A calorimetric investigation of a series of mixed-chain polyunsaturated phosphatidylcholines: effect of sn-2 chain length and degree of unsaturation

The importance of omega-3 polyunsaturated fatty acids as high-quality food in freshwater ecosystems with implications of global change

Traditionally, trophic ecology research on aquatic ecosystems has focused more on the quantity of dietary energy flow within food webs rather than food quality and its effects on organisms at various trophic levels. Recent studies emphasize that food quality is central to consumer growth and reproduction, and the importance of food quality for aquatic ecosystems has become increasingly well recognized. It is timely to synthesise these findings and identify potential future research directions. We conducted a systematic review of omega-3 polyunsaturated fatty acids (ω3-PUFAs) as a crucial component of high-quality food sources in freshwater ecosystems to evaluate their impact on a variety of consumers, and explore the effects of global change on these high-quality food sources and their transfer to higher trophic consumers within and across ecosystems. In freshwater ecosystems, algae rich in ω3 long-chain PUFAs, such as diatoms, dinoflagellates and cryptophytes, represent important high-quality food sources for consumers, whereas cyanobacteria, green algae, terrestrial vascular plants and macrophytes low in ω3 long-chain PUFAs are low-quality food sources. High-quality ω3-PUFA-containing food sources usually lead to increased growth and reproduction of aquatic consumers, e.g. benthic invertebrates, zooplankton and fish, and also provide ω3 long-chain PUFAs to riparian terrestrial consumers via emergent aquatic insects. Consumers feeding on high-quality ω3-PUFA-containing foods in turn represent high-quality food for their own predators. However, the ω3-PUFA content of food sources is sensitive to global environmental changes. Warming, eutrophication, increased light intensity (e.g. from loss of riparian shading), and pollutants potentially inhibit the synthesis of algal ω3-PUFAs while at the same time promoting the growth of lower-quality foods, such as cyanobacteria and green algae. These factors combined could lead to a significant reduction in the availability of ω3-PUFAs for consumers and constrain their overall fitness. Although the effect of individual environmental factors on high-quality ω3-PUFA-containing food sources has been investigated, multiple environmental factors (e.g. climate change, human activities, pollution) will act in combination and any synergistic effects on aquatic food webs remain unclear. Identifying the sources and fate of ω3-PUFAs within and across ecosystems could represent an important approach to understand the impact of multiple environmental factors on trophic relationships and the implications for populations of freshwater and riparian consumers. Maintaining the availability of high-quality ω3-PUFA-containing food sources may also be key to mitigating freshwater biodiversity loss due to global change.

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Smaller herring larval size-at-stage in response to environmental changes is associated with ontogenic processes and stress response

Global change puts coastal systems under pressure, affecting the ecology and physiology of marine organisms. In particular, fish larvae are sensitive to environmental conditions, and their fitness is an important determinant of fish stock recruitment and fluctuations. To assess the combined effects of warming, acidification and change in food quality, herring larvae were reared in a control scenario (11°C*pH 8.0) and a scenario predicted for 2100 (14°C*pH 7.6) crossed with two feeding treatments (enriched in phosphorus and docosahexaenoic acid or not). The experiment lasted from hatching to the beginning of the post-flexion stage (i.e. all fins present) corresponding to 47 days post-hatch (dph) at 14°C and 60 dph at 11°C. Length and stage development were monitored throughout the experiment and the expression of genes involved in growth, metabolic pathways and stress responses were analysed for stage 3 larvae (flexion of the notochord). Although the growth rate was unaffected by acidification and temperature changes, the development was accelerated in the 2100 scenario, where larvae reached the last developmental stage at a smaller size (-8%). We observed no mortality related to treatments and no effect of food quality on the development of herring larvae. However, gene expression analyses revealed that heat shock transcripts expression was higher in the warmer and more acidic treatment. Our findings suggest that the predicted warming and acidification environment are stressful for herring larvae, inducing a decrease in size-at-stage at a precise period of ontogeny. This could either negatively affect survival and recruitment via the extension of the predation window or positively increase the survival by reducing the larval stage duration.

Omega-3 versus Omega-6: Are We Underestimating the Ecological Significance of Arachidonic Acid in Aquatic Systems?

The long-chain polyunsaturated fatty acids (PUFA) eicosapentaenoic acid (EPA, ω-3, or n-3) and arachidonic acid (ARA, ω-6 or n-6) are known to have distinct physiological functions, yet can both support growth and reproduction of consumers, raising the question of whether EPA and ARA are ecologically substitutable dietary resources. We explored the relative importance of EPA and ARA for the growth and reproduction of the freshwater keystone herbivore Daphnia in a life-history experiment. Both PUFA were supplemented in a concentration-dependent manner to a PUFA-free diet, separately and in combination (50% EPA: 50% ARA mixture). The growth-response curves obtained with EPA, ARA, and the mixture were virtually congruent and the thresholds for PUFA limitation did not differ, indicating that EPA (n-3) and ARA (n-6) were substitutable dietary resources under the applied experimental conditions. The actual requirements for EPA and ARA might change with growth conditions, e.g., under the influence of parasites or pathogens. The higher retention of ARA in Daphnia suggests that EPA and ARA are subject to different turnover rates, which also implies different physiological functions. Studies on the ARA requirements of Daphnia could provide valuable information on the presumably underestimated ecological importance of ARA in freshwater food webs.

Molecular Approaches Reduce Saturates and Eliminate trans Fats in Food Oils

Vegetable oils composed of triacylglycerols (TAG) are a major source of calories in human diets. However, the fatty acid compositions of these oils are not ideal for human nutrition and the needs of the food industry. Saturated fatty acids contribute to health problems, while polyunsaturated fatty acids (PUFA) can become rancid upon storage or processing. In this review, we first summarize the pathways of fatty acid metabolism and TAG synthesis and detail the problems with the oil compositions of major crops. Then we describe how transgenic expression of desaturases and downregulation of the plastid FatB thioesterase have provided the means to lower oil saturates. The traditional solution to PUFA rancidity uses industrial chemistry to reduce PUFA content by partial hydrogenation, but this results in the production of trans fats that are even more unhealthy than saturated fats. We detail the discoveries in the biochemistry and molecular genetics of oil synthesis that provided the knowledge and tools to lower oil PUFA content by blocking their synthesis during seed development. Finally, we describe the successes in breeding and biotechnology that are giving us new, high-oleic, low PUFA varieties of soybean, canola and other oilseed crops.

Do Eicosapentaenoic Acid and Docosahexaenoic Acid Have the Potential to Compete against Each Other?

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are n-3 polyunsaturated fatty acids (PUFAs) consumed in low abundance in the Western diet. Increased consumption of n-3 PUFAs may have beneficial effects for a wide range of physiological outcomes including chronic inflammation. However, considerable mechanistic gaps in knowledge exist about EPA versus DHA, which are often studied as a mixture. We suggest the novel hypothesis that EPA and DHA may compete against each other through overlapping mechanisms. First, EPA and DHA may compete for residency in membrane phospholipids and thereby differentially displace n-6 PUFAs, which are highly prevalent in the Western diet. This would influence biosynthesis of downstream metabolites of inflammation initiation and resolution. Second, EPA and DHA exert different effects on plasma membrane biophysical structure, creating an additional layer of competition between the fatty acids in controlling signaling. Third, DHA regulates membrane EPA levels by lowering its rate of conversion to EPA's elongation product n-3 docosapentaenoic acid. Collectively, we propose the critical need to investigate molecular competition between EPA and DHA in health and disease, which would ultimately impact dietary recommendations and precision nutrition trials.

