Evidence for acyl chain trans/gauche isomerization during the thermal pretransition of dipalmitoyl phosphatidylcholine bilayer dispersions

Raman Microscopy Investigation of GLP-1 Peptide Association with Supported Phospholipid Bilayers

A wide range of important biological processes occur at phospholipid membranes including cell signaling, where a peptide or small molecule targets a membrane-localized receptor protein. In this work, we report the adaptation of confocal Raman microscopy to quantify populations of unlabeled glucagon-like peptide-1 (GLP-1), a membrane-active 30-residue incretin peptide, in supported phospholipid bilayers deposited on the interior surfaces of wide-pore porous silica particles. Quantification of lipid bilayer-associated peptide is achieved by measuring the Raman scattering intensity of the peptide relative to that of the supported lipid bilayer, which serves as an internal standard. The dependence of the bilayer-associated GLP-1 population on the solution concentration of GLP-1 produces an isotherm used to determine the equilibrium constant for peptide-bilayer association and the maximum peptide surface coverage. The maximum coverage of GLP-1 in the lipid bilayer was found to be only 1/5th of a full monolayer based on its hydrodynamic radius. The saturation coverage, therefore, is not limited by the size of GLP-1 but by the ability of the bilayer to accommodate the peptide at high concentrations within the bilayer. Raman spectra show that GLP-1 association with the supported bilayer is accompanied by structural changes consistent with the intercalation of the peptide into the bilayer, where the observed increase in acyl-chain order would increase the lipid density and provide free volume needed to accommodate the peptide. These results were compared with previous measurements of the association of fluorescently labeled GLP-1 with a planar-supported bilayer; the unlabeled peptide exhibits a 3-fold greater affinity for the lipid bilayer on the porous silica support, suggesting that the fluorescent label alters the GLP-1 lipid bilayer association.

Hybrid-Supported Bilayers Formed with Mixed-Charge Surfactants on C18-Functionalized Silica Surfaces

Mixtures of cationic-anionic surfactants have been shown to spontaneously form ordered monolayers at hydrophobic-hydrophilic boundaries, including air-water and oil-water interfaces. In this work, confocal Raman microscopy is used to investigate the structure of hybrid-supported surfactant bilayers (HSSBs) formed by deposition of a distal leaflet of mixed cationic-anionic surfactants onto a proximal leaflet of n-alkane (C₁₈) chains on the interior surfaces of chromatographic silica particles. The surface coverage of the two surfactants in a hybrid bilayer was determined from carbon analysis and the relative Raman scattering of their respective head-groups. Within the measurement uncertainty, the stoichiometric ratio of the two surfactants is one-to-one, equivalent to mixed-charge-surfactant monolayers at air-water and oil-water interfaces and consistent with the role of the head-group electrostatic interactions in their formation. When self-assembled on the hydrophobic surface, pairs of oppositely charged n-alkyl chain surfactants resemble a phospholipid (phosphatidylcholine) molecule, with its zwitterionic head-group and two hydrophobic acyl chain tails. Indeed, the structure of these hybrid-supported surfactant bilayers on C₁₈-modified silica surfaces is similar to that of hybrid-supported lipid bilayers (HSLBs) on the same supports, but with denser and more-ordered n-alkyl chains. Hybrid-supported surfactant bilayers exhibit a melting phase transition (gel to liquid-crystalline phase) with structural and energetic characteristics similar to those of hybrid-supported bilayers prepared from a zwitterionic phospholipid of the same alkyl chain length. These mixed-charge surfactants on n-alkane-modified silica are stable in water over time (months), results that suggest the potential use of these hybrid bilayers for generating supported lipid-bilayer-like surfaces or for separation applications.

