Natural Biomembranes Research Articles

The presence of free palmitic acid modulates the effects of lutein on structural and dynamic properties of lipid membranes

Free fatty acids, like palmitic acid (PA), and xanthophyll pigments, like lutein (LUT) are the natural membrane compounds in plants. To study the effect of PA on LUT and their organization, a model membrane of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine (DMPC) enriched with 2 mol% PA and 1 mol% LUT was formed. Molecular mechanisms underlying the interaction between these two compounds were examined with application of molecular spectroscopy techniques, e.g., visible spectroscopy, electron paramagnetic resonance and Fourier transform infrared. We determined the monomeric/dimeric organization of LUT in the membrane. We proved that the presence of PA in the lipid phase facilitated and stabilized the formation of LUT structures in the membrane. Lutein with PA did not form strong molecular aggregates like H- and J-structures. We presented the simplified model membrane that could be a suitable representation of the physiological process of de-esterification of PA from LUT appearing in natural biomembranes in humans.

Biomembrane-based nanostructures for cancer targeting and therapy: From synthetic liposomes to natural biomembranes and membrane-vesicles.

The translational success of liposomes in chemotherapeutics has already demonstrated the great potential of biomembrane-based nanostructure in effective drug delivery. Meanwhile, increasing efforts are being dedicated to the application of naturally derived lipid membranes, including cellular membranes and extracellular vesicles in anti-cancer therapies. While synthetic liposomes support superior multifunctional flexibility, natural biomembrane materials possess interesting biomimetic properties and can also be further engineered for intelligent design. Despite being remarkably different from each other in production and composition, the phospholipid bilayer structure in common allows liposomes, cell membrane-derived nanomaterials, and extracellular vesicles to be modified, functionalized, and exploited in many similar manners against challenges posed by tumor-targeted drug delivery. This review will summarize the recent advancements in engineering the membrane-derived nanostructures with "intelligent" modules to respond, regulate, and target tumor cells and the microenvironment to fight against malignancy. We will also discuss perspectives of combining engineered functionalities with naturally occurring activity for enhanced cancer therapy.

The influence of phosphatidylserine localisation and lipid phase on membrane remodelling by the ESCRT-II/ESCRT-III complex.

The endosomal sorting complex required for transport (ESCRT) organises in supramolecular structures on the surface of lipid bilayers to drive membrane invagination and scission of intraluminal vesicles (ILVs), a process also controlled by membrane mechanics. However, ESCRT association with the membrane is also mediated by electrostatic interactions with anionic phospholipids. Phospholipid distribution within natural biomembranes is inhomogeneous due to, for example, the formation of lipid rafts and curvature-driven lipid sorting. Here, we have used phase-separated giant unilamellar vesicles (GUVs) to investigate the link between phosphatidylserine (PS)-rich lipid domains and ESCRT activity. We employ GUVs composed of phase separating lipid mixtures, where unsaturated DOPS and saturated DPPS lipids are incorporated individually or simultaneously to enhance PS localisation in liquid disordered (Ld) and/or liquid ordered (Lo) domains, respectively. PS partitioning between the coexisting phases is confirmed by a fluorescent AnnexinV probe. Ultimately, we find that ILV generation promoted by ESCRTs is significantly enhanced when PS lipids localise within Ld domains. However, the ILVs that form are rich in Lo lipids. We interpret this surprising observation as preferential recruitment of the Lo phase beneath the ESCRT complex due to its increased rigidity, where the Ld phase is favoured in the neck of the resultant buds to facilitate the high membrane curvature in these regions of the membrane during the ILV formation process. Ld domains offer lower resistance to membrane bending, demonstrating a mechanism by which the composition and mechanics of membranes can be coupled to regulate the location and efficiency of ESCRT activity.

Aβ-Amyloid Fibrils Are Self-Triggered by the Interfacial Lipid Environment and Low Peptide Content.