Polyunsaturated Phospholipid Modified Membrane Degradation Catalyzed by a Secreted Phospholipase A2

To optimize the compositions of the lipid-based nanomedicine and to advance understanding of the roles of polyunsaturated phospholipids in biological membranes, this study examined the effects of polyunsaturated phospholipids on the degradation of giant unilamellar vesicles catalyzed by a secreted phospholipase A2 (sPLA₂) using fluorescence microscopy. Molecular interfacial packing, interaction, and degradation of the films containing various mixing ratios of saturated and polyunsaturated phospholipids were quantified using a Langmuir trough integrated with synchrotron X-ray surface scattering techniques. It was found that a high molar fraction (0.63 and above) of polyunsaturated phospholipids not only enhanced the rate of sPLA₂-catalyzed vesicle degradation but also changed the vesicle deformation process and degradation product morphology. Hydrolysis of the saturated phospholipids generated highly ordered liquid crystal domains, which was reduced or prohibited by the presence of the polyunsaturated phospholipids in the reactant film.

Differential Scanning Calorimetry of Protein-Lipid Interactions

Differential scanning calorimetry (DSC) is a highly sensitive nonperturbing technique used for studying the thermodynamic properties of thermally induced transitions. Since these properties might be affected by ligand binding, DSC is particularly useful for the characterization of protein interactions with biomimetic membranes. The advantages of this technique over other methods consist in the direct measurement of intrinsic thermal properties of the samples, requiring no chemical modifications or extrinsic probes. This chapter describes the basic theory of DSC and provides the reader with an understanding of the capabilities of DSC instrumentation and the type of information that can be achieved from DSC studies of lipid-protein interactions. In particular, the chapter provides a detailed analysis of DSC data to assess the effects of proteins on biomimetic membranes.

Unperturbed hydrocarbon chains and liquid phase bilayer lipid chains: a computer simulation study

In this work, the properties of saturated and unsaturated fatty acid acyl chains 16:0, 18:0, 18:1(n-9)cis, 18:2(n-6)cis, 18:3(n-3)cis, 18:4(n-3)cis, 18:5(n-3)cis, 20:4(n-6)cis, 20:5(n-3)cis and 22:6(n-3)cis in a bilayer liquid crystalline state and similar hydrocarbon chains (with CH[Formula: see text] terminal groups instead of C=O groups) in the unperturbed state characterised by a lack of long-range interaction were investigated. The unperturbed hydrocarbon chains were modelled by Monte Carlo simulations at temperature [Formula: see text] K; sixteen fully hydrated homogeneous liquid crystalline phosphatidylcholine bilayers containing these chains were studied by molecular dynamics simulations at the same temperature. To eliminate effects of the simulation parameters, the molecular dynamics and Monte Carlo simulations were carried out using the same structural data and force field coefficients. From these computer simulations, the average distances between terminal carbon atoms of the chains (end-to-end distances) were calculated and compared. The trends in the end-to-end distances obtained for the unperturbed chains were found to be qualitatively similar to those obtained for the same lipid chains in the bilayers. So, for understanding of a number of processes in biological membranes (e.g., changes in fatty acid composition caused by environmental changes such as temperature and pressure), it is possible to use, at least as a first approximation, the relationships between the structure and properties for unperturbed or isolated hydrocarbon chains.

Docosahexaenoic and eicosapentaenoic acids segregate differently between raft and nonraft domains

Omega-3 polyunsaturated fatty acids (n-3 PUFA), enriched in fish oils, are increasingly recognized to have potential benefits for treating many human afflictions. Despite the importance of PUFA, their molecular mechanism of action remains unclear. One emerging hypothesis is that phospholipids containing n-3 PUFA acyl chains modify the structure and composition of membrane rafts, thus affecting cell signaling. In this study the two major n-3 PUFA found in fish oils, eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids, are compared. Using solid-state (2)H NMR spectroscopy we explored the molecular organization of 1-[(2)H(31)]palmitoyl-2-eicosapentaenoylphosphatidylcholine (PEPC-d(31)) and 1-[(2)H(31)]palmitoyl-2-docosahexaenoylphosphatidylcholine (PDPC-d(31)) in mixtures with sphingomyelin (SM) and cholesterol (chol). Our results indicate that whereas both PEPC-d(31) and PDPC-d(31) can accumulate into SM-rich/chol-rich raftlike domains, the tendency for DHA to incorporate into rafts is more than twice as great as for EPA. We propose that DHA may be the more bioactive component of fish oil that serves to disrupt lipid raft domain organization. This mechanism represents an evolution in the view of how PUFA remodel membrane architecture.

Retailoring docosahexaenoic acid-containing phospholipid species during impaired neurogenesis following omega-3 alpha-linolenic acid deprivation

Diminished levels of docosahexaenoic acid (22:6n-3), the major fatty acid (FA) synthesized from alpha-linolenic acid (18:3n-3), have been implicated in functional impairment in the developing and adult brain. We have now examined the changes in phospholipid (PL) molecular species in the developing postnatal cortex, a region recently shown to be affected by a robust aberration in neuronal cell migration, after maternal diet alpha-linolenic acid deprivation (Yavin et al. (2009)Neuroscience162(4),1011). The frontal cortex PL composition of 1- to 4-week-old rats was analyzed by gas chromatography and electrospray ionization/tandem mass spectrometry. Changes in the cortical PL molecular species profile by dietary means appear very specific as 22:6n-3 was exclusively substituted by docosapentaenoic acid (22:5n-6). However, molecular species were conserved with respect to the combination of specific polar head groups (i.e. ethanolamine and serine) in sn-3 and defined saturated/mono-unsaturated FA in sn-1 position even when the sn-2 FA moiety underwent diet-induced changes. Our results suggest that substitution of docosahexaenoic acid by docosapentaenoic acid is tightly regulated presumably to maintain a proper biophysical characteristic of membrane PL molecular species. The importance of this conservation may underscore the possible biochemical consequences of this substitution in regulating certain functions in the developing brain.

Comparative effects of DHA and EPA on cell function

Fish oil supplementation has been reported to be generally beneficial in autoimmune, inflammatory and cardiovascular disorders. Most researchers have attributed these beneficial effects to the high content of omega-3 fatty acids in fish oil (FO). The effects of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are not differentiated in most studies. In fact, up to 1990, purified DHA was not available for human use and there was no study regarding its effects on human immune response. In this review, the differences in the effects of these two fatty acids on cell function are discussed. Studies have shown that EPA and DHA have also different effects on leukocyte functions such as phagocytosis, chemotactic response and cytokine production. DHA and EPA modulate differently expression of genes in lymphocytes. Activation of intracellular signaling pathways involved with lymphocyte proliferation is also differently affected by these two fatty acids. In relation to insulin producing cell line RINm5F, DHA and EPA are cytotoxic at different concentrations and the proteins involved with cell death are differently modulated by these two fatty acids. Substantial improvement in the therapeutic usage of omega-3 fatty acid-rich FO will be possible with the discovery of the different mechanisms of actions of DHA and EPA.