Confocal Raman Microscopy Investigation of Phospholipid Monolayers Deposited on Nitrile-Modified Surfaces in Porous Silica Particles

Phospholipid bilayers deposited on a variety of surfaces provide models for investigation of the lipid membrane structure and supports for biocompatible sensors. Hybrid-supported phospholipid bilayers (HSLBs) are stable membrane models for these investigations, typically prepared by self-assembly of a lipid monolayer over an n-alkane-modified surface. HSLBs have been prepared on n-alkyl chain-modified silica and used for lipophilicity-based chromatographic separations. The structure of these hybrid bilayers differs from vesicle membranes where the lipid head group spacing is greater due to interdigitation of the lipid acyl chains with the underlying n-alkyl chains bound to the silica surface. This interdigitated structure exhibits a broader melting transition at a higher temperature due to strong interactions between the lipid acyl chains and the immobile n-alkyl chains bound to silica. In the present work, we seek to reduce the interactions between a lipid monolayer and its supporting substrate by self-assembly of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) on porous silica functionalized with nitrile-terminated surface ligands. The frequency of Raman scattering of the surface -C≡N stretching mode at the lipid-nitrile interface is consistent with an n-alkane-like environment and insensitive to lipid head group charge, indicating that the lipid acyl chains are in contact with the surface nitrile groups. The head group area of this lipid monolayer was determined from the within-particle phospholipid concentration and silica specific surface area and found to be 54 ± 2 Ų, equivalent to the head group area of a DMPC vesicle bilayer. The structure of these nitrile-supported phospholipid monolayers was characterized below and above their melting transition by confocal Raman microscopy and found to be nearly identical to DMPC vesicle bilayers. Their narrow gel-to-fluid-phase melting transition is equivalent to dispersed DMPC vesicles, suggesting that the acyl chain structure on the nitrile support mimics the outer leaflet structure of a vesicle membrane.

Lactobacillus bulgaricus improves antioxidant capacity of black garlic in the prevention of gestational diabetes mellitus: a randomized control trial

Objectives:Lactobacillus bulgaricus may improve antioxidant capacity of black garlic in the prevention of gestational diabetes mellitus (GDM).

Methods: Black garlic was prepared with or without L. bulgaricus. Volatile and polysaccharides were analyzed by using LC-MS, Fourier Transform infrared (FTIR) and ¹³C nuclear magnetic resonance (NMR). The study design was parallel randomized controlled trial and 226 GDM patients were randomly assigned into BG (black garlic and L. bulgaricus) and CG (black garlic) groups, and allocation ratio was 1:1. The treatment duration was 40 weeks. Fasting blood glucose (FBG) and 1- and 2-h blood glucose (1hBG and 2hBG) after oral glucose tolerance test (OGTT) were detected. Antioxidant function of black garlic was determined by measuring plasma malondialdehyde (MDA), superoxide dismutase (SOD), glutathione peroxidase (GSH-PX) and total antioxidant capacity (T-AOC) in GDM patients. The comparison between two groups was made using two independent samples t test.

Results: The intake of nutrients was similar between two groups (P>0.05). L. bulgaricus promoted the transformation of the glucopyranoside to glucofuranoside. L. bulgaricus increased the abilities of black garlic for scavenging hydroxyl radicals, 2,2′-azino-bis (3-ethylbenzenthiazoline-6-sulfonic) acid (ABTS) and DPPH free radicals. L. bulgaricus reduced the levels of FBG, 1hBG and 2hBG, and incidence of perinatal complications (P<0.01). Plasma MDA level in the BG group was lower than in the CG group, whereas the levels of SOD, GSH-PX and T-AOC in the BG group were higher than in the CG group (P<0.01).

Conclusions:L. bulgaricus improves antioxidant capacity of black garlic in the prevention of GDM.

Imaging phospholipid conformational disorder and packing in giant multilamellar liposome by confocal Raman microspectroscopy

Liposomes are closed phospholipid bilayer systems that have profound applications in fundamental cell biology, pharmaceutics and medicine. Depending on the composition (pure or mixture of phospholipids, presence of cholesterol) and preparation protocol, intra- and inter-chain molecular interactions vary leading to changes in the quality (order and packing) of liposomes. So far it is not possible to image conformational disorders and packing densities within a liposome in a straightforward manner. In this study, we utilized confocal Raman microspectroscopy to visualize structural disorders and packing efficiency within a giant multilamellar liposome model by focusing mainly on three regions in the vibrational spectrum (CC stretching, CH deformation and CH stretching). We estimated properties such as trans/gauche isomers and lateral packing probability. Interestingly, our Raman imaging studies revealed gel phase rich domains and heterogeneous lateral packing within the giant multilamellar liposome.