We studied the surface properties of Aβ(1-40) amyloid peptides mixed with 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC) (liquid state) or 1,2-disteraoyl-phosphatidylcholine (DSPC) (solid state) phospholipids by using nanostructured lipid/peptide films (Langmuir monolayers). Pure Aβ(1-40) amyloid peptides form insoluble monolayers without forming fibril-like structures. In a lipid environment [phospholipid/Aβ(1-40) peptide mixtures], we observed that both miscibility and stability of the films depend on the peptide content. At low Aβ(1-40) amyloid peptide proportion (from 2.5 to 10% of peptide area proportion), we observed the formation of a fibril-like structure when mixed only with POPC lipids. The stability acquired by these mixed films is within 20-35 mN·m-1 compatible with the equivalent surface pressure postulated for natural biomembranes. Fibrils are clearly evidenced directly from the monolayers by using Brewster angle microscopy. The so-called nanostructured fibrils are thioflavin T positive when observed by fluorescence microscopy. The amyloid fibril network at the surface was also evidenced by atomic force microscopy when the films are transferred onto a mica support. Aβ(1-40) amyloid mixed with the solid DSPC lipid showed an immiscible behavior in all peptide proportions without fibril formation. We postulated that the amyloid fibrillogenesis at the membrane can be dynamically nano-self-triggered at the surface by the quality of the interfacial environment, that is, the physical state of the water-lipid interface and the relative content of amyloid protein present at the interface.

Izozte-urtze ziklo bidezko mintz-ereduen fusioa

Mintz ereduak, bai zelularen bai zelulako organuluen arkitektura imita dezaketenez, erabilgarriak izan daitezke proteina-lipido elkarrekintzak, geruza bikoitzean txertatutako proteinen egiturak edo mintzen dinamika ikertzeko. Gainera, mintz ereduen diseinu kontrolatu bat egiteko aukera dagoenez, konposizio fisiko-kimiko eta tamaina jakineko egitura lipidikoak sor daitezke. Horrela, bada, mintz eredu hauek izan ditzaketen erabilerak gero eta ugariagoak izaten ari dira; horien artean ditugu, hain zuzen ere, fusio prozesua ikertzeko entseguak. Jakina da zeluletan fusioa oso prozesu garrantzitsua dela; espermatozoideek egiten duten obuluaren ernalketa, exozitosi konstitutibo eta arautua edo organuluen biogenesia, esaterako, fusio prozesuetan oinarritzen dira. Ikerketa honetan, izozte-urtze ziklo bidezko mintz ereduen fusio kontrolatu bat egiteko teknikaren efikazia neurtu da. tamaina (SUV, LUV eta MLV) eta konposizio molekular ezberdineko (fosfatidilkolina edota fosfatidilkolina, esfingomielina, fosfatidiletanolamina eta kolesterol kantitate ezberdineko) mintz ereduak erabili dira. Fusio maila erresonantzia bidezko energia transferentzia (FRET) izeneko teknikaz neurtu da, horretarako NBD-PE eta Rho-PE markatzaile fluoreszenteak mintz ereduetan txertatuz. Horrela, bada, ikerketa honetan honako hauek ikertu dira: saiakuntza mota hauetan erabili beharko liratekeen mintz eredu eta lipido mota egokienak, markatzaile fluoreszente kantitate eta izozte-urtze ziklo kopuru optimoak, eta fusio osteko egitura lipidikoen morfologia. Azkenik, teknika hau GUV deritzen mintz ereduak sortzeko baliagarria izan daitekeela frogatu da.