Adsorption behaviors of DPPC/MO aggregates on SiO2 surfaces

The adsorption kinetics of extruded 1,2-dipalmitoyl- sn-glycero-3-phosphatidylcholine (DPPC)/1-(cis-9-octadecenoyl)- rac-glycerol (monoolein, MO) aggregates on SiO 2 surface at 25 degrees C is investigated in real time, using the dissipative quartz crystal microbalance (QCM) technique. Four adsorption pathways have been identified depending on the molar fraction of MO in the DPPC/MO system: (I) intact vesicle adsorption, (II) vesicle reorganization on a SiO 2 surface, (III) supported lipid bilayer (SLB) formation, and (IV) cubosome adsorption. The results can be understood by the fact that DPPC is a lamellar phase-forming lipid, whereas MO prefers the cubic phase. Therefore, the incorporation of MO in DPPC increases the packing parameter. Equally important, MO also increases the mobility of lipid molecules and lateral pressure in the bilayers as a result of the presence of a unique cis- double bond. Before extrusion, the vesicles size increases with the MO content when X MO <or= 0.7 and cubosomes are formed for X MO >or= 0.8. The extruded DPPC/MO suspensions consist of reformed vesicles for X MO <or= 0.7 and filtered cubosomes for X MO >or= 0.8, all with a uniform diameter of approximately 100 nm. Differential scanning calorimetry (DSC) further indicates that the addition of MO lowers the main phase transition temperature of DPPC and thus makes the hydrophobic interior more fluid.

Molecular dynamics simulations of a lipovitellin-derived amphiphilic beta-sheet homologous to apoB-100 beta-sheets at a hydrophobic decane-water interface

Lipovitellin, an egg-yolk lipoprotein, transports lipids in a pocket surrounded by amphiphilic beta-sheets. Its X-ray structure provides possibilities to study interactions between lipophilic beta-sheets and lipids at the atomic level. Here, we studied a 67-residue-long amphiphilic beta-sheet of lipovitellin previously suggested a suitable working model for studies of the lipid-binding behaviour of amphiphilic beta-sheet regions in apolipoprotein B-100 (apoB-100). We performed four molecular dynamics simulations with different starting configurations to define characteristics of the amphiphilic beta-sheet model at a decane-water interface. In each simulation the model beta-sheet bound keenly to the decane layer via its hydrophobic surface. The structural profiles showed unchanged secondary structure of the beta-sheet during the attachment. Also, aromatic side chains, especially tryptophans and tyrosines, mediated the attachment to the hydrophobic layer and influenced the orientation of the decane molecules that are in contact with the beta-sheet. In conclusion, the present simulations reveal high affinity of a lipovitellin-derived amphiphilic beta-sheet to a hydrophobic decane layer. They lay thereby the basis for further studies of the interaction between amphiphilic beta-sheets and lipids in complex molecular systems, like LDL particles, in which the large apoB-100 is the main protein component.

The cell membrane-shielding function of eicosapentaenoic acid for Escherichia coli against exogenously added hydrogen peroxide

The colony-forming ability of catalase-deficient Escherichia coli mutant genetically modified to produce eicosapentaenoic acid (EPA) showed less decrease than in a control strain producing no EPA, when treated with 0.3mM hydrogen peroxide (H(2)O(2)) under non-growth conditions. H(2)O(2)-induced protein carbonylation was enhanced in cells lacking EPA. The amount of fatty acids was decreased more significantly for cells lacking EPA than for those producing EPA. Much lower intracellular concentrations of H(2)O(2) were detected for cells with EPA than those lacking EPA. These results suggest that cellular EPA can directly protect cells against oxidative damage by shielding the entry of exogenously added H(2)O(2).

Preferable stimulation of PON1 arylesterase activity by phosphatidylcholines with unsaturated acyl chains or oxidized acyl chains at sn-2 position

To examine the effect of phospholipids on PON1 activities, purified PON1 was exposed to phospholipids prior to the determination of arylesterase and paraoxonase activities. Phosphatidylcholines with saturated acyl chains (C10-C16) showed a stimulation of both activities, chain length-dependent, with a greater stimulation of arylesterase activity, suggesting the implication of lipid bilayer in the stimulatory action. Such a preferable stimulation of arylesterase activity was more remarkable with phosphatidylcholines with polyunsaturated acyl chains or oxidized chains at sn-2 position, implying that the packing degree of acyl chain may be also important for the preferable stimulation of arylesterase activity. Separately, 1-palmitoyl-lysoPC also stimulated arylesterase activity preferably, indicating that the micellar formation of lipids around PON1 also contributes to the stimulatory action. Additionally, phosphatidylglycerols slightly enhanced arylesterase activity, but not paraoxonase activity. In contrast, phosphatidylserine and phosphatidic acid (> or =0.1 mM) inhibited both activities Further, such a preferable stimulation of arylesterase activity by phosphatidylcholines was also reproduced with VLDL-bound PON1, although to a less extent. These data indicate that phosphatidylcholines with polyunsaturated acyl chains or oxidized chain, or lysophosphatidylcholine cause a preferable stimulation of arylesterase activity, thereby contributing to the decrease in the ratio of paraoxonase activity to arylesterase activity.

Specific volumes of unsaturated phosphatidylcholines in the liquid crystalline lamellar phase

The specific volumes of seven 1,2-diacyl-sn-glycero-3-phosphocholines with symmetric, unbranched acyl chains containing one, four, or six cis double bonds per chain, or with a saturated sn-1 chain and one, four, or six cis double bonds in the sn-2 chain were determined by the neutral buoyancy method. Experiments were conducted in the liquid crystalline lamellar phase over the temperature range from 5 to 35 degrees C. It is demonstrated that the molecular volume of phosphatidylcholines can be well approximated as the sum of a constant volume of the polar lipid head region and the temperature-dependent volumes of hydrocarbon chain CH2, CH, and terminal CH3 groups. A linear dependence of chain segment volumes on temperature was observed. A self-consistent set of partially temperature-dependent volumes is obtained that allows prediction of phosphatidylcholine molecular volumes within very tight error margins.

Dietary fats and membrane function: implications for metabolism and disease

Lipids play varied and critical roles in metabolism, with function dramatically modulated by the individual fatty acid moities in complex lipid entities. In particular, the fatty acid composition of membrane lipids greatly influences membrane function. Here we consider the role of dietary fatty acid profile on membrane composition and, in turn, its impact on prevalent disease clusters of the metabolic syndrome and mental illness. Applying the classical physiological conformer-regulator paradigm to quantify the influence of dietary fats on membrane lipid composition (i.e. where the membrane variable is plotted against the same variable in the environment--in this case dietary fats), membrane lipid composition appears as a predominantly regulated parameter. Membranes remain relatively constant in their saturated (SFA) and monounsaturated (MUFA) fatty acid levels over a wide range of dietary variation for these fatty acids. Membrane composition was found to be more responsive to n-6 and n-3 polyunsaturated fatty acid (PUFA) levels in the diet and most sensitive to n-3 PUFA and to the n-3/n-6 ratio. These differential responses are probably due to the fact that both n-6 and n-3 PUFA classes cannot be synthesised de novo by higher animals. Diet-induced modifications in membrane lipid composition are associated with changes in the rates of membrane-linked cellular processes that are major contributors to energy metabolism. For example, in the intrinsic activity of fundamental processes such as the Na+/K+ pump and proton pump-leak cycle. Equally, dietary lipid profile impacts substantially on diseases of the metabolic syndrome with evidence accruing for changes in metabolic rate and neuropeptide regulation (thus influencing both sides of the energy balance equation), in second messenger generation and in gene expression influencing a range of glucose and lipid handling pathways. Finally, there is a growing literature relating changes in dietary fatty acid profile to many aspects of mental health. The understanding of dietary lipid profile and its influence on membrane function in relation to metabolic dysregulation has exciting potential for the prevention and treatment of a range of prevalent disease states.