Confocal Raman Microscopy of Hybrid-Supported Phospholipid Bilayers within Individual C18-Functionalized Chromatographic Particles

Measuring lipid-membrane partitioning of small molecules is critical to predicting bioavailability and investigating molecule-membrane interactions. A stable model membrane for such studies has been developed through assembly of a phospholipid monolayer on n-alkane-modified surfaces. These hybrid bilayers have recently been generated within n-alkyl-chain (C18)-modified porous silica and used in chromatographic retention studies of small molecules. Despite their successful application, determining the structure of hybrid bilayers within chromatographic silica is challenging because they reside at buried interfaces within the porous structure. In this work, we employ confocal Raman microscopy to investigate the formation and temperature-dependent structure of hybrid-phospholipid bilayers in C18-modified, porous-silica chromatographic particles. Porous silica provides sufficient surface area within a confocal probe volume centered in an individual particle to readily measure, with Raman microscopy, the formation of an ordered hybrid bilayer of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) with the surface C18 chains. The DMPC surface density was quantified from the relative Raman scattering intensities of C18 and phospholipid acyl chains and found to be ∼40% of a DMPC vesicle membrane. By monitoring Raman spectra acquired versus temperature, the bilayer main phase transition was observed to be broadened and shifted to higher temperature compared to a DMPC vesicle, in agreement with differential scanning calorimetry (DSC) results. Raman scattering of deuterated phospholipid was resolved from protonated C18 chain scattering, showing that the lipid acyl and C18 chains melt simultaneously in a single phase transition. The surface density of lipid in the hybrid bilayer, the ordering of both C18 and lipid acyl chains upon bilayer formation, and decoupling of C18 methylene C-H vibrations by deuterated lipid acyl chains all suggest an interdigitated acyl chain structure. The simultaneous melting of both layers is also consistent with an interdigitated structure, where immobility of surface-grafted C18 chains decreases the cooperativity and increases the melting temperature compared to a vesicle bilayer.

Interaction of choline salts with artificial biological membranes: DSC studies elucidating cellular interactions

To better understand the relationship between the relative cytotoxicity of diluted ionic liquids and their specific interaction with biological membranes, the thermotropic behavior of model lipid membrane systems formulated in a series of choline based organic salts was investigated. Unilamellar vesicles prepared from dipalmitoylphosphatidylcholine were exposed to a series of choline phosphate salts at a concentration of 10mM at pH7.40, and the gel to liquid-crystalline state transition was examined using differential scanning calorimetry. The choline salts that were observed to have a low relative toxicity in previous studies induced minimal changes in the lipid phase transition behavior of these model membranes. In contrast, the salts choline bis(2,4,4-trimethylpentyl)phosphinate and choline bis(2-ethylhexyl)phosphate, both of which were observed to have high relative toxicity, caused distinct disruptions in the lipid phase transition behavior, consistent with penetration of the salts into the acyl chains of the phospholipids. choline bis(2,4,4-trimethylpentyl)phosphinate reduced the Tm and enthalpy of the main transition of dipalmitoylphosphatidylcholine while choline bis(2-ethylhexyl)phosphate induced the equilibration of alternate phases.

Sclera as a surrogate marker for determining AGE-modifications in Bruch's membrane using a Raman spectroscopy-based index of aging

PURPOSE

Raman spectroscopy is an effective probe of advanced glycation end products (AGEs) in Bruch's membrane. However, because it is the outermost layer of the retina, this extracellular matrix is difficult to analyze in vivo with current technology. The sclera shares many compositional characteristics with Bruch's membrane, but it is much easier to access for in vivo Raman analysis. This study investigated whether sclera could act as a surrogate tissue for Raman-based investigation of pathogenic AGEs in Bruch's membrane.

METHODS

Human sclera and Bruch's membrane were dissected from postmortem eyes (n = 67) across a wide age range (33-92 years) and were probed by Raman spectroscopy. The biochemical composition, AGEs, and their age-related trends were determined from data reduction of the Raman spectra and compared for the two tissues.