A Mechanism of the Interaction of Metal Oxide Nanoparticles with Biological Membranes

Abstract—Advances in nanotechnologies and artificial methods for the synthesis of metal oxide nanoparticles with sizes of 1–100 nm raise the issue of their biological safety. Nanoparticles with a size of less than 10 nm are capable of penetrating into the bloodstream, followed by their entry into various organs, such as the brain, liver, kidneys, lungs, and spleen. Nanoparticles are a pathogenic exogenous factor that disturbs the structure and functions of natural biomembranes. We have previously shown using in vitro studies that nanoparticles with a diameter below a critical value of Rcr permeate the membrane. As a result, the microviscosity of the lipid bilayer of biomembranes decreases. This process is accompanied by the formation of through pores, which can combine into cracks and destroy the membrane. If the radius of the nanoparticles is greater than a critical value they are adsorbed on the membrane surface, thus increasing their microviscosity. In both cases, nanoparticles disrupt the normal functioning of natural biomembranes and cells. This paper presents a thermodynamic model of the direct (not due to endocytosis) penetration of nanoparticles into the lipid bilayer of the membrane. It has been shown that the Rcr value increases with an increase in the Zeta potential of the nanoparticles.

Optimization of the Inverted Emulsion Method for High-Yield Production of Biomimetic Giant Unilamellar Vesicles.

In the field of bottom‐up synthetic biology, lipid vesicles provide an important role in the construction of artificial cells. Giant unilamellar vesicles (GUVs), due to their membrane's similarity to natural biomembranes, have been widely used as cellular mimics. So far, several methods exist for the production of GUVs with the possibility to encapsulate biological macromolecules. The inverted emulsion‐based method is one such technique, which has great potential for rapid production of GUVs with high encapsulation efficiencies for large biomolecules. However, the lack of understanding of various parameters that affect production yields has resulted in sparse adaptation within the membrane and bottom‐up synthetic biology research communities. Here, we optimize various parameters of the inverted emulsion‐based method to maximize the production of GUVs. We demonstrate that the density difference between the emulsion droplets, oil phase, and the outer aqueous phase plays a crucial role in vesicle formation. We also investigated the impact that centrifugation speed/time, lipid concentration, pH, temperature, and emulsion droplet volume has on vesicle yield and size. Compared to conventional electroformation, our preparation method was not found to significantly alter the membrane mechanical properties. Finally, we optimize the parameters to minimize the time from workbench to microscope and in this way open up the possibility of time‐sensitive experiments. In conclusion, our findings will promote the usage of the inverted emulsion method for basic membrane biophysics studies as well as the development of GUVs for use as future artificial cells.

Thermo-chemical modification of a natural biomembrane to induce mucoadhesion, pH sensitivity and anisotropic mechanical properties.

In the present study due to the distinctive mechanochemical/biological characteristics of natural biomembranes, we state the preparation, characterization and cytocompatibility of modified eggshell membrane (ESM) by citric acid (CA) for biomedical and pharmaceutical applications. FTIR spectroscopy and CHNS analysis demonstrated the successful reaction of ESM with CA. Also, successful modification of the ESM was observed by the change in thermogravimetric analysis. SEM micrographs of neat ESM and ESM-CA gave further insight into membranes morphology and revealed that aligned oriented fibrous frameworks were prepared using thermo-chemical process. The ESM-CA displayed dense and orderly shapes with tailorable architectures to mimic the intended tissue. Moreover, mechanical analyzes for ESM-CA indicated anisotropic mechanical properties and proved that the ESM-CA could induce enhanced mucoadhesion, because of the existence of an enormous amounts of functional groups. It was found that by modification of ESM the swelling behavior was significantly changed. Indomethacin release from the ESM-CA showed enhanced pH sensitivity. The modified membranes have clearly presented adequate mucoadhesion, pH sensitivity and cell viability which can be tailored for potential use in controlled lipophilic drug delivery systems and tissue engineering.

