Orientation studies of hydrated dipalmitoylphosphatidylcholine multibilayers by polarized FTIR-ATR spectroscopy

Sum-frequency vibrational spectroscopy, a tutorial: Applications for the study of lipid membrane structure and dynamics

Planar supported lipid bilayers (PSLBs) are an ideal model for the study of lipid membrane structures and dynamics when using sum-frequency vibrational spectroscopy (SFVS). In this paper, we describe the construction of asymmetric PSLBs and the basic SFVS theory needed to understand and make measurements on these membranes. Several examples are presented, including the determination of phospholipid orientation and measuring phospholipid transmembrane translocation (flip-flop).

Incorporation of acridine orange and methylene blue in Langmuir monolayers mimicking releasing nanostructures

The efficiency of methylene blue (MB) and acridine orange (AO) for photodynamic therapy (PDT) is increased if encapsulated in liposomes. In this paper we determine the molecular-level interactions between MB or AO and mixed monolayers of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC), 1,2-dipalmitoyl-sn-glycero-3-phospho-(1'-rac-glycerol) (DPPG) and cholesterol (CHOL) using surface pressure isotherms and polarization-modulated infrared reflection absorption spectroscopy (PM-IRRAS). To increase liposome stability, the effects from adding the surfactants Span® 80 and sodium cholate were also studied. Both MB and AO induce an expansion in the mixed monolayer, but this expansion is less significant in the presence of either Span® 80 or sodium cholate. The action of AO and MB occurred via coupling with phosphate groups of DPPC or DPPG. However, the levels of chain ordering and hydration of carbonyl and phosphate in headgroups depended on the photosensitizer and on the presence of Span® 80 or sodium cholate. From the PM-IRRAS spectra, we inferred that incorporation of MB and AO increased hydration of the monolayer headgroup, except for the case of the monolayer containing sodium cholate. This variability in behaviour offers an opportunity to tune the incorporation of AO and MB into liposomes which could be exploited in the release necessary for PDT.

Two-dimensional polymers with versatile functionalities via gemini monomers

Two-dimensional synthetic polymers (2DSPs) are sheet-like macromolecules consisting of covalently linked repeat units in two directions. Access to 2DSPs with controlled size and shape and diverse functionality has been limited because of the need for monomers to retain their crystallinity throughout polymerization. Here, we describe a synthetic strategy for 2DSPs that obviates the need for crystallinity, via the free radical copolymerization of amphiphilic gemini monomers and their monomeric derivatives arranged in a bilayer at solid-liquid interfaces. The ease of this strategy allowed the preparation of 2DSPs with well-controlled size and shape and diverse functionality on solid templates composed of various materials with wide-ranging surface curvatures and dimensions. The resulting 2DSPs showed remarkable mechanical strength and have multiple applications, such as nanolithographic resist and antibacterial agent. The broad scope of this approach markedly expands the chemistry, morphology, and functionality of 2DSPs accessible for practical applications.

Effects of heat treatment under low moisture conditions on the protein and oil in soybean seeds

The effects of autoclave and microwave heating on the protein and oil in soybean seeds were investigated under low moisture conditions. The nitrogen solubility index (NSI) decreased on heating. The reduction in the NSI was accompanied by an increase in the size and deformation of the oil bodies in the cellular tissue of soybean seeds. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that lipoxygenase was susceptible to heat denaturation, but 7S and 11S globulins were only partially denatured. The partial denaturation of the proteins was confirmed by Fourier transform infrared spectroscopy measurements. The ratio of oil to protein peaks increased with increasing heating, suggesting the exudation of oil to the surface or outside of oil bodies. Microwave heating is more efficient in changing the oil distribution in soybean seeds than autoclave heating. On the other hand, the degree of protein denaturation is lower after microwave heating.

Physical Effects of Buckwheat Extract on Biological Membrane In Vitro and Its Protective Properties

Buckwheat is a valuable source of many biologically active compounds and nutrients. It has properties that reduce blood cholesterol levels, and so reduces the risk of atherosclerosis, seals the capillaries, and lowers blood pressure. The aim of the study was to determine quantitative and qualitative characteristics of polyphenols contained in extracts from buckwheat husks and stalks, the biological activity of the extracts, and biophysical effects of their interaction with the erythrocyte membrane, treated as a model of the cell. An analysis of the extract’s composition has shown that buckwheat husk and stalk extracts are a rich source of polyphenolic compounds, the stalk extracts showing more compounds than the husk extract. The study allowed to determine the location which incorporated polyphenols occupy in the erythrocyte membrane and changes in the membrane properties caused by them. It was found that the extracts do not induce hemolysis of red blood cells, causing an increase in osmotic resistance of erythrocytes. They affect mainly the hydrophilic region by changing the degree of order of the polar heads of lipids, but do little to change the fluidity of the membrane and its hydration. The results showed also that polyphenolic substances included in the extracts well protect the membranes of red blood cells against oxidation and exhibit anti-inflammatory effect.