RESULTS

Raman microscopy demonstrated that Bruch's membrane and sclera are composed of a similar range of biomolecules but with distinct relative quantities, such as in the heme/collagen and the elastin/collagen ratios. Both tissues accumulated AGEs, and these correlated with chronological age (R(2) = 0.824 and R(2) = 0.717 for sclera and Bruch's membrane, respectively). The sclera accumulated AGE adducts at a lower rate than Bruch's membrane, and the models of overall age-related changes exhibited a lower rate (one-fourth that of Bruch's membrane) but a significant increase with age (P < 0.05).

CONCLUSIONS

The results suggest that the sclera is a viable surrogate marker for estimating AGE accumulation in Bruch's membrane and for reliably predicting chronological age. These findings also suggest that sclera could be a useful target tissue for future patient-based, Raman spectroscopy studies.

Conformational Structure of Triblock Copolymers by FT-Raman and FTIR Spectroscopy

Fourier transform Raman (FT-Raman) and Fourier transform infrared (FTIR) spectra of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers as pure solids or liquids and in aqueous solutions have been examined. The qualitative features in Raman and FTIR spectra of these copolymers have been presented. The Raman and FTIR spectra of PEO-PPO-PEO triblock copolymers are very sensitive to the structural and conformational changes. It shows that the relative intensities of several peaks in Raman spectra are dependent on the PPO/PEO ratios and the conformation of the copolymers. From Raman and FTIR spectra, Pluronic F68 and F88 assume helical structures with a few trans conformers. Other block copolymers exhibit that the disordered structure increases with increasing PPO/PEO ratio. Comparison of Raman spectra of PEO-PPO-PEO triblock copolymers as pure solids or liquids with those in aqueous solutions has been presented. Copyright 1999 Academic Press.

A Raman spectroscopic study of acetylcholine receptor-rich membranes from Torpedo marmorata. Interaction of the receptor with carbamylcholine and (+)-tubocurarine

Raman spectroscopy is used to determine structural features of alkali-treated subsynaptic membrane fragments from Torpedo marmorata electric organ, rich in native functional AcChR. Distinct vibrations attributable to the membrane proteins and lipids were identified and studied before and after addition of the agonist carbamylcholine and the competitive antagonist (+)-tubocurarine. The protein secondary structure determined by using amide-I polypeptide vibrational analysis, indicates 47% alpha-helices, 25% beta-sheets, 18% turns and 11% undefined structure. The secondary structure of the AcChR molecule was not subject to large modifications upon addition of carbamylcholine. But, the presence of the (+)-tubocurarine leads to detectable changes in the amide-I region which might be interpreted as reflecting different contributions of alpha-helices and turns in the secondary structure. In addition, Raman spectra provide information about the environment of aromatic amino acids (tyrosine and tryptophan), the (C-C) bonds, the CH2 and CH3 groups of aliphatic side chains, as well as the disulfide (S-S) and cystein (C-S) bonds. The tyrosines seem 'exposed' to the aqueous medium. The Raman spectra of the AcChR-carbamylcholine complex suggest 'exposed' tryptophans, while those of the unliganded membrane-bound AcChR or of the receptor with (+)-tubocurarine are shown 'buried'. The disulfide bridges in the AcChR subunits show identical conformation in the absence and presence of carbamylcholine. On the contrary, considerable changes are found in the AcChR-(+)-tubocurarine complex. Carbamylcholine and especially (+)-tubocurarine decrease lipid fluidity.

