and for c greater than 10% the depression of the transition point was a non-linear function of c. The infrared and Raman spectra showed a perturbation of the hydrophobic regions, including the terminal methyl groups of the acyl tails.

Study of the effect of poly(L-lysine) on phosphatidic acid and phosphatidylcholine/phosphatidic acid bilayers by raman spectroscopy

The effect of polylysine (PLL) on dimyristoylphosphatidic acid (DMPA), on dimyristoyl-phosphatidylcholine (DMPC), and on mixtures of these lipids was investigated by Raman spectroscopy. These results show that long polylysine (Mr approximately 200,000) increases the stability of the acyl chain matrix of DMPA to form a more closely packed structure with a stoichiometry of one lysine residue per PA molecule. On the other hand, short PLL (Mr 4000) destabilizes the PA bilayer, and the complex formed undergoes a gel to liquid-crystalline transition at a lower temperature than of the pure lipid. For both cases, we have observed that bound polylysine adopts a beta-sheet conformation as opposed to the alpha-helical structure previously found for dipalmitoylphosphatidylglycerol/long PLL complexes [Carrier, D., & Pézolet, M. (1984) Biophys. J. 46, 497-506]. The difference in the thermal behavior of complexes of DMPA with long and short polylysines is believed to be associated with the fact that in the complex the long polypeptide adopts the beta-sheet conformation over the whole range of temperatures investigated while the short one undergoes a change of conformation from beta-sheet of random coil upon heating. Therefore, the conformation of the lipid-bound polypeptides depends on the nature of the polar head group of the lipid, not only on its net charge, and it affects considerably the thermotropism of the lipid. On the other hand, both long and short polylysines show no affinity for phosphatidylcholine since the temperature profiles of DMPC and of DMPC/PLL complexes exhibit exactly the same behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of alcohols on the phase transitions of dihexadecylphosphatidylcholine

We have systematically investigated the effect of short chain alcohols (methanol to n-propanol) on the phase transitions of 1,2-dihexadecylphosphatidylcholine (DHPC), a lipid that forms a stable interdigitated gel phase (L beta I) in aqueous solution. The temperature of the low-temperature L beta I to P beta' phase transition of DHPC was found to increase with alcohol concentration, showing that alcohol interacts preferentially with the interdigitated phase relative to the non-interdigitated gel. The main transition of DHPC exhibited a biphasic effect of alcohol concentration similar to that previously observed with DPPC (Rowe, E.S. (1983) Biochemistry 22,3299-3305). As alcohol concentration is increased the lower L beta I to P beta' and main P beta' to L alpha transitions of DHPC merge at the threshold concentration of the biphasic effect, so that above this concentration there is one phase transition from L beta I directly to L alpha. This is analogous to DPPC above its biphasic threshold. Similar to DPPC, the transition between L beta I and L alpha exhibits marked hysteresis.

Structures and mechanisms of lipid phase transitions in nonaqueous media. Dipalmitoylphosphatidylethanolamine in fused salt

The phase transitions for dipalmitoylphosphatidylethanolamine (DPPE) dispersed in water and in N-ethylammonium nitrate (EAN) were examined using differential scanning calorimetry and time-resolved x-ray diffraction. Subgel, pre-, and main-phase transitions were observed for DPPE in water, whereas only the pre- and main transitions were observed for DPPE in EAN. Hysteresis was observed for both dispersions upon cooling. In addition, the lamellar (L alpha) to hexagonal (H alpha) phase transition was observed for DPPE dispersed in EAN when using time-resolved x-ray diffraction but not when using calorimetry. This low enthalpy process occurred at 73-77 degrees C, which is significantly lower than that observed for DPPE in water. The presence of EAN stabilizes the existence of the H alpha phase in DPPE by its influence on the bilayer interfacial properties, primarily on the area per lipid head group.