Flexibility of “Polyunsaturated Fatty Acid Chains” and Peptide Backbones: A Comparative ab Initio Study

The conformational properties of omega-3 type of polyunsaturated fatty acid (PUFA) chains and their fragments were studied using Hartree-Fock (RHF/3-21G) and DFT (B3LYP/6-31G(d)) methods. Comparisons between a unit (U) fragment of the PUFA chain and a mono N-Ac-glycine-NHMe residue show that both structures have the same sequence of sp2-sp3-sp2 atoms. The flexibility of PUFA originates in the internal rotation about the above pairs of sigma bonds. Therefore, potential energy surfaces (PESs) were generated by a scan around the terminal dihedral angles (phi t1 and phi t2) as well as the phi 1 and psi 1 dihedrals of both 1U congeners (Me-CHCH-CH2-CHCHMe and MeCONH-CH2-CONHMe) at the RHF/3-21G level of theory. An interesting similarity was found in the flexibility between the cis allylic structure and the trans peptide models. A flat landscape can be seen in the cis 1U (hepta-2,5-diene) surface, implying that several conformations are expected to be found in this (PES). An exhaustive search carried out on the 1U and 2U models revealed that straight chain structures such as trans and cis beta (phi 1 approximately psi 1 approximately 120 degrees; phi 2 approximately psi 2 approximately -120 degrees) or trans and cis extended (phi 1 approximately psi 1 approximately phi 2 approximately psi 2 approximately 120 degrees) can be formed at the lowest energy of both isomers. However, forming helical structures, such as trans helix (phi 1 approximately -120 degrees, psi 1 approximately 12 degrees; phi 2 approximately -120 degrees, psi 2 approximately 12 degrees) or cis helix (phi 1 approximately -130 degrees, psi 1 approximately 90 degrees; phi 2 approximately -145 degrees, psi 2 approximately 90 degrees) will require more energy. These six conformations, found in 2U, were selected to construct longer chains such as 3U, 4U, 5U, and 6U to obtain the thermochemistry of secondary structures. The variation in the extension or compression of the chain length turned out to be a factor of 2 between the helical and nonhelical structures. The inside diameter of the "tube" of cis helix turned out to be 3.5 A after discounting the internal H atoms. Thermodynamic functions were computed at the B3LYP/6-311+G(2d,p)//B3LYP/6-31G(d). The cis-trans isomerization energy of 1.7 +/- 0.2 kcal mol(-1) unit(-1) for all structure pairs indicates that the conformer selection was consistent. A folding energy of 0.5 +/- 0.1 kcal mol(-1) unit(-1) has been extracted from the energy comparison of the helices and most extended nonhelical structures. The entropy change associated with the folding (Delta S(folding)) is decreases faster with the degree of polymerization (n) for the cis than for the trans isomer. As a consequence, the linear relationships between (Delta G(folding)) and n for the cis and trans isomer crossed at about n = 3. This suggested that the naturally occurring cis isomer less ready to fold than the trans isomer since a greater degree of organization is exhibited by the cis isomer during the folding process. The result of this work leads to the question within the group additivity rule: could the method applied in our study of the folding of polyallylic hydrocarbons be useful in investigating the thermochemistry of protein folding?

Acyl chain unsaturation in PEs modulates phase separation from lipid raft molecules

By one hypothesis, phospholipids containing unsaturated fatty acids may be involved in phase separation from the lipid raft molecules sphingomyelin (SM) and cholesterol (CHOL). We tested the effect of increasing the number of double bonds in the acyl chains of phosphatidylethanolamines (PEs) on phase separation from SM/CHOL. The detergent extraction method was employed on various homoacid and heteroacid PEs in mixed vesicles composed of PE/SM/CHOL (1:1:1mol). The disaturated homoacid 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (16:0-16:0PE) showed the least solubility upon detergent extraction whereas maximal solubility was observed for the polyunsaturated homoacid 1,2-didocosahexaenoyl-sn-glycero-3-phosphoethanolamine (22:6-22:6PE). Increasing the number of double bonds in the sn-2 position of heteroacid saturated-unsaturated PEs resulted in an increase in detergent solubility, which correlated with a general decrease in the gel-to-liquid crystalline phase transition temperature of the PEs. Our findings demonstrate that increasing unsaturation in PEs results in increased phase separation from SM/CHOL membranes, which may have implications for cellular signaling.

Docosahexaenoic acid: membrane properties of a unique fatty acid

Docosahexaenoic acid (DHA) with 22-carbons and 6 double bonds is the extreme example of an omega-3 polyunsaturated fatty acid (PUFA). DHA has strong medical implications since its dietary presence has been positively linked to the prevention of numerous human afflictions including cancer and heart disease. The PUFA, moreover, is essential to neurological function. It is remarkable that one simple molecule has been reported to affect so many seemingly unrelated biological processes. Although details of a molecular mode of action remain elusive, DHA must be acting at a fundamental level common to many tissues that is related to the high degree of conformational flexibility that the multiple double bonds have been identified to confer. One likely target for DHA action is at the cell membrane where the fatty acid is known to readily incorporate into membrane phospholipids. Once esterified into phospholipids DHA has been demonstrated to significantly alter many basic properties of membranes including acyl chain order and "fluidity", phase behavior, elastic compressibility, permeability, fusion, flip-flop and protein activity. It is concluded that DHA's interaction with other membrane lipids, particularly cholesterol, may play a prominent role in modulating the local structure and function of cell membranes.

The structure of DHA in phospholipid membranes

Early experiments and molecular simulations of PUFA favored a rigid arrangement of double bonds in U-shaped or extended conformations such as angle-iron or helical. Although results of recent solid-state NMR measurements and molecular simulations have confirmed the existence of these structural motifs, they portray an image of DHA (22:6n-3) as a highly flexible molecule with rapid transitions between large numbers of conformers on the time scale from picoseconds to hundreds of nanoseconds. The low barriers to torsional rotation about C-C bonds that link the cis-locked double bonds with the methylene carbons between them are responsible for this unusual flexibility. Both the amplitude and frequency of motion increase toward the terminal methyl group of DHA.

Molecular dynamics simulations of hydrated unsaturated lipid bilayers in the liquid-crystal phase and comparison to self-consistent field modeling

Molecular dynamics simulations, using the collision dynamics method, were carried out for hydrated bilayers of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (18:0/18:1 omega 9cis PC, SOPC) and 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphatidylcholine (18:0/22:6 omega 3cis PC, SDPC). The simulation cells of the two bilayers consisted of 96 SOPC (or SDPC) molecules and 2304 water molecules: 48 lipid molecules per layer and 24 H2O molecules per lipid. The water was modeled by explicit TIP3P water molecules. The C-H bond-order-parameter -S(CH) profiles of the hydrocarbon tails, the bond orientation distribution functions and the root-mean-square values of the positional fluctuations of the lipid chain carbons were calculated. Simulation results are compared to the available experimental data and to other computer investigations of these lipid molecules. Several results of molecular-level self-consistent field calculations of these bilayers are also presented. Both theoretical methods reveal the same main characteristic features of the order-parameter profiles for the given bilayers. Some aspects of the physical properties of unsaturated lipids and their biological significance are discussed.