Morphological and Nanomechanical Behavior of Supported Lipid Bilayers on Addition of Cationic Surfactants

The addition of surfactants to lipid bilayers is important for the modulation of lipid bilayer properties (e.g., in protein reconstitution and development of nonviral gene delivery vehicles) and to provide insight on the properties of natural biomembranes. In this work, the thermal behavior, organization, and nanomechanical stability of model cationic lipid-surfactant bilayers have been investigated. Two different cationic surfactants, hexadecyltrimethylammonium bromide (CTAB) and a novel derivative of the amino acid serine (Ser16TFAc), have been added (up to 50 mol %) to both liposomes and supported lipid bilayers (SLBs) composed by the zwitterionic phospholipid DPPC. The thermal phase behavior of mixed liposomes has been probed by differential scanning calorimetry (DSC), and the morphology and nanomechanical properties of mixed SLBs by atomic force microscopy-based force spectroscopy (AFM-FS). Although DSC thermograms show different results for the two mixed liposomes, when both are deposited on mica substrates similar trends on the morphology and the mechanical response of the lipid-surfactant bilayers are observed. DSC thermograms indicate microdomain formation in both systems, but while CTAB decreases the degree of organization on the liposome bilayer, Ser16TFAc ultimately induces the opposite effect. Regarding the AFM-FS studies, they show that microphase segregation occurs for these systems and that the effect is dependent on the surfactant content. In both SLB systems, different microdomains characterized by their height and breakthrough force Fb are formed. The molecular organization and composition is critically discussed in the light of our experimental results and literature data on similar lipid-surfactant systems.

Microscopy and Image Analysis

Microscopy and Image Analysis

Surface glycosylation of polymeric membranes

Surface glycosylation of polymeric membranes has been inspired by the structure of natural biomembranes. It refers to that glycosyl groups are introduced onto the membrane surface by various strategies, which combine the separation function of the membrane with the biological function of the saccharides in one system. In this review, progress in the surface glycosylation of polymeric membranes is highlighted in two aspects, i.e. the glycosylation methods and the potential applications of the surface-glycosylated membranes.

A Molecular Toolkit for Highly Insulating Tethered Bilayer Lipid Membranes on Various Substrates

Tethered bilayer lipid membranes (tBLMs) are promising model architectures that mimic the structure and function of natural biomembranes. They provide a fluid, stable, and electrically sealing platform for the study of membrane related processes, specifically, the function of incorporated membrane proteins. This paper presents a generic approach toward the synthesis of functional tBLMs adapted for application to various surfaces. The central element of a tethered membrane consists of a lipid bilayer. Its proximal layer is covalently attached via a spacer unit to a solid support, either gold or silicon oxide. The membranes are characterized optically by using surface plasmon resonance spectroscopy (SPR) or ellipsometry and electrically by using electrochemical impedance spectroscopy (EIS). The bilayer membranes obtained show high electrical barrier properties and can be used to incorporate and study small membrane proteins in a functional form.

A Multidimensional 1H NMR Investigation of the Conformation of Methionine-Enkephalin in Fast-Tumbling Bicelles

Enkephalins are pentapeptides found in the central nervous system. It is believed that these neuropeptides interact with the nerve cell membrane to adopt a conformation suitable for their binding to an opiate receptor. In this work, we have determined the three-dimensional structure of methionine-enkephalin (Menk) in fast-tumbling bicelles using multidimensional 1H NMR. Bicelles were selected as model membranes because both their bilayer organization and composition resemble those of natural biomembranes. The effect of the membrane composition on the peptide conformation was explored using both zwitterionic (PC bicelles) and negatively charged bicelles (Bic/PG). Pulsed field gradient experiments allowed the determination of the proportion of Menk bound to the model membranes. Approximately 60% of the water-soluble enkephalin was found to associate to the bicellar systems. Structure calculations from torsion angle and NOE-based distance constraints suggest the presence of both μ- and δ-selective conformers of Menk in each system and slightly different conformers in PC bicelles and Bic/PG. As opposed to previous studies of enkephalins in membrane mimetic systems, our results show that these opiate peptides could adopt several conformations in a membrane environment, which is consistent with the flexibility and poor selectivity of enkephalins.