Long-Range Lipid-Water Interaction as Observed by ATR-FTIR Spectroscopy

It is commonly assumed that the structure of water at a lipid-water interface is influenced mostly in the first hydration layer. However, recent results from different experimental methods show that perturbation extends through several hydration layers. Due to its low light penetration depth, attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy is specifically suited to study interlamellar water structure in multibilayers. Results obtained by this technique confirm the long-range water structure disturbance. Consequently, in confined membrane environments nearly all water molecules can be perturbed. It is important to note that the behavior of confined water molecules differs significantly in samples prepared in excess water and in partially hydrated samples. We show in what manner the interlamellar water perturbation is influenced by the hydration level and how it is sequentially modified with a step-by-step dehydration of samples either by water evaporation or by osmotic pressure. Our results also indicate that besides different levels of hydration the lipid-water interaction is modulated by different lipid headgroups and different lipid phases as well. Therefore, modification of interlamellar water properties may clarify the role of water-mediated effects in biological processes.

Tracking molecular interactions in membranes by simultaneous ATR-FTIR-AFM

In situ atomic force microscopy (AFM) is an exceedingly powerful and useful technique for characterizing the structure and assembly of proteins in real-time, in situ, and especially at model membrane interfaces, such as supported planar lipid bilayers. There remains, however, a fundamental challenge with AFM-based imaging. Conclusions are inferred based on morphological or topographical features. It is conventionally very difficult to use AFM to confirm specific molecular conformation, especially in the case of protein-membrane interactions. In this case, a protein may undergo subtle conformational changes upon insertion in the membrane that may be critical to its function. AFM lacks the ability to directly measure such conformational changes and can, arguably, only resolve features that are topographically distinct. To address these issues, we have developed a platform that integrates in situ AFM with attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy. This combination of tools provides a unique means of tracking, simultaneously, conformational changes, not resolvable by in situ AFM, with topographical details that are not readily identified by conventional spectroscopy. Preliminary studies of thermal transitions in supported lipid bilayers and direct evidence of lipid-induced conformational changes in adsorbed proteins illustrates the potential of this coupled in situ functional imaging strategy.

New insights into lung surfactant monolayers using vibrational sum frequency generation spectroscopy

At the air-water interface, interfacial molecular structure, intermolecular interactions, film relaxation and film respreading of model lung surfactant monolayers were studied using vibrational sum frequency generation (VSFG) spectroscopy combined with a Langmuir film balance. Chain-perdeuterated dipalmitoylphosphatidylcholine (DPPC-d62), palmitoyloleoyl-phosphatidylglycerol (POPG), palmitic acid (PA) and tripalmitin were investigated. In the DPPC-d62-PA binary monolayer, PA showed a condensing effect on the DPPC chains. On the contrary, in the DPPC-d62-POPG binary monolayer, POPG showed a fluidizing effect on the DPPC chains. In the ternary monolayer system of DPPC-d62-POPG-PA, the balance between the fluidizing and the condensing effect was also observed. In addition, the film relaxation behavior of DPPC-d62 and the enhanced film stability of DPPC-d62 caused by the addition of tripalmitin were observed. Real-time VSFG was also employed to study the respreading properties of a complex lung surfactant mixture containing DPPC-d62, POPG, PA and KL4 (a mimic of SP-B) peptide, which revealed DPPC enrichment after film compression.

Directed Assembly of Binary Monolayers with a High Protein Affinity: Infrared Reflection Absorption Spectroscopy (IRRAS) and Surface Plasmon Resonance (SPR)

Infrared reflection absorption spectroscopy (IRRAS) and surface plasmon resonance (SPR) techniques have been employed to investigate human serum albumin (HSA) binding to binary monolayers of zwitterionic dipalmitoylphosphatidylcholine (DPPC) and cationic dioctadecyldimethylammonium bromide (DOMA). At the air-water interface, the favorable electrostatic interaction between DPPC and DOMA leads to a dense chain packing. The tilt angle of the hydrocarbon chains decreases with increasing mole fraction of DOMA (X(DOMA)) in the monolayers at the surface pressure 30 mN/m: DPPC ( approximately 30 degrees ), X(DOMA) = 0.1 ( approximately 15 degrees ), and X(DOMA) = 0.3 ( approximately 0 degrees ). Negligible protein binding to the DPPC monolayer is observed in contrast to a significant binding to the binary monolayers. After HSA binding, the hydrocarbon chains at X(DOMA) = 0.1 undergo an increase in tilt angle from 15 degrees to 25 approximately 30 degrees , and the chains at X(DOMA) = 0.3 remain almost unchanged. The two components in the monolayers deliver through lateral reorganization, induced by the protein in the subphase, to form multiple interaction sites favorable for protein binding. The surfaces with a high protein affinity are created through the directed assembly of binary monolayers for use in biosensing.

Condensing effect of palmitic acid on DPPC in mixed Langmuir monolayers

The interaction between deuterated dipalmitoylphosphatidylcholine (DPPC-d62) and palmitic acid (PA) in mixed Langmuir monolayers is studied using vibrational sum frequency generation (VSFG) spectroscopy. Palmitic acid is an additive in exogenous lung surfactant preparations such as Survanta and Surfaxin. The effect of PA on the chain conformation and orientation of DPPC in the liquid-expanded and condensed phases is explored. A condensing effect of PA on DPPC is observed with VSFG. At 12 mN/m, DPPC-d62 alone is in the liquid-expanded phase. Adding PA increases the conformational ordering of DPPC chains and causes DPPC to transition from the expanded phase into the condensed phase. At 42 mN/m, DPPC-d62 and PA form a mixed structure in the condensed phase. The presence of PA decreases the chain tilt angle of DPPC, increasing the orientational ordering of DPPC chains. At 42 mN/m, there is also evidence from the frequency red shift of the PO2- symmetric stretch that the carboxyl group of PA forms a hydrogen bond with the phosphate group of DPPC in the condensed phase. From this work the effect of PA on DPPC is 2-fold: (1) PA increases the chain ordering of DPPC and promotes the LE and TC phase separation and (2) due to the miscibility between DPPC and PA in the condensed phase, PA decreases the collapse pressure.