Perspective and limitations of cryo-electron microscopy. From model systems to biological specimens

We investigated the possibility of vitrifying temperature-sensitive lipid phases as well as (small) biological specimens. From a suspension of unilamellar vesicles, prepared from dipalmitoyl-phosphatidylcholine (DPPC), thin aqueous films were formed at various temperatures. With cryo-electron microscopy vesicles were found to be smooth, rippled and faceted or faceted only, depending on the temperature of thin-film formation (318, 312 and 296 K respectively). The morphology and the electron diffraction patterns indicate that membranes can by physically fixed by vitrification in their high-temperature configuration and studied at low temperature by cryo-electron microscopy. This finding suggests that it may also be possible to preserve, in their original state, the more complex membrane systems found in living organisms by initiating rapid-cooling at a physiological temperature. This was explored by vitrification of thin films formed on specimen grids with (human) blood platelets adhering to collagen fibres. Low-temperature observation with an acceleration voltage of 120 kV revealed subcellular details, More details were observed when using higher accelerating voltages (200 and 300 kV) of the electron beam. The results presented in this paper illustrate the great potential of cryo-electron microscopy in the study of membrane dynamics, both in relatively simple model membrane systems and in more complex biological membrane systems.

Partitioning behavior of indocarbocyanine probes between coexisting gel and fluid phases in model membranes

Gel-fluid partition coefficients, Kp, were measured for a series of indocarbocyanine dyes in multilamellar lipid vesicles. The dyes examined had alkyl chain lengths from 12 to 22 carbons. Fluorescence quenching by a spin-labeled phosphatidylcholine-enriched fluid phase created a large difference in quantum yield for indocarbocyanine fluorescence between fluid and gel phases, enabling reliable Kp determinations. The values range from Kp = 8 for the 12-carbon chain, favoring a fluid phase over a Ca2-phosphatidylserine rigid phase, to Kp = 0.02 for the 20-carbon chain dye, favoring a distearoylphosphatidylcholine-rich gel phase over the fluid phase.

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

A method is proposed and demonstrated for the direct determination of conformational disorder (trans-gauche isomerization) as a function of acyl-chain position in phospholipid bilayer membranes. Three specifically deuterated derivatives of dipalmitoylphosphatidylcholine (DPPC), namely 4,4,4',4'-d4-DPPC (4-d4-DPPC), 6,6,6',6'-d4-DPPC (6-d4-DPPC), and 10,10,10',10'-d4-DPPC (10-d4-DPPC), have been synthesized. The CD2 rocking modes in the Fourier transform infrared (FT-IR) spectrum have been monitored as a function of temperature for each derivative. A method originally applied by Snyder and Poore [(1973) Macromolecules 6, 708-715] as a specific probe of hydrocarbon chain conformation in alkanes has been used to analyze the data. The rocking modes appear at 622 cm-1 for a CD2 segment surrounded by a trans C-C-C skeleton and between 645 and 655 cm-1 for segments surrounded by particular gauche conformers. The integrated band intensities of these modes have been used to monitor trans-gauche isomerization in the acyl chains at particular depths in the bilayer. At 48 degrees C, above the gel-liquid-crystal phase transition, the percentage of gauche rotamers present is 20.7 +/- 4.2, 32.3 +/- 2.3, and 19.7 +/- 0.8 for 4-d4-DPPC, 6-d4-DPPC, and 10-d4-DPPC, respectively. The gel phase of the latter two molecules is highly ordered. In contrast, a substantial population of gauche rotamers was observed for the 4-d4-DPPC. The conformational analysis yields a range of 3.6-4.2 gauche rotamers/acyl chain of DPPC above the phase transition. This range is in excellent accord with the dilatometric data of Nagle and Wilkinson [(1978) Biophys. J. 23, 159-175]. The significant advantages of the FT-IR approach are discussed.

Calculation of intermolecular interaction strengths in the P beta' phase in lipid bilayers. Implications for theoretical models

The existence of the P beta' phase in certain lipid bilayers is evidence that molecular interactions between lipids are capable of producing unusual large-scale structures at or near biological conditions. The problem of identifying the specific intermolecular interactions responsible for the structures requires construction of theoretical models capable of clear predictions of the observable consequences of postulated intermolecular interactions. To this end we have carried out a twofold modeling effort aimed at understanding the ripple phase. First, we have performed detailed numerical calculations of potential energies of interaction between pairs and triplets of lipid molecules having different chain tilt angles and relative vertical alignments. The calculations support the notion that chain tilting in the gel phase is a result of successive 3-5-A displacements of neighboring molecules perpendicular to the bilayer plane rather than long-range cooperative chain tilting. Secondly, we have used these results as a guide to formulate a new lattice model for lipid bilayer condensed phases. The new model is less complex than our earlier model and it includes interactions which are, based on the energy calculations, more likely to be responsible for the ripple phase. In a certain limit the model maps onto the chiral clock model, a model of much interest in condensed matter theory. In this limit the model exhibits a chain-tilted ordered phase followed by (as temperature increases) a modulated phase followed by a disordered phase. Within this limit we discuss the properties of the model and compare structures of the modulated phase exhibited by the model with experimental data for the P beta' phase in lipid bilayers.