‘New dimensions for manufacturing’ — A UK strategy for nanotechnology

This chapter discusses the nanotechnology and nano engineered membranes. Self-assembly of molecules or large groups of molecules is a key feature in nanotechnology. Nanotechnology deals with structures having at least 1D that is sufficiently small, of the order of about one to several hundred nanometers. It is concerned with materials and systems whose structures and components exhibit novel and improved physical, chemical, and biological properties. It enables the exploitation of novel phenomena and processes because of their nanoscale size. The chapter presents some examples of nano and micro engineered membrane technology and its use for nano engineered devices. A technique to pattern surfaces is to evaporate a target material through a thin membrane (shadow mask) with well-defined perforations. Photonic crystals are artificially three-dimensional structures fabricated in an optical material (crystal or amorphous) with unit cells whose dimensions are comparable to the optical wavelength. Nano and microsieves can be used as stencils for patterning, as photonic crystals, but they can also be used as a proper solid support for thin functional membranes. Nanotubes are the hollow cylindrical structures. The chapter discusses the nano printing and etching with phase separated membranes. Research goal in biomembrane science is to study the function of natural biomembranes and to engineer new types of biomembrane materials on the nanometer scale.

Submicron Structure in l- α-Dipalmitoylphosphatidylcholine Monolayers and Bilayers Probed with Confocal, Atomic Force, and Near-Field Microscopy

Langmuir-Blodgett (LB) monolayers and bilayers of l- α-dipalmitoylphosphatidylcholine (DPPC), fluorescently doped with 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (diIC 18), are studied by confocal microscopy, atomic force microscopy (AFM), and near-field scanning optical microscopy (NSOM). Beyond the resolution limit of confocal microscopy, both AFM and NSOM measurements of mica-supported lipid monolayers reveal small domains on the submicron scale. In the NSOM studies, simultaneous high-resolution fluorescence and topography measurements of these structures confirm that they arise from coexisting liquid condensed (LC) and liquid expanded (LE) lipid phases, and not defects in the monolayer. AFM studies of bilayers formed by a combination of LB dipping and Langmuir-Schaefer monolayer transfer exhibit complex surface topographies that reflect a convolution of the phase structure present in each of the individual monolayers. NSOM fluorescence measurements, however, are able to resolve the underlying lipid domains from each side of the bilayer and show that they are qualitatively similar to those observed in the monolayers. The observation of the small lipid domains in these bilayers is beyond the spatial resolving power of confocal microscopy and is complicated in the topography measurements taken with AFM, illustrating the utility of NSOM for these types of studies. The data suggest that the small LC and LE lipid domains are formed after lipid transfer to the substrate through a dewetting mechanism. The possible extension of these measurements to probing for lipid phase domains in natural biomembranes is discussed.

1064 nm FT-Raman spectra of hog kidney mitochondrial outer membranes: effects of NaCl and temperature on 1100 cm-1 region spectra

In order to obtain the experimental basis for the cause of the previously reported intensity decrease of the 1080 cm-1 band in the presence of NaCl, FT-Raman spectra were measured with 1064 nm excitation for aqueous dispersions of hog kidney mitochondrial outer membrane preparations in phosphate and Tris–HCl buffer (pH 7.5) in the absence and presence of NaCl. It was found that the relative intensity of the ∽1084 cm-1 Raman band with respect to the ∽1064 cm-1 band decreased considerably in the presence of NaCl in phosphate buffer and that the intensity decrease in the presence of NaCl was not appreciable in the spectrum in Tris–HCl buffer. It was also found that the presence of NaCl decreased the relative intensities of the ∽1126 cm-1 band with respect to the ∽1064 and ∽1134 cm-1 bands in both phosphate and Tris–HCl buffer. The spectral changes in the same region due to the temperature increase were studied for the membrane preparation of ultracentrifuged pellets for the elucidation of the vibrational assignments. It was found that the intensity ratio of the 1084 cm-1 band with respect to the ∽1064 cm-1 band was increased with a temperature increase from 8 to 20°C. An unexpected increase was observed in the relative intensity of the ∽1126 cm-1 band with respect to the ∽1064 cm-1 band with temperature elevation. It was concluded that the decrease in I1084/I1064 in phosphate buffer (pH 7.5) in the presence of NaCl could be attributed to the decrease in the gauche conformation of phospholipid acyl chains as predicted previously. The NaCl-caused decrease of the gauche conformation was found to be small in Tris–HCl buffer (pH 7.5). The increase in I∽1126/I∽1064 in the temperature increase experiment was interpreted in terms of a fairly large contribution of another chain conformation containing gauche bonds, the so-called almost fully extended form against the fully extended form, i.e. all-trans conformation, to the ∽1126 cm-1 Raman band in the fluid state, in which the usual natural biomembranes exist. The observed decrease in I∽1126/I∽1064 and I∽1126/I∽1134 in the presence of NaCl in both phosphate and Tris–HCl buffers could result from a decrease in this conformation in the acyl chains due to the interchain tighter packing induced by the non-specific screening effect on the membrane surface charges in the presence of NaCl. © 1997 John Wiley & Sons, Ltd.