DPPC Langmuir monolayer at the air-water interface: probing the tail and head groups by vibrational sum frequency generation spectroscopy

Dipalmitoylphosphatidylcholine (DPPC) is the predominant lipid component in lung surfactant. In this study, the Langmuir monolayer of deuterated dipalmitoylphosphatidylcholine (DPPC-d62) in the liquid-expanded (LE) phase and the liquid-condensed (LC) phase has been investigated at the air-water interface with broad bandwidth sum frequency generation (BBSFG) spectroscopy combined with a Langmuir film balance. Four moieties of the DPPC molecule are probed by BBSFG: the terminal methyl (CD3) groups of the tails, the methylene (CD2) groups of the tails, the choline methyls (CH3) in the headgroup, and the phosphate in the headgroup. BBSFG spectra of the four DPPC moieties provide information about chain conformation, chain orientation, headgroup orientation, and headgroup hydration. These results provide a comprehensive picture of the DPPC phase behavior at the air-water interface. In the LE phase, the DPPC hydrocarbon chains are conformationally disordered with a significant number of gauche configurations. In the LC phase, the hydrocarbon chains are in an all-trans conformation and are tilted from the surface normal by 25 degrees. In addition, the orientations of the tail terminal methyl groups are found to remain nearly unchanged with the variation of surface area. Qualitative analysis of the BBSFG spectra of the choline methyl groups suggests that these methyl groups are tilted but lie somewhat parallel to the surface plane in both the LE and LC phases. The dehydration of the phosphate headgroup due to the LE-LC phase transition is observed through the frequency blue shift of the phosphate symmetric stretch in the fingerprint region. In addition, implications for lung surfactant function from this work are discussed.

Conformation of the synaptobrevin transmembrane domain

The synaptic vesicle protein synaptobrevin (also called VAMP, vesicle-associated membrane protein) forms part of the SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex, which is essential for vesicle fusion. Additionally, the synaptobrevin transmembrane domain can promote lipid mixing independently of complex formation. Here, the conformation of the transmembrane domain was studied by using circular dichroism and attenuated total reflection Fourier-transform infrared spectroscopy. The synaptobrevin transmembrane domain has an alpha-helical structure that breaks in the juxtamembrane region, leaving the cytoplasmic domain unstructured. In phospholipid bilayers, infrared dichroism data indicate that the transmembrane domain adopts a 36 degrees angle with respect to the membrane normal, similar to that reported for viral fusion peptides. A conserved aromatic/basic motif in the juxtamembrane region may be causing this relatively high insertion angle.

Identification of Lipid Aggregate Structures on TiO2 Surface Using Headgroup IR Bands

Attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy was used to study the nature of the dipalmitoylphosphatidylcholine (DPPC) aggregated structures adsorbed on TiO(2). DPPC molecules were assembled on TiO(2) using Langmuir-Blodgett (LB) deposition methods or by directly flowing the DPPC liposome solution across the TiO(2)-coated ATR crystal. We found that there is a direct correlation between the intensity and frequency position of the zwitterionic headgroup IR bands and the nature of LB films. Specifically, we have shown that the bands due to PO(2)(-) modes are sensitive to changes in the degree of hydration of the LB films and that the symmetric deformation vibrational mode (delta(s) (+)N-CH(3)) is sensitive to interaction with oppositely charged surface sites. Using this information, we found that the liposomes adsorbed on TiO(2) remain intact as vesicles and that the vesicles are stable and not removed in flowing water. We have also shown that the antisymmetric deformation vibrational (delta(as) (+)N-CH(3)) modes are sensitive to changes in lateral-lateral DPPC interactions. This information was used to show that there is a lateral interaction between each positively charged (+)N(CH(3))(3) headgroup and negatively charged PO(2)(-) headgroup of the adjacent DPPC molecule in the adsorbed vesicles and LB films. This study provides a framework for the use of this IR technique in studies of adsorption and transport of molecules across membrane interfaces.

The interaction of the penetration enhancer DDAIP with a phospholipid model membrane

Differential scanning calorimetry (DSC) was used to investigate the mechanism of action of a proprietary skin penetration enhancer, dodecyl-2-(N,N-dimethylamino)propionate (DDAIP) in dipalmitoylphosphatidylcholine (DPPC) liposomes. Furthermore, the effect of enhancer concentration on lipid thermotropic transitions was investigated. With increasing concentrations of DDAIP (from 5 to 50 mol%), the main transition peak shifted to lower temperatures and became more broad. The pretransition peak also shifted to lower temperatures with increasing concentrations of DDAIP and disappeared completely above an enhancer concentration of 20 mol%. Main transition and pretransition enthalpies of reaction decreased with increasing DDAIP concentration, indicating that enhancer treatment destabilized both rippled gel and liquid crystal phases within the bilayer. At and above a DDAIP concentration of 33.3 mol%, an additional transition was evident, indicating the presence of two phases of enhancer-lipid complex. Results suggest that DDAIP enhances drug transport by interacting with the polar region of the phospholipid bilayer and also by increasing the motional freedom of lipid hydrocarbon chains.