Mixed-chain phospholipids: structures and chain-melting behavior

It has long been established that diacyl phospholipids isolated from animal cell membranes are predominantly of a mixed-chain variety, meaning that the sn-1 and sn-2 acyl chains are saturated and unsaturated acyl chains, respectively. In general, monoenoic and dienoic acids are found in the sn-2 acyl chain of phosphatidylcholine (PtdCho), whereas polyenoic acids are in phosphatidylethanolamine (PtdEth). These unsaturated chains contain only cis-double bonds, which are always methylene-interrupted. In recent years, the structures and the chain-melting behavior of mixed-chain PtdCho and PtdEth have been systematically studied in this laboratory. Specifically, we have examined the effects of chain unsaturation of the sn-2 acyl chain on the phase transition temperature (Tm) of many PtdCho and PtdEth by high-resolution differential scanning calorimetry (DSC). The Tm values, for instance, obtained from all-unsaturated mixed-chain PtdEth derived from a common precursor can be grouped together according to their chemical formula to form a Tm-diagram. Hence, all the Tm values can be compared simply, systematically, and simultaneously using the Tm-diagram. In addition, the energy-minimized structures of mixed-chain phospholipids containing different numbers/positions of methylene-interrupted cis-double bonds have been simulated by molecular mechanics calculations (MM). In this review, the results of our MM and DSC studies carried out with various mixed-chain phospholipids are summarized. In addition, we emphasize that the combined approach of MM and DSC yields unique information that can correlate the various Tm-profiles seen in the Tm-diagram with the structural variation of mixed-chain lipids as caused by the introduction of different numbers/positions of methylene-interrupted cis-double bonds.

Liquid crystalline/gel state phase separation in docosahexaenoic acid-containing bilayers and monolayers

The phase behavior of lipid mixtures containing 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (18:0, 22:6 PC) with 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) was studied with bilayers using differential scanning calorimetry (DSC), and with monolayers monitoring pressure/area isotherms and surface elasticity, and lipid domain formation followed by epifluorescence microscopy. From DSC studies it is concluded that DPPC/18:0, 22:6 PC phase separates into DPPC-rich and 18:0, 22:6 PC-rich phases. In monolayers, phase separation is indicated by changes in pressure-area isotherms implying phase separation where 18:0, 22:6 PC is 'squeezed out' of the remaining DPPC monolayer. Phase separation into lipid domains in the mixed PC monolayer is quantified by epifluorescence microscopy using the fluorescently labeled phospholipid membrane probe, 1, 2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(lissamine rhodamine B sulfonyl). These results further describe the ability of docosahexaenoic acid to participate in lipid phase separations in membranes.

Plasma total cholesterol concentrations do not predict cerebrospinal fluid neurotransmitter metabolites: implications for the biophysical role of highly unsaturated fatty acids

Low concentrations of a metabolite of serotonin found in cerebrospinal fluid (CSF), 5-hydroxyindolacetic acid (5-HIAA), are strongly associated with suicidal and violent behaviors. Although lowering of plasma total cholesterol has been suggested to increase mortality from suicide and violence by decreasing concentrations of CSF 5-HIAA via changes in membrane biophysical properties, highly unsaturated fatty acids may play a more important role. Violent and nonviolent comparison groups, early- and late-onset alcoholics, and healthy comparison subjects were studied to control for alcohol use and predisposition to violence. Fasting plasma total cholesterol and CSF were assayed under stringently controlled conditions. When all groups were combined (n = 234), plasma cholesterol concentrations had a weak positive correlation with CSF 5-HIAA (r = 0.18, P < 0.01). However, age correlated with both plasma total cholesterol and CSF 5-HIAA concentrations. When age was included in multiple regression models, the correlation between cholesterol and CSF 5-HIAA concentrations was not significant. Cholesterol correlated weakly with CSF 5-HIAA concentrations only in late-onset alcoholics after age was controlled for, but the relation was not significant after correction for multiple testing. CSF homovanillic acid did not correlate with plasma total cholesterol in any group. Plasma total cholesterol had no apparent relation to CSF neurotransmitter metabolites in any group of subjects. Highly unsaturated essential fatty acids, which are also critical determinants of membrane biophysical properties and may be linked to brain serotonin concentrations, should also be considered in studies examining the effect of lowering fat intake on the incidence of suicide and violence.

Calorimetric and molecular mechanics studies of the thermotropic phase behavior of membrane phospholipids

In this review, we summarize the results of recent studies on the main phase transition behavior of phospholipid bilayers using the combined approaches of molecular mechanics simulations and high-resolution differential scanning calorimetry. Following a brief overview of the phase transition phenomenon exhibited by the lipid bilayer, we begin with the review by showing how several structural parameters underlying various phospholipids including phosphatidylcholine, phosphatidylethanolamine, and phosphatidylglycerol are defined and determined. Specifically, these structural parameters are obtained with saturated lipids packed in the gel-state bilayer using computer-based molecular mechanics calculations. Then we proceed to present the calorimetric data obtained with the lipid bilayer composed of saturated phospholipids as it undergoes the gel-to-liquid-crystalline phase transition in excess water. The general equations that can correlate the gel-to-liquid-crystalline phase transition temperature (T(m)) of the lipid bilayer with the structural parameters of the lipid molecule constituting the lipid bilayer are subsequently presented. From these equations, two tables of predicated T(m) values for well over 400 molecular species of saturated phosphatidylcholine and saturated phosphatidylethanolamine are generated. We further review the structure and chain-melting behavior of a large number of sn-1 saturated/sn-2 unsaturated phospholipids. Two T(m)-diagrams are shown, from which the effects of the number and the position of one to five cis carbon-carbon double bonds on T(m) can be viewed simultaneously. Finally, in the last part of this review, simple molecular models that have been invoked to interpret the characteristic T(m) trends exhibited by lipid bilayers composed of unsaturated lipids with different numbers and positions of cis carbon-carbon double bonds as seen in the T(m)-diagram are presented.

Membranes as possible pacemakers of metabolism

Basal metabolic rate (BMR) varies dramatically among vertebrate species, both (i) being several fold higher in the endothermic mammals and birds compared to the ectothermic reptiles, amphibians and fish, and (ii) being much greater, on a body mass basis, in small vertebrates compared to large vertebrates. These differences in whole animal BMR are also manifest at the cellular level with substantial contributions to basal metabolic activity from the maintenance of various trans-membrane gradients. The percentage contribution of various processes to basal metabolism is remarkably consistent between different vertebrates and when BMR varies, the components of metabolic activity vary in relative unison. Membrane composition also varies between vertebrates and the degree of polyunsaturation of membrane phospholipids is correlated with cellular metabolic activity. In general, the tissue phospholipids and thus membrane bilayers of endotherms are more polyunsaturated than those from similar-sized ectotherms. In mammals membrane polyunsaturation is allometrically related to body mass. We suggest that membranes can act as pacemakers for overall metabolic activity. We propose that such membrane polyunsaturation increases the molecular activity of many membrane-bound proteins and consequently some specific membrane leak-pump cycles and cellular metabolic activity. We hypothesize a possible mechanistic basis for this effect that is based on a greater transfer of energy during intermolecular collisions of membrane proteins with the unsaturated two carbon units (C=C) of polyunsaturates compared to the single carbon units of saturated acyl chains, as well as the more even distribution of such units throughout the depth of the bilayer when membranes contain polyunsaturated acyl chains compared to monounsaturated ones. The proposed pacemaker role of differences in membrane bilayer composition is briefly discussed with respect to the brain (and sensory cells), evolution of mammalian endothermic metabolism, and its clinical implications for humans.