A 13C-NMR study of 10,12-tricosadiynoic acid and the corresponding phospholipid and phospholipid polymer

Diacetylene phospholipids are presently being studied because of their potential to polymerise in vesicles, multilayers and natural biomembranes. 13C-NMR spectra and spin-lattice relaxation times have now been obtained of a diacetylene phospholipid present in a sonicated dispersion in water. Similar data have been obtained of a monoacetylene phospholipid and a saturated phospholipid. For further comparison the spectrum of a diacetylenic fatty acid in benzene-d 6 was also examined and relaxation data obtained. A comparison of the various relaxation data provides an indication of the restricted motion associated with the two conjugated triple bonds of the diacetylene phospholipid within the lipid bilayer structure. A proximity interaction between diacetylene groups occurs and a conformation for the diacetylene part of the lipid in the bilayer is deduced. The 13C-NMR spectrum of a soluble phospholipid polymer in C 2HCl 3, obtained by ultraviolet irradiation of the diacetylene phospholipid, shows that the two conjugated triple bonds of the monomer is replaced in the polymer by an alternating double and triple bonded conjugated structure.

The biosynthetic incorporation of diacetylenic fatty acids into the biomembranes of Acholeplasma laidlawii a cells and polymerisation of the biomembranes by irradiation with ultraviolet light

Acholeplasma laidlawii A has been grown in media containing synthetic, long chain C 20- and C 23-fatty acids possessing a diacetylene group in their acyl chains. Growth on the C 23 diacetylenic acid was poor but was good on the C 20 acid. Biosynthetic incorporation of the fatty acids occurs; as much as 90% of the membrane lipid fatty acyl chains consisting of the C 20-diacetylene fatty acid, the remainder being shorter chain, saturated fatty acids. The thermal phase transition of this biomembrane has been studied and a differential scanning calorimetry heating curve shows the presence of an endotherm corresponding to a membrane lipid phase transition occurring at about 26°C. The lipid class composition of membranes containing the C 20-diacetylene lipids was examined and found to be similar to membranes from cells grown on oleic acid-containing medium. (The ratio of monoglucosyl- to diglucosyldiacylglycerols was the same but the ratio of glycolipid to phosphatidylglycerol was higher in the cells grown with diacetylene fatty acids). Upon irradiation with ultraviolet light the cells and isolated biomembranes become coloured, either red or yellow depending upon their thermal history. The colour change indicates that extensive cross-linking of the lipids of the biomembranes of A. laidlawii has occurred and that a conjugated polymeric structure has been formed. Analysis of the extracted lipids from the biomembranes by GLC indicates that extensive cross-linking of the lipid chains within the biomembrane of a natural cell system has been achieved. The monoglucosyldiacylglycerols cross-link more readily that do the phosphatidylglycerol lipids. The effect of such lipid cross-linking or polymerisation on the activity at 35°C of an intrinsic membrane-bound enzyme, NADH oxidase, and ribonuclease, an extrinsic membrane-bound enzyme, was studied. The NADH oxidase activity decreased rapidly upon cross-linking of the lipid environment whereas ribonuclease activity was unaffected. The potential for future studies of polymerised model and natural biomembranes is discussed.