Orientation and dynamics of an antimicrobial peptide in the lipid bilayer by solid-state NMR spectroscopy

The orientation and dynamics of an 18-residue antimicrobial peptide, ovispirin, has been investigated using solid-state NMR spectroscopy. Ovispirin is a cathelicidin-like model peptide (NH(2)-KNLRRIIRKIIHIIKKYG-COOH) with potent, broad-spectrum bactericidal activity. (15)N NMR spectra of oriented ovispirin reconstituted into synthetic phospholipids show that the helical peptide is predominantly oriented in the plane of the lipid bilayer, except for a small portion of the helix, possibly at the C-terminus, which deviates from the surface orientation. This suggests differential insertion of the peptide backbone into the lipid bilayer. (15)N spectra of both oriented and unoriented peptides show a reduced (15)N chemical shift anisotropy at room temperature compared with that of rigid proteins, indicating that the peptide undergoes uniaxial rotational diffusion around the bilayer normal with correlation times shorter than 10(-4) s. This motion is frozen below the gel-to-liquid crystalline transition temperature of the lipids. Ovispirin interacts strongly with the lipid bilayer, as manifested by the significantly reduced (2)H quadrupolar splittings of perdeuterated palmitoyloleoylphosphatidylcholine acyl chains upon peptide binding. Therefore, ovispirin is a curved helix residing in the membrane-water interface that executes rapid uniaxial rotation. These structural and dynamic features are important for understanding the antimicrobial function of this peptide.

Interaction of Zn2+ with phospholipid membranes

To characterize the specificity of zinc binding to phospholipid membranes in terms of headgroup structure, hydration and phase behavior we studied the zwitterionic lipid 1-palmitoyl-2-oleoyl-phosphatidylcholine as a function of hydration at 30 degreesC in the presence and absence of ZnCl2. Zinc forms a 2:1-1:1 complex with the lipid, and in particular with the negatively charged phosphate groups. Zn2(+)-bridges between neighboring lipid molecules stabilize the gel phase of the lipid relative to the liquid-crystalline state. Upon Zn2+ binding the C-O-P-O-C- backbone of the lipid headgroup changes from a gauche/gauche into the trans/trans conformation and it loses roughly 50% of the hydration shell. The ability of the Zn2(+)-bound phosphate groups to take up water is distinctly reduced, meaning that the headgroups have become less hydrophilic. The energetic cost (on the scale of Gibbs free energy) for completely dehydrating the lipid headgroups is decreased by approximately 10 kJ/mole in the presence of Zn2+. The interaction of phospholipid headgroups with Zn2+ is conveniently described by a hydrated zinc-phosphate complex the key energy contribution of which is more covalent than electrostatic in nature. Dehydration of phospholipid headgroups due to complexation with zinc cations is suggested to increase fusogenic potency of lipid membranes. Zinc appears to be one of the most potent divalent cation in inducing membrane fusion.

Conformational mapping of the N-terminal segment of surfactant protein B in lipid using 13C-enhanced Fourier transform infrared spectroscopy

Synthetic peptides based on the N-terminal domain of human surfactant protein B (SP-B1-25; 25 amino acid residues; NH2-FPIPLPYCWLCRALIKRIQAMIPKG) retain important lung activities of the full-length, 79-residue protein. Here, we used physical techniques to examine the secondary conformation of SP-B1-25 in aqueous, lipid and structure-promoting environments. Circular dichroism and conventional, 12C-Fourier transform infrared (FTIR) spectroscopy each indicated a predominate alpha-helical conformation for SP-B1-25 in phosphate-buffered saline, liposomes of 1-palmitoyl-2-oleoyl phosphatidylglycerol and the structure-promoting solvent hexafluoroisopropanol; FTIR spectra also showed significant beta- and random conformations for peptide in these three environments. In further experiments designed to map secondary structure to specific residues, isotope-enhanced FTIR spectroscopy was performed with 1-palmitoyl-2-oleoyl phosphatidylglycerol liposomes and a suite of SP-B1-25 peptides labeled with 13C-carbonyl groups at either single or multiple sites. Combining these 13C-enhanced FTIR results with energy minimizations and molecular simulations indicated the following model for SP-B1-25 in 1-palmitoyl-2-oleoyl phosphatidylglycerol: beta-sheet (residues 1-6), alpha-helix (residues 8-22) and random (residues 23-25) conformations. Analogous structural motifs are observed in the corresponding homologous N-terminal regions of several proteins that also share the 'saposin-like' (i.e. 5-helix bundle) folding pattern of full-length, human SP-B. In future studies, 13C-enhanced FTIR spectroscopy and energy minimizations may be of general use in defining backbone conformations at amino acid resolution, particularly for peptides or proteins in membrane environments.