Effects of various numbers and positions of cis double bonds in the sn-2 acyl chain of phosphatidylethanolamine on the chain-melting temperature

In an attempt to investigate systematically the effects of various single and multiple cis carbon-carbon double bonds in the sn-2 acyl chains of natural phospholipids on membrane properties, we have de novo synthesized unsaturated C20 fatty acids comprised of single or multiple methylene-interrupted cis double bonds. Subsequently, 15 molecular species of phosphatidylethanolamine (PE) with sn-1 C20-saturated and sn-2 C20-unsaturated acyl chains were semi-synthesized by acylation of C20-lysophosphatidylcholine with unsaturated C20 fatty acids followed by phospholipase D-catalyzed base-exchange reaction in the presence of excess ethanolamine. The gel-to-liquid crystalline phase transitions of these 15 mixed-chain PE, in excess H2O, were investigated by high resolution differential scanning calorimetry. In addition, the energy-minimized structures of these sn-1 C20-saturated/sn-2 C20-unsaturated PE were simulated by molecular mechanics calculations. It is shown that the successive introduction of cis double bonds into the sn-2 acyl chain of C(20):C(20)PE can affect the gel-to-liquid crystalline phase transition temperature, Tm, of the lipid bilayer in some characteristic ways; moreover, the effect depends critically on the position of cis double bonds in the sn-2 acyl chain. Specifically, we have constructed a novel Tm diagram for the 15 species of unsaturated PE, from which the effects of the number and the position of cis double bonds on Tm can be examined simultaneously in a simple, direct, and unifying manner. Interestingly, the characteristic Tm profiles exhibited by different series of mixed-chain PE with increasing degree of unsaturation can be interpreted in terms of structural changes associated with acyl chain unsaturation.

Dietary menhaden and corn oils and the red blood cell membrane lipid composition and fluidity in hyper- and normocholesterolemic miniature swine

Fatty acids in the diet are readily incorporated into lipids in various tissues. However, it is not clear whether all tissues have the same level of incorporation. Second, (n-6) unsaturated fatty acids increase the fluidity of membranes, but this has not been shown for (n-3) fatty acids. In this study, we measured the incorporation of (n-6) and (n-3) fatty acids into erythrocyte membrane lipids and studied their effects on the fluidity of erythrocyte membranes. One group of female miniature swine was made hypercholesterolemic by feeding the swine cholesterol and lard for 2 mo; the other group served as controls and was fed a stock diet. Both groups were then fed either corn oil or menhaden oil or a mixture of the two for 23 additional weeks. Blood was collected at 0, 2, 4, 12 and 23 wk after initialization of the experimental diets, and fatty acid composition of phospholipids was assessed. Membrane phospholipids of pigs fed menhaden oil had elevated (n-3) fatty acids (20:5 and 22:6), and lower 18:2 than those fed corn oil. There was no difference in 20:4 content. The fatty acid changes occurred as early as 2 wk after consumption of the corn oil or menhaden oil in pigs previously fed a stock diet, but it took longer in pigs previously fed lard + cholesterol, indicating residual effects of pretreatment. Menhaden oil increased anisotropy (indicating decreased fluidity) more than corn oil for the nonpolar probe diphenylhexatriene (DPH) at earlier time points, but not at 23 wk. Erythrocyte membrane fluidity was significantly related to membrane polyunsaturate content, with (n-6) fatty acids having a greater influence than (n-3) fatty acids. A comparison of the present red blood cell fatty acid compositions with brain synaptosome fatty acid compositions for the same animals showed poor correlations for some of the fatty acids. There was no significant direct relationship between docosahexaenoate (DHA) concentrations in erythrocyte membranes with DHA concentrations in brain synaptosomes from cerebellum, forebrain and caudate nucleus.

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.

Molecular order and dynamics in bilayers consisting of highly polyunsaturated phospholipids

The time-resolved fluorescence emission and decay of fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene (DPH) was used to characterize equilibrium and dynamic bilayer structural properties of symmetrically substituted phosphatidylcholines (PCs) with acyl chains containing no, one, four, or six double bonds and mixed-chain phosphatidylcholines with a saturated sn-1 chain and one, four, or six double bonds in the sn-2 chain. Both the Brownian rotational diffusion (BRD) model and the wobble-in-cone model were fit to all differential polarization data, and the descriptions of the data provided by the BRD model were found to be statistically superior. Global analysis of differential polarization data revealed two statistically equivalent solutions. The solution corresponding to a bimodal orientational distribution function, f(theta), was selected based on the effects of temperature on f(theta) and previous measurements on fixed, oriented bilayers. The overall equilibrium acyl chain order in these bilayers was analyzed by comparing the orientational probability distribution for DPH, f(theta) sin theta, with a random orientational distribution. Orientational order decreased and probe dynamics increased in mixed-chain species as the unsaturation of the sn-2 chain was increased. The degree of orientational order dropped dramatically in the dipolyunsaturated species compared with the mixed-chain phosphatidylcholines, which contained a polyunsaturated sn-2 chain. In terms of both orientational order and probe dynamics, the differences between the highly polyunsaturated species and the monounsaturated species were much greater than the differences between the monounsaturated species and a disaturated PC.

Structure and dynamic properties of diunsaturated 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine lipid bilayer from molecular dynamics simulation

Unsaturated fatty acid chains are known to be an essential structural part of biomembranes, but only monounsaturated chains have been included in the molecular dynamics (MD) simulations of membrane systems. Here we present a 1-ns MD simulation for a diunsaturated 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphatidylcholine (PLPC; 16:0/18:2[delta9,12]) bilayer. The structural behavior of the phosphatidylcholine headgroup, the glycerol backbone, and the hydrating water were assessed and found to be consistent with the existing information about similar systems from both experimental and computational studies. Further analysis was focused on the structure of the double bond region and the effects of the diunsaturation on the bilayer interior. The behavior of the diunsaturated sn-2 chains is affected by the tilted beginning of the chain and the four main conformations of the double bond region. The double bonds of the sn-2 chains also influenced the characteristics of the saturated chains in the sn-1 position. Furthermore, extreme conformations of the sn-2 chains existed that are likely to be related to the functional role of the double bonds. The results here point out the importance of polyunsaturation for the biological interpretations deduced from the membrane MD simulations.