Intrinsic protein-lipid interactions: Infrared spectroscopic studies of gramicidin A, bacteriorhodopsin and Ca 2+-ATPase in biomembranes and reconstituted systems

Infrared spectroscopy has been applied to the study of a number of aqueous systems of model and natural biomembranes. The absorption bands arising from water and buffer solutions were eliminated by means of an infrared spectrometer data station. Spectra were examined using H 2O and 2H 2O aqueous buffer systems. Pure lecithin-water systems, and various model biomembranes containing cholesterol, gramicidin A, bacteriorhodopsin or Ca 2+-ATPase were examined. The infrared spectra of the reconstituted biomembranes were compared with those of the corresponding natural biomembranes, i.e. the purple membrane of Halobacterium halobium and also sarcoplasmic reticulum membranes, respectively. Changes in lipid chain conformation caused by the various intrinsic molecules incorporated within the model lipid bilayer structures were monitored by studying the shifts in frequency (cm −1) of the CH 2 symmetric and asymmetric absorption bands arising from the lipid chains. The effect of gramicidin A and also the intrinsic proteins, as indicated by the shift of band frequencies, are quite different from that of cholesterol at temperatures above the main lipid transition temperature t c. Cholesterol causes a reduction in gauche isomers which increases with concentration of cholesterol within the lipid bilayer. Whilst gramicidin A and the intrinsic proteins at low concentration cause a reduction of gauche isomers, at higher concentrations of these molecules, however, there is little difference in gauche isomer content when the intrinsic molecule is present compared with that of the fluid lipid alone. These results are considered and compared with previously published studies using deuterium nuclear magnetic resonance spectroscopy on similar model biomembrane systems. Below the lipid t c value, all the intrinsic molecules produce an increase in gauche isomers presumably by disturbing the lipid chain packing in the crystalline lipid arrangement. Information about the polypeptide structure within gramicidin A. the reconstituted proteins and also the proteins in the natural biomembranes was obtained by examining the region of the infrared spectrum between 1600 and 1700 cm −1 associated with the amide I and amide II bands. An examination of the infrared band frequencies of the different systems in this region leads to the conclusions: (1) that gramicidin A within a phospholipid bilayer structure probably has a single helix rather than a double helix structure; (2) that there are differences in band widths of the reconstituted Ca 2+-ATPase and bacteriorhodopsin compared with the spectra of the corresponding sarcoplasmic reticulum and purple membrane; (3) different membrane proteins adopt different conformations as evinced by a comparison of the spectra of the sarcoplasmic reticulum and purple membrane; (4) the polypeptide arrangement in the purple membrane is mainly helical but the abnormal frequency of the amide I band suggests that some distortion of the helix occurs: and (5) the sarcoplasmic reticulum membrane contains unordered as well as α-helix polypeptide arrangements.

Phospholipid polymers—Synthesis and spectral characteristics

A new approach has been developed for the study of model and natural biomembranes. This involves the cross-linking of diacetylene groups after ultraviolet irradiation. For the study of model biomembranes, pure phospholipids (phosphatidylcholines) have been synthesized containing diacetylene groups in each acyl chain. The physical properties of these lipids have been examined and the conditions under which they polymerise have been determined. Polymerisation occurs when the lipid is in a crystalline phase, either compressed in KBr, dispersed in water (liposomes) or deposited on a suitable support (multilayers). The resultant polymer contains a conjugated backbone and is coloured. The visible spectrum of the phospholipid polymer is sensitive to its environment. Preliminary experiments show that similar polymerisation can be induced in Acholeplasma laidlawii cells grown on diacetylenic fatty acid.