Hydration of phosphatidyl serine multilayers and its modulation by conformational change induced by correlated electrostatic interaction

Hydration of the various residues of phospholipids was inferred from the shift in the wave number of their vibration bands, obtained from the amplitudes of their positive and negative peaks in the difference spectra between those of the hydrated and the dry phospholipid multibilayers. The effect of aligned phospholipid layers on the orientation of their hydrating water molecules was inferred from the dichroic ratio of the OH stretching band, measured by polarized attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with a germanium prism, as a function of the water-to-lipid ratio in the surface film. The results indicate that about seven water molecules are oriented by one phosphatidyl serine molecule in the surface film. About 8 to 11 additional water molecules contribute to the hydration of the polar residues as revealed by the effect on the difference spectra. The hydration appears to be cooperative. A water molecule that initiates hydration of a site facilitates access of additional water molecules, until the hydration of the whole site composed of many different interacting polar residues is completed.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Hydration of the dienic lipid dioctadecadienoylphosphatidylcholine in the lamellar phase--an infrared linear dichroism and x-ray study on headgroup orientation, water ordering, and bilayer dimensions

In the phospholipid 1,2-bis(2,4-octadecadienoyl)-sn-glycero-3-phosphorylcholine (DODPC) in each of the fatty acid chains, a rigid diene group is inserted in a position near the polar/apolar boundary that is exceptionally sensitive for membrane stability. DODPC transforms upon gradual dehydration from the liquid-crystalline to a metastable gel state, which rearranges into two subgel phases at low and intermediate degrees of hydration. The molecular dimensions of the respective bilayers were determined by means of x-ray diffraction. Infrared linear dichroism of selected vibrations of the phosphate and trimethylammonium groups and of the nu13(OH) band of water adsorbed onto the lipid was used to study the molecular order in the polar part of the bilayers in macroscopically oriented samples. The dense packing of the tilted acyl chains in the subgel causes the in-plane orientation of the phosphatidylcholine headgroups with direct interactions between the phosphate and trimethylammonium groups, and a strong orientation of adsorbed water molecules. In the more disordered gel, the thickness of the polar part of the bilayer increases and the lateral interactions between the lipid headgroups weaken. The higher order in the headgroup region of the subgels correlates with shorter decay lengths of the repulsive forces acting between opposite membrane surfaces. This result can be understood if the work to dehydrate the lipid is determined to a certain degree by the work to break up the lipid-water interactions without compensation by adequate lipid-lipid contacts. Almost similar area compressibility moduli are found in the liquid-crystalline and solid phases. Obviously, the lipid avoids lateral stress by the structural rearrangement.

Order in phospholipid Langmuir-Blodgett monolayers determined by total internal reflection fluorescence

Orientational order parameters of two diphenylhexatriene (DPH)-based fluorescent probes, 2-(3-(diphenylhexatrienyl)propanoyl)-1-hexadecanoyl-sn-glycero-3-p hosphocholine (DPHpPC) and 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH), in dipalmitoylphosphatidylcholine (DPPC) Langmuir-Blodgett monolayers on quartz have been determined by total internal reflection fluorescence (TIRF). From these order parameters orientation distributions were reconstructed by the maximum-entropy method. For monolayers transferred from the liquid-condensed phase, preferential tilt angles with respect to the substrate normal around 14 degrees in the tail region and 5 degrees near the glycerol-acyl chain linkage were found, as reflected by the DPHpPC and TMA-DPH probes, respectively. The degree of ordering near the headgroup region seems to be larger than that further away from the surface. A substantial fraction of the TMA-DPH probes have a flat orientation and are probably located between the phospholipid headgroups and the substrate surface. Monolayers transferred from the liquid-expanded phase show a more random ordering, and most of the probe molecules (DPHpPC) are more or less flat on the surface. The results are consistent with earlier atomic force microscopy measurements on identical monolayers and are in reasonable agreement with previously published data on other organized phospholipid systems.

Differences in the physical properties of lipid monolayers and bilayers on a spherical solid support

A monolayer of 1,2-dipalmitoyl-d62-glycero-3-phosphocholine (DPPC-d62) coated onto silanized silica beads (spherical supported monolayer: SSM) is studied by 2H-NMR and DSC. The results are compared with those obtained from a single bilayer on the same solid support (spherical supported vesicles: SSV) and from multilamellar vesicles (MLV). The phase transition temperature (Tm) of the SSMs is significantly higher than that of the bilayer systems and the extent of this difference depends on the lipid density in the monolayer that is determined during its preparation. 2H-NMR reveals a gel and fluid phase coexistence in the SSM transition region. A comparison of the 2H-NMR line shapes suggests the presence of highly curved structures for the fluid phase of the SSM samples. From a comparison of SSM and SSV transverse relaxation in the fluid phase we can conclude that the lateral diffusion coefficient D1 in supported monolayers is similar to that in bilayers.

Oriented 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine/ganglioside membranes: a Fourier transform infrared attenuated total reflection spectroscopic study. Band assignments; orientational, hydrational, and phase behavior; and effects of Ca2+ binding