Influence of the positions of cis double bonds in the sn-2-acyl chain of phosphatidylethanolamine on the bilayer's melting behavior

In an attempt to examine the effects of different numbers and positions of cis double bonds in the sn-2-acyl chain of phosphatidylethanolamine (PE) on the bilayer's melting behavior, 21 molecular species of PE were first semisynthesized, and their Tm and DeltaH values were subsequently determined by high resolution differential scanning calorimetry. In the plot of Tm versus the number of the cis double bond, some characteristic profiles were observed for the various series of PEs. For instance, if the cis double bond was first introduced into the sn-2-acyl chain of C(20):C(20)PE at the Delta5-position, the Tm was observed to reduce drastically. Subsequent stepwise additions of up to five cis double bonds at the methylene-interrupted positions toward the methyl end resulted in a progressive yet smaller decrease in Tm. If, on the other hand, the cis double bonds were introduced sequentially at the Delta11-, Delta11,14-, and Delta11,14,17-positions along the sn-2-acyl chain of C(20):C(20)PE, the Tm profile in the Tm versus the number of the cis double bond showed a down-and-up trend. Most interestingly, for positional isomers of C(20):C(20:3Delta5,8,11)PE, C(20):C(20:3Delta8,11,14)PE, and C(20):C(20:3Delta11,14,17)PE, an inverted bell-shaped Tm profile was detected in the plot of Tm against the position of the omega-carbon for these isomers. Similar Tm profiles were also observed for C(18):C(20)PE, C(20):C(18)PE, and their unsaturated derivatives. This work thus demonstrated that both the positions and the numbers of cis double bonds in the sn-2 acyl chain could exert noticeable influence on the gel-to-liquid crystalline phase transition behavior of the lipid bilayer. Finally, a molecular model was presented, with which the behavior of the gel-to-liquid crystalline phase transition observed for lipid bilayers composed of various sn-1-saturated/sn-2-unsaturated lipids can be rationalized.

Water permeability of polyunsaturated lipid membranes measured by 17O NMR

Diffusion-controlled water permeation across bilayers of polyunsaturated phospholipids was measured by 17O nuclear magnetic resonance. In 100-nm extruded liposomes containing 50 mM MnCl2, water exchange between internal and external solutions was monitored via changes in the linewidth of the 17O water resonance of external water. Liposome size and shape were characterized by light scattering methods and determination of liposome trapped volume. At 25 degrees C, the following water permeability coefficients were determined: 18:0-18:1n-9 PC, 155 +/- 24 microns/s; 18:0-18:3n-3 PC, 330 +/- 88 microns/s; and 18:0-22:6n-3 PC, 412 +/- 91 microns/s. The addition of 1 M ethanol reduced permeability coefficients to 66 +/- 15 microns/s for 18:0-18:1n-9 PC and to 239 +/- 67 microns/s for 18:0-22:6n-3 PC. Furthermore, the addition of 50 mol% 18:1n-9-18:1n-9 PE reduced the water permeability from 122 +/- 21 microns/s for pure 18:1n-9-18:1n-9 PC to 74 +/- 15 microns/s for the mixture. The significant increase in water permeation for membranes with polyunsaturated hydrocarbon chains correlates with looser packing of polyunsaturated lipids at the lipid-water interface and the suggested deeper penetration of water into these bilayers. Ethanol may block water diffusion pathways by occupying points of water entry into bilayers at the interface. The addition of dioleoylphosphatidylethanolamine increases lipid packing density and, consequently, reduces permeation rates.

Influence of cis double bonds in the sn-2 acyl chain of phosphatidylethanolamine on the gel-to-liquid crystalline phase transition

We have semisynthesized 19 species of mixed-chain phosphatidylethanolamines (PEs) in which the sn-1 acyl chain is derived from saturated fatty acids with varying chain lengths and the sn-2 acyl chain has different chain lengths but contains 0, 1, and 2 cis double bond(s). The gel-to-liquid crystalline phase transition temperatures (Tm) of lipid bilayers prepared from these 19 mixed-chain PEs were determined calorimetrically. When the Tm values are compared with those of saturated and monounsaturated counterparts, a common Tm profile is observed in the plot of Tm versus the number of cis double bonds. Specifically, a marked stepwise decrease in Tm is detected as the number of cis double bonds in the sn-2 acyl chain of the mixed-chain PE is successively increased from 0 to 1 and then to 2. The large Tm-lowering effect of the acyl chain unsaturation can be attributed to the increase in Gibbs free energy of the gel-state bilayer as a result of weaker lateral chain-chain interactions. In addition, we have applied molecular mechanics calculations to simulate the molecular structure of dienoic mixed-chain C(X):C(Y:2 delta n,n+3)PE in the gel-state bilayer, thus enabling the three independent structural parameters (N, delta C, and LS) to be calculated in terms of X, Y, and n, which are intrinsic quantities of C(X):C(Y:2 delta n,n+3)PE. When the Tm values and the corresponding N and delta C values of all dienoic mixed-chain PEs under study are first codified and then analyzed statistically by multiple regressions, the dependence of Tm on the structural parameters can be described quantitatively by a simple and general equation. The physical meaning and the usefulness of this simple and general equation are explained.

X-ray diffraction study of bilayer to non-bilayer phase transitions in aqueous dispersions of di-polyenoic phosphatidylethanolamines

The low temperature phase properties of aqueous dispersions of di-18:2 and di-18:3 phosphatidylethanolamine are strongly influenced by the presence of ice. In the presence of cryoprotectants to inhibit ice formation, these lipids persist in the H(II) phase down to at least -50 degrees C. Ice formation, however, leads to a drastic reduction in the amount of available free water and a rapid reduction in the diameter of the inverted cylindrical micelles of the H(II) phase. The resulting increase in surface curvature of the micelles induces an imbalance in the forces acting in the lipid surface and the hydrophobic core which is relieved by formation of the L(alpha) phase. On reheating the lipid samples undergo an abrupt L(alpha) --> H(II) phase transition at about -20 degrees C. The radius of the water core of the inverted micelles at their point of formation is estimated to be 0.9 nm. This increases with temperature as more unfrozen water becomes available until the normal equilibrium radius of about 2.3 nm is reached at 0 degrees C when the bulk water in the sample finally melts. A small proportion of the H(II) phase lipid enters an as yet unidentified cubic phase on freezing. The spacings of the (10) planes of the H(II) phase, the (111) planes of the cubic phase and the d-spacing of the L(alpha) phase were found to be almost identical at the phase transition temperature. The cubic phase appears to disappear at low temperature but to reform on heating. Freeze-fracture studies revealed no unequivocal evidence for cubic phase lipid but the presence of residual non-bilayer lipid structures was observed even at temperatures as low as -80 degrees C. The presence of intersecting stacks of lamellar sheets in the replicas strongly suggest the existence of an epitaxial relationship between the L(alpha) and H(II) phases in these systems.

A combined SAXS/WAXS investigation of the phase behaviour of di-polyenoic membrane lipids

Real-time measurements of the SAXS/WAXS diffraction patterns of aqueous dispersions (1:1 wt/wt) of the di-polyenoic lipids di-18:2 PC, di-18:3 PC, di-18:2 PE and di-18:3 PE were made over the temperature range 10 degrees to about -80 degrees C. The results of these measurements were compared to similar measurements performed on the corresponding di-18:0 and di-18:1 derivatives. SAXS measurements of the temperature dependence of lamellar repeat distances show that the di-polyenoic lipids undergo broad second-order transitions between their gel and liquid-crystal lamellar phases spanning 30-40 degrees C. The di-18:1 and di-18:0 derivatives, in contrast, undergo abrupt first-order transitions. The gel phases of the di-18:0 derivatives are characterised by two-component WAXS patterns with a sharp component close to 0.42 nm and a broader component at narrower spacings. On cooling, these lipids appear to undergo an initial transition to an L beta, phase followed by a conversion to an Lc phase. The gel phases of the di-18:1 derivatives also show two-component patterns but with the sharp component centred closer to 0.44 nm. The di-polyenoic lipids, in contrast, are characterised by a single broad peak centred at a spacing of about 0.42 nm, close to that of conventional L beta phases. The changes in lamellar repeat distance accompanying the transitions in the di-monoenoic and di-polyenoic lipids, all of which occur in the frozen state, are very similar, indicating that the acyl chains of the polyenoic lipids are close to their maximum extension in the gel state. The WAXS patterns of the polyenoic lipids suggest that the saturated upper parts of the acyl chains are packed on a regular hexagonal lattice while their polyunsaturated termini remain relatively disordered.