Structure and mechanical properties of giant lipid (DMPC) vesicle bilayers from 20 degrees C below to 10 degrees C above the liquid crystal-crystalline phase transition at 24 degrees C

We have used micromechanical tests to measure the thermoelastic properties of the liquid and gel phases of dimyristoylphosphatidylcholine (DMPC). We have found that the rippled P beta' phase is only formed when a vesicle is cooled to temperatures below the main acyl chain crystallization transition, Tc, under zero or very low membrane tension. We also found that the P beta' surface ripple or superlattice can be pulled flat under high membrane tension into a planar structure. For a ripple structure formed by acyl chains perpendicular to the projected plane, the projected area change that results from a flattening process is a direct measure of the molecular crystal angle. As such, the crystal angle was found to increase from about 24 degrees just below Tc to about 33 degrees below the pretransition. It was also observed that the P beta' superlattice did not form when annealed L beta' phase vesicles were heated from 5 degrees C to Tc; likewise, ripples did not form when the membrane was held under large tension during freezing from the L alpha phase. Each of these three procedures could be used to create a metastable planar structure which we have termed L*beta' since it is lamellar and plane-crystalline with acyl chains tilted to the bilayer plane. However, we show that this structure is not as condensed as the L beta' phase below 10 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

The ripple phase of phosphatidylcholines: effect of chain length and cholesterol

Saturated phosphatidylcholine (PC) bilayers display a rippled surface in the temperature region between the pre- and main transitions. Ripple repeat distance was measured from freeze-fracture electron micrographs. All of the lipids examined (C13PC to C16PC; C14C16PC and equimolar C14PC/C16PC) showed a bimodal distribution of ripple repeat distances with the two dominant values being in the ratio of 1:2. Within this series, chain length was a weak determinant of the actual repeat distance. The introduction of increasing concentrations of cholesterol eliminated the bimodal distribution and led to the appearance of a single distribution of increasing repeat distance and decreasing amplitude. Ripples disappeared above a cholesterol concentration of 15 mol%. These observations are discussed within the framework of a model which links the genesis of the ripples (vertical displacement of lipid molecules) to the trans-gauche isomerization known to occur at the pre-transition.

Infrared spectroscopic characterization of the interaction of lipid bilayers with phenol, salicylic acid and o-acetylsalicylic acid

The interaction of phenol (PHE), salicylic acid (SA) and o-acetylsalicylic acid (ASA) with bilayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was investigated by infrared spectrometry. The temperature of the main gel to liquid crystal phase transition of DPPC is markedly depressed in the presence of the three guest molecules. The temperature depression depends on the nature and concentration of the additives. The temperature of the pretransition is also affected by these guest molecules and the depression in temperature is even more pronounced than that of the main transition temperature. Possible modes of interaction of these guest molecules with the lipid bilayers are discussed.

Effects of anesthetic tetradecenols on phosphatidylcholine phase transitions. Implications for the mechanism of the bilayer pretransition