Fourier transform infrared (FTIR) attenuated total reflection (ATR) spectroscopy was used to elucidate the hydration behavior and molecular order of phospholipid/ganglioside bilayers. We examined dry and hydrated films of the gangliosides GM1, deacetyl-GM1, lyso-GM1, deacetyllyso-GM1, and GM3 and oriented mixed films of these gangliosides with 1,2-dimyristoyl-sn-glycero-3-phosphorylcholine (DMPC) using polarized light. Analysis of the amide I frequencies reveals that the amide groups are involved in intermolecular interactions via hydrogen bonds of varying strengths. The tilt angle of the acyl chains of the lipids in mixed films was determined as a function of ganglioside structure. Deacetylation of the sialic acid in the headgroup has a stronger influence on the tilt angle than the removal of the ganglioside fatty acid. The phase behavior was examined by FTIR ATR spectroscopy and by differential scanning calorimetry (DSC) measurements on lipid suspensions. At the same molar concentration, lyso-gangliosides have less effect on changes of transition temperature compared to the double-chain analogs. Distinct differences in the amide band shapes were observed between mixtures with lyso-gangliosides and normal double-chain gangliosides. Determined from the dicroic ratio RATR, the orientation of the COO- group in all DMPC/ganglioside mixtures was found to be relatively fixed with respect to the membrane normal. In 4:1 mixtures of DMPC with GM1 and deacetyl-GM1, the binding of Ca2+ leads to a slight decrease in chain tilt in the gel phase, probably caused by a dehydration of the membrane-water interface. In mixtures of DMPC with GM3 and deacetyl-lyso-GM1, a slight increase in chain tilt is observed. The chain tilt in DMPC/lyso-GM1 mixtures is unchanged. Analysis of the COO- band reveals that Ca2+ does not bind to the carboxylate group of the sialic acid of GM1 and deacetyl-GM1, the mixtures in which a decrease in chain tilt was observed. Binding to the sialic acid was only observed for mixtures of DMPC with GM3, lyso-GM1, and deacetyl-lyso-GM1. Ca2+ obviously accumulates at the bilayer-water interface and leads to partial dehydration of the headgroup region in the gel as well as in the liquid-crystalline phase. This can be concluded from the changes in the amide I band shapes. With the exception of DMPC/deacetyl-GM1, the effects on the ester C==O bands are small. The addition of Ca2+ has minor effects on the phase behavior, with the exception of the DMPC/GM1 mixture.

Conformation and molecular topography of the N-terminal segment of surfactant protein B in structure-promoting environments

Although the effects of surfactant protein B (SP-B) on lipid surface activity in vitro and in vivo are well known, the relationship between molecular structure and function is still not fully understood. To further characterize protein structure-activity correlations, we have used physical techniques to study conformation, orientation, and molecular topography of N-terminal SP-B peptides in lipids and structure-promoting environments. Fourier transform infrared (FTIR) and CD measurements of SP-B1-25 (residues 1-25) in methanol, SDS micelles, egg yolk lecithin (EYL) liposomes, and surfactant lipids indicate the peptide has a dominant helical content, with minor turn and disordered components. Polarized FTIR studies of SP-B1-25 indicate the long molecular axis lies at an oblique angle to the surface of lipid films. Truncated peptides were similarly examined to assign more accurately the discrete conformations within the SP-B1-25 sequence. Residues Cys-8-Gly-25 are largely alpha-helix in methanol, whereas the N-terminal segment Phe-1-Cys-8 had turn and helical propensities. Addition of SP-B1-25 spin-labeled at the N-terminal Phe (i.e., SP-B1-25) to SDS, EYL, or surfactant lipids yielded electron spin resonance spectra that reflect peptide bound to lipids, but retaining considerable mobility. The absence of characteristic radical broadening indicates that SP-B1-25 is minimally aggregated when it interacts with these lipids. Further, the high polarity of SP-B1-25 argues that the reporter on Phe-1 resides in the headgroup of the lipid dispersions. The blue-shift in the endogenous fluorescence of Trp-9 near the N-terminus of SP-B1-25 suggests that this residue also lies near the lipid headgroup. A summary model based on the above physical experiments is presented for SP-B1-25 interacting with lipids.

Orientational order determination by internal reflection infrared spectroscopy

PATIR-FTIR spectroscopy is a powerful technique for the determination of molecular order in thin films such as supported lipid membranes, but it relies on electromagnetic theory which is incomplete and potentially misleading. A complete derivation of the current theory for two, three and four phase system has been reported. The two phase and thin film approximations most commonly used in practice have been shown to represent the thickness-dependent expressions from which they are derived with a high degree of accuracy. However, these expressions are based on the macroscopic behavior of dielectric materials, and may not be accurate when applied to microscopic circumstances. The potential error introduced is qualitatively and quantitatively significant. Further experimental and theoretical work is needed to verify the accuracy of this theory, or to refute and refine it. This effort to do this is warranted by the power and increasing popularity of the technique.

Lipid transfer between small unilamellar vesicles and single bilayers on a solid support: self-assembly of supported bilayers with asymmetric lipid distribution

The transfer of lipids between small unilamellar vesicles of either dimyristoylphosphatidylcholine (DMPC), dimyristoylphosphatidylglycerol (DMPG), or dioctadecyl diammonium bromide (DODAB) and a single bilayer on a solid support of chain perdeuterated DMPC-d54 has been studied by time-resolved ATR infrared spectroscopy, deuterium NMR, and DSC. The IR method was used for measuring the transfer kinetics and the amount of lipid transferred to the supported bilayer, while NMR was employed for the assessment of molecular order and for the occurrence of lipid asymmetries due to the transfer. We find that the composition of a supported planar dimyristoylphosphatidylcholine (DMPC-d54) bilayer can be modified by incubation with high concentrations of sonicated vesicles consisting of the donor lipid. Three cases were studied. First, the incubation was done with DMPC as donor lipid. The kinetics of this process is double exponential and comparatively slow, with a half-time in the range of several hours. The activation energy was estimated as 50 +/- 2 kJ/mol. In a second set of measurements, cationic DODAB or anionic DMPG was used as donor lipid. The kinetics of this transfer is 1 order of magnitude faster than for DMPC and can be described by a single exponential. For DMPG transfer, we obtained an activation energy of 35 +/- 2 kJ/mol. Independent of the headgroup charge of the donor lipid, 25-35% of the (acceptor) DMPC in the supported bilayer is not accessible for exchange with the donor lipid. The transfer of either DMPG or DODAB causes drastic changes of the phase transition behavior of the supported bilayer without significantly altering the lipid packing density of the lipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Local anesthetics destabilize lipid membranes by breaking hydration shell: infrared and calorimetry studies