Effect of unsaturation on the chain order of phosphatidylcholines in a dioleoylphosphatidylethanolamine matrix

The properties of phosphatidylcholines (PCs) having a perdeuterated stearic acid, 18:0d35, in the sn-1 position and the fatty acid 18:0, 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 at the sn-2 position were investigated in a matrix of dioleoylphosphatidylethanolamine (DOPE) by 2H and 31P NMR spectroscopy. At a mole ratio of DOPE/PC = 5:1, the lipids form liquid crystalline lamellar phases below 40 degrees C and coexisting lamellar, inverse hexagonal (Hll), and cubic phases at higher temperatures. The sn-1 chain of the PCs in a DOPE matrix is appreciably more ordered than in pure PCs, corresponding to an increase in the hydrophobic bilayer thickness of approximately 1 A. Distearoylphosphatidylcholine in the DOPE matrix has a higher sn-1 chain order than the unsaturated PCs. We observed distinct differences in the lipid order of upper and lower sections of the hydrocarbon chains caused by changes of temperature, unsaturation, headgroups, and ethanol. Unsaturation lowers chain order, mostly in the lower third of the hydrocarbon chains. By contrast, the increase in chain order caused by the DOPE matrix and the decrease in order with increasing temperature have a constant magnitude for the upper two-thirds of the chain and are smaller for the lower third. Addition of 2 M ethanol reduced order parameters, in effect reversing the increase in chain order caused by the DOPE matrix.

Interaction between alpha-tocopherol and heteroacid phosphatidylcholines with different amounts of unsaturation

Differential scanning calorimetry was used to study the influence of a alpha-tocopherol on the thermotropic properties of model membranes composed by a series of heteroacid phosphatidylcholines with different amount of unsaturation in the sn-2 chain. The effect of alpha-tocopherol on 1,2-distearoylglycerophosphocholine (18:0,18:0), 1-stearoyl-2-oleoylgylcerophosphocholine (18:0,18:1), 1-stearoyl-2-linoleoylglycerophosphocholine (18:0,18:2), 1-stearoyl-2-linolenoylglycerophosphocholine (18:0,18:3), and 1-stearoyl-2-arachidonoylglycerophosphocholine (18:0,20:4) was determined. The addition of alpha-tocopherol perturbed the thermotropic gel to liquid-crystalline phase transition of these phospholipids. alpha-Tocopherol broadened the endotherm, lowered the transition temperature and decreased the associated enthalpy change. Partial phase diagrams showed the presence of fluid immiscibilities giving rise to lateral phase separation of domains containing different amounts of alpha-tocopherol. We suggest that, in these alpha-tocopherol-rich domains, the influence exerted by the vitamin on the phospholipids is strong enough to alter their thermotropic properties such that an additional endotherm appears in the thermogram, a characteristic not observed in homoacid phosphatidylcholines. alpha-Tocopherol caused a concentration-dependent removal of the detectable phase transition in all cases. The magnitude of the influence of alpha-tocopherol on phospholipid was dependent on the degree of unsaturation of the sn-2 acyl chain. These results are explained on the basis of the effect of alpha-tocopherol which will reduce the differences between gel and liquid crystalline states, the magnitude of these differences depending on the type of phospholipid considered, which are probably related to the change of molecular shape of phosphatidylcholines containing a polyunsaturated acyl chain.

Nuclear magnetic resonance investigation of hydrocarbon chain packing in bilayers of polyunsaturated phospholipids

2H nuclear magnetic resonance (NMR) on chain-deuterated phospholipids has been used to study the influence of the degree of unsaturation on lipid chain packing and on area per molecule at the lipid water interface. Order and motions of deuterated stearic acid in position sn-1 of phosphatidylcholines (PC) containing 18:0, 18:1n-9, 18:2n-6, 18:3n-3, 20:4n-6, 20:5n-3, or 22:6n-3 in position sn-2 were investigated in pure PC and in mixtures of PC in a phosphatidylethanolamine (PE) matrix. Results reveal that lipid packing in bilayers is mainly controlled by packing requirements at the lipid water interface. Increasing degrees of unsaturation lower chain order and increase area per PC molecule, whereas inclusion of PE in model membranes has the opposite effect. Chain order and motions in highly unsaturated lipid membranes are less sensitive to changes in temperature. Temperature sensitivity decreases further upon incorporation of PC into a PE matrix. Unsaturation induces chain disordering, which may be interpreted as an increase in area per molecule of lipids toward the center of the bilayer. This may result in a lower packing density of unsaturated lipids at the lipid water interface. We hypothesize that these differences in lipid packing and dynamics may influence activity of membrane proteins.

2H nuclear magnetic resonance order parameter profiles suggest a change of molecular shape for phosphatidylcholines containing a polyunsaturated acyl chain

Solid-state 2H nuclear magnetic resonance spectroscopy was used to determine the orientational order parameter profiles for a series of phosphatidylcholines with perdeuterated stearic acid, 18:0d35, in position sn-1 and 18:1 omega 9, 18:2 omega 6, 18:3 omega 3, 20:4 omega 6, 20:5 omega 3, or 22:6 omega 3 in position sn-2. The main phase transition temperatures were derived from a first moment analysis, and order parameter profiles of sn-1 chains were calculated from dePaked nuclear magnetic resonance powder patterns. Comparison of the profiles at 37 degrees C showed that unsaturation causes an inhomogenous disordering along the sn-1 chain. Increasing sn-2 chain unsaturation from one to six double bonds resulted in a 1.6-kHz decrease in quadrupolar splittings of the sn-1 chain in the upper half of the chain (or plateau region) and maximum splitting difference of 4.4 kHz at methylene carbon 14. The change in chain order corresponds to a decrease in the 18:0 chain length of 0.4 +/- 0.2 A with 18:2 omega 6 versus 18:1 omega 9 in position sn-2. Fatty acids containing three or more double bonds in sn-2 showed a decrease in sn-1 chain length of 0.7 +/- 0.2 A compared with 18:1 omega 9. The chain length of all lipids decreased with increasing temperature. Highly unsaturated phosphatidylcholines (three or more double bonds in sn-2) had shorter sn-1 chains, but the chain length was somewhat less sensitive to temperature. The profiles reveal that the sn-1 chain exhibits a selective increase in motional freedom in a region located toward the bottom half of the chain as sn-2 unsaturation is increased. This corresponds to an area increase around carbon atom number 14 that is three to four times greater than the increase for the top part of the chain. A similar asymmetric decrease in order, largest toward the methyl end of the chain, was observed when 1 -palmitoyl-2-oleoylphosphatidylethanolamine goes from a lamellar to an inverse hexagonal (H,,) phase. This is consistent with a change to a more wedge-shaped space available for the acyl chain.