The effects of cis- and trans-9,10-tetradecenols on the phase transitions of dimyristoyl-, dipalmitoyl-, and distearoyl-phosphatidylcholines were investigated using high sensitivity scanning calorimetry and Raman spectroscopy. Both alcohols lowered the gel to liquid crystalline phase transition temperatures for all three phosphatidylcholines, with cis-tetradecenol showing a considerably greater effect than trans-tetradecenol in each case. While both alcohols increased the temperature of the dimyristoylphosphatidylcholine pretransition, and decreased the temperature of the distearoylphosphatidylcholine pretransition, cis-tetradecenol lowered the temperature of the dipalmitoylphosphatidylcholine pretransition, while trans-tetradecenol dramatically raised the pretransition temperature. These results are interpreted in terms of the reduction in gel (L beta) phase chain tilt and changes in the ease of acyl chain trans-gauche isomerization which are introduced by the alcohols, and the consequent effects of these changes on the pretransition and the gel to liquid crystalline phase transition. The data clearly show that caution is necessary in applying information on lipid-anesthetic interactions obtained from model membranes to the problem of clinical anesthesia, since qualitatively different results may be obtained when lipids of differing acyl chain lengths are employed. Superficial interpretation of such data might lead to erroneous conclusions.

Raman spectroscopic study of the interaction of poly-L-lysine with dipalmitoylphosphatidylglycerol bilayers

The interaction of the basic polypeptide poly-L-lysine with the negatively charged phospholipid dipalmitoylphosphatidylglycerol was studied using Raman spectroscopy. The nature of the interaction appeared to depend on the molar ratio of the constituents. At up to one lysine group per lipid molecule, the bilayer was stabilized by the polypeptide that underwent a conformational transition toward an ordered alpha-helical structure, in which the electrostatic interactions were probably maximal. The stabilization of the bilayer was detected by an increase in both the temperature of the thermotropic transition of the lipid and the interchain vibrational coupling of the methylene C-H vibrations. At higher poly-L-lysine concentration, hydrophobic interactions must have been involved to explain the binding of excess polypeptide. There seemed to be a penetration of poly-L-lysine in the bilayer that increased with the polypeptide concentration. Under these conditions, the chain-packing lattice gradually changed from hexagonal to either orthorhombic or monoclinic symmetry. We believe that this change of structure is associated with the interdigitation of the acyl chains.

Theory of the intermediate rippled phase of phospholipid bilayers

A mathematical model of the periodic, rippled P beta' phase of phosphatidylcholine is described. In this model the P beta' phase shows a periodic variation in membrane properties corresponding to a lateral periodic variation between fluid-like and crystal-like states. The model is based on a curvature-dependent Landau-de Gennes free-energy functional. Our proposed form of the curvature-dependent free energy remedies a subtle mathematical flaw in an earlier model having similar physical content [Falkovitz, M. F., Seul, M., Frisch, H. L. & McConnell, H. M. (1982) Proc. Natl. Acad. Sci. USA 79, 3918-3921].

Raman spectroscopic and X-ray diffraction studies of the effect of temperature and Ca2+ on phosphatidylethanolamine dispersions

Raman spectroscopy and X-ray diffraction are used to study the effect of heat and Ca2+ on dimyristoylphosphatidylethanolamine dispersions. Unlike phosphatidylcholine dispersions, dimyristoylphosphatidylethanolamine bilayers (at pH 8) require heating above Tm in order for hydration to occur and apparently bind Ca2+ at very low levels. These results are related to models for membrane fusion.

Comparative effects of melittin and its hydrophobic and hydrophilic fragments on bilayer organization by Raman spectroscopy

For bilayer systems consisting of 1,2-dimyristoyl phosphatidylcholine (DMPC) incubated with melittin, a polypeptide capable of integrating itself within the membrane, temperature profiles derived from Raman spectroscopic data indicate the existence of an immobilized lipid annulus surrounding the polypeptide. In particular, temperature profiles derived from C--H, C--D and C--C stretching mode parameters for 25:1, 14:1 and 10:1 lipid:protein mole ratios exhibit two order-disorder transitions. The primary (lower) gel to liquid crystalline phase transition is depressed when polypeptide concentration is increased. The concentration-independent higher temperature transition is associated with a fluidization of the immobilized boundary lipids present at the lipid-polypeptide interface within the bilayer. We estimate that five to seven lipids are involved in this discrete boundary layer around the inserted membrane component. The behavior of the intrinsic hydrophobic (residues 1-19) and of the extrinsic hydrophilic (residues 20-26) portions of melittin in the bilayer is compared with the properties of the intact polypeptide. We emphasize evidence that both intrinsic and extrinsic components immobilize lipids contiguous to the polypeptide.