Differential scanning calorimetry (DSC) showed that local anesthetics decreased the pretransition (L beta'-->P beta') temperature of dipalmitoylphosphatidylcholine (DPPC) vesicle membranes four- to five-fold more than the main transition (P beta'-->L alpha) temperature. Because pretransition is mainly a change in the hydrophilic head property (tilted-rippled), the stronger effect on the pretransition suggests that the primary action site of local anesthetics is the lipid-water interface. The interfacial effect was analyzed by Fourier-transform infrared spectroscopy (FTIR) in water-in-oil (CCl4) reversed micelles. FTIR showed that the local anesthetics released hydrogen-bonded water molecules from the phosphate (P = O bands) and glycerol (sn-2 C = O) moieties. The N-H stretching band of the local anesthetics was deconvoluted into two bands: hydrogen bonded to the phosphate moiety of the lipid and free (unbound to lipid). The formation constants between lipid P = O and anesthetic N-H were estimated in CCl4 from the spectral changes: 110 M-1 for lidocaine and 250 M-1 for dibucaine. This small difference in the formation constants cannot explain the ten-fold stronger effect on the phase-transition temperature of dibucaine over lidocaine. By comparing the local anesthetic adsorption to the air/water interface in the presence and absence of lipid monolayers, we have previously shown (Lin et al. (1980) Biochim. Biophys. Acta 598, 51-65) that lipid-anesthetics interaction involves three forces: lipophilic effect, hydrophobic effect, and anesthetic-anesthetic interaction. The anesthetic potency depends mainly on the hydrophobic effect (the difference in the standard molar free energies of local anesthetics in water and at the interface) and anesthetic-anesthetic interaction energy. The anesthetic-anesthetic interaction means cooperativity of local anesthetics for the interfacial density: local anesthetics condense at the membrane surface when there are enough anesthetic molecules present at the interface to attract more anesthetics. The present data suggest that anesthetic action is directed to the interface between water and macromolecule, whether it is lipid membranes or proteins.

Role of disulfide linkages in tachyplesin-lipid interactions

In order to elucidate the role of the two disulfide linkages of tachyplesin I (T-SS), a membrane-acting cyclic antimicrobial peptide from Tachypleus tridentatus, we synthesized the acyclic analog (T-Acm) with the four SH groups protected by acetamidomethyl groups and also investigated the interactions of these peptides with lipid bilayers. T-SS induced leakage of calcein from egg yolk L-alpha-phosphatidylglycerol large unilamellar vesicles (PG LUVs) at peptide concentrations 1 order of magnitude smaller than those at which leakage was induced by T-Acm, which coincides with the stronger antimicrobial activities of T-SS. The micellization of PG LUVs was also more efficient for the cyclic peptide. Fluorescence titration studies revealed that binding affinities of both peptides to the PG membranes were similar. Fourier transform infrared polarized attenuated total reflection spectroscopy and fluorescence quenching experiments demonstrated that T-SS and T-Acm both form amphiphilic antiparallel beta-sheet structures in the membranes. They are formed in such a way that the sheet planes lie parallel to the membrane surface with the sheet hydrophobic surfaces penetrating slightly into the hydrophobic region of the bilayers. Furthermore, the observation that the linear T-Acm, the weaker membrane permeabilizer, caused a far more serious membrane disruption suggests the possibility that the mechanisms of membrane permeabilization by the cyclic peptide are different from those by the linear peptide, the latter being the disruption of the lipid organization.

Interaction of myelin basic protein with single bilayers on a solid support: an NMR, DSC and polarized infrared ATR study

The interaction of myelin basic protein (MBP) with single bilayers on a solid support (planar and spherical support) is studied by deuterium nuclear magnetic resonance (2H-NMR), differential scanning calorimetry (DSC) and polarized attenuated total reflection infrared spectroscopy (ATR-IR). The single bilayer consisted of either DMPC or of a binary mixture of DMPC with 10-20 mol% of an acidic phospholipid (DMPG, DMPS or DMPA). All methods applied indicate that MBP strongly interacts with the binary lipid systems but not with the pure DMPC bilayers. The interaction is predominantly electrostatic in nature and does not depend on the choice of a particular acidic lipid (for the binary systems). In particular, the results give no indication for a hydrophobic interaction of MBP with the membrane. Our data provide evidence that, in contrast to previous findings, no demixing and/or domain formation in the binary systems is induced due to the MBP coupling. The infrared order parameter was determined for both lipid components of the binary systems and shows a remarkable change for both lipids due to the interaction with MBP while the NMR order parameter remained essentially unchanged. This is discussed in terms of the different timescales characteristic for both methods. The single supported bilayer responds to the MBP coupling as a whole although only 50% of the bilayer surface is accessible to the protein, indicating a strong coupling between the two bilayer leaflets via the hydrophobic chain region. Moreover, the asymmetric coupling of MBP to the single supported bilayer does not result in a significant redistribution of lipids between the two bilayer leaflets. NMR relaxation time measurements in the headgroup and chain region of DMPG and DMPC suggest that the lateral diffusion coefficient of the acidic lipid decreases significantly due to the coupling with MBP while the zwitterionic DMPC is not affected.

A comparative study on interactions of alpha-aminoisobutyric acid containing antibiotic peptides, trichopolyn I and hypelcin A with phosphatidylcholine bilayers

Interactions of alpha-aminoisobutyric acid containing antibiotic peptides, trichopolyn I and hypelcin A with phosphatidylcholine bilayers were investigated to obtain some basic information on their bioactive mechanisms. Trichopolyn I as well as hypelcin A induced the leakage of a fluorescent dye, calcein, entrapped in sonicated egg yolk L-alpha-phosphatidylcholine vesicles. A quantitative analysis revealed that both the binding affinity and the 'membrane-perturbing activity' of trichopolyn I to the vesicles are about one-third of those of hypelcin A. The conformations and the orientations of the peptide and lipid molecules in the membranes were studied using polarized Fourier transform infrared-attenuated total reflection spectroscopy, circular dichroism, and differential scanning calorimetry. In phosphatidylcholine bilayers, both peptides mainly conformed to helical structures irrespective of the membrane physical state (gel or liquid-crystalline). The helix axes, penetrating the hydrophobic region of the bilayers, were oriented neither parallel nor perpendicular to the membrane normal. The disruption in the lipid packing induced by the peptide insertion seems to be responsible for the leakage by these peptides.

Polarized infrared attenuated total reflectance spectroscopy of the Ca(2+)-ATPase of sarcoplasmic reticulum

The mean orientations of the transition dipole moments associated with vibrational modes of the proteins and phospholipids of sarcoplasmic reticulum were determined on dry and hydrated membrane multilayers deposited on germanium or zinc selenide crystals, using polarized infrared attenuated total reflectance spectroscopy (P-IR-ATR). For preservation of the enzymatic activity of the Ca(2+)-ATPase the films were prepared from solutions containing 0.05 M KCl, 5 mM imidazole (pH 7.4), 0.5 mM MgCl2, 1-10 mM trehalose and dithiothreitol. The anisotropy was highest in dry films containing congruent to 7.5 micrograms protein/cm2, and decreased with increasing membrane thickness or hydration. The dichroic ratio of the CH2 vibrations (2923 cm-1) of extracted sarcoplasmic reticulum phospholipids on Ge plate was 1.56, compared with a dichroic ratio of 1.68 obtained on dry films of whole sarcoplasmic reticulum. The dichroic ratios of the amide I band (1650 cm-1) of the Ca(2+)-ATPase in the Ca2-E1 state and in the EGTA and vanadate stabilized E2-V state were nearly identical (1.60 vs. 1.62). The dichroism of the amide I, amide II and lipid CH2 vibrations was not affected by changes in the concentration of KCl (25-100 mM) or Ca2+ (approximately equal to 10(-8)-10(-4) M) and by the addition of vanadate (1 mM) or Pi (5 mM) in a calcium-free medium containing 0.5 mM EGTA. The dichroic ratio of the C-C (1033 cm-1) or CO stretching band (1046 cm-1) of trehalose incorporated into SR films was 1.2 on Ge plate; this corresponds to a mean angle of approximately 70 degrees between the plane of the trehalose ring and the normal of the film plane, suggesting that the trehalose molecules are surprisingly well oriented in the polar headgroup region of the phospholipids. The orientation of the trehalose was not affected by the presence of Ca(2+)-ATPase.

Orientation of specifically 13C=O labeled phosphatidylcholine multilayers from polarized attenuated total reflection FT-IR spectroscopy

Oriented multilayers of 1-myristoyl-2(1-13C)-myristoyl-sn-glycero-3-phosphatidylcholine (2[1-13C]DMPC) and 1-palmitoyl-2(1-13C)-palmitoyl-sn-glycero-3-phosphatidylcholine (2[1-13C]DPPC) were investigated by use of attenuated total reflection infrared spectroscopy with polarized light. Experiments were performed with the aim to determine the orientation of the two ester groups in these phospholipids in the solid state and in the hydrated state at temperatures below and above the respective gel to liquid-crystalline phase transitions. Substitution of the naturally occurring 12C carbonyl carbon atom by 13C in the ester group of the sn-2 chain of DMPC and DPPC shifts the infrared absorption of the carbonyl double bond stretching vibration to lower frequency. This results in two well-resolved ester C=O bands which can be assigned unequivocally to the sn-1 and sn-2 chains as they are separated by more than 40 cm-1. The two ester CO-O single bond stretching vibrations of the molecular fragments-CH2CO-OC-are also affected and the corresponding infrared absorption band shifts by 20 cm-1 on 13C-labeling of the carbonyl carbon atom. From the dichroic ratios of the individual ester bands in 2(1-13C)DMPC and 2(1-13C)DPPC we were able to demonstrate that the sn-1 and sn-2 ester C=O groups are similarly oriented with respect to the bilayer plane, with an angle greater than or equal to 60 degrees relative to the bilayer normal. The two CO-O single bonds on the other hand have very different orientations. The CH2CO-OC fragment of the sn-1 chain is oriented along the direction of the all-trans methylene chain, whereas the same molecular segment of the sn-2 carbon chain is directed toward the bilayer plane. This orientation of the ester groups is retained in the liquid-crystalline phase. The tilt angle of the hydrocarbon all-trans chains, relative to the membrane normal, is 25 degrees in the solid state of DMPC and DPPC multibilayers. In the hydrated gel state this angle varies between 26 degrees and 30 degrees, depending on temperature. Neither the orientation of the phosphate group, nor that of the choline group varies significantly in the different physical states of these phospholipids.