Insights on PUFA-containing lipid membranes probed by MD simulations

Indiana University-Purdue University Indianapolis (IUPUI)The cell membrane serves as a barrier between the interior and exterior of a living cell. Its main structural component is the lipid bilayer, which is composed of various kinds of lipids that segregate into domains. These lipid domains, distinguished in composition and physical properties from the bulk lipids that surround them, are believed to modulate the function of resident proteins by providing an appropriate lipid environment. Polyunsaturated fatty acids (PUFA) are a type of fatty acid that contain multiple C=C double bonds. They have a lot of health benefits, which may originate in part due to their incorporation into lipids in the plasma membrane. Hypotheses that PUFA-containing lipids laterally separate into domains and/or modulate the structure of existing domains have been raised to explain the fundamental role played by PUFA. In our research, we use molecular dynamics (MD) simulations to simulate model membranes composed of PUFA-containing phospholipids and to investigate their interaction with cholesterol and vitamin E that are influential membrane constituents. The presumptive function for vitamin E in membranes is to protect PUFA against oxidation. Although the chemistry of the process is well established, the role played by the molecular structure that we address with atomistic molecular dynamics (MD) simulations remains controversial. We compared the behavior of vitamin E in lipid bilayers composed of 1-stearoyl-2-docosahexaenoylphosphatidylcholine (SDPC, 18:0-22-6PC) and 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18:0-18:1PC) via all-atom MD simulations at 37° C. SDPC represents a PUFA-containing lipid, and SOPC serves as monounsaturated control. From the calculation of van der Waals energy of interaction between vitamin E and fatty acid (FA) chains, we found higher probability that the PUFA chains surround the chromanol head group on vitamin E. This is further demonstrated by probability density maps of acyl chains around vitamin E molecules. Also, an ability to more easily penetrate deep into the PUFA containing bilayer of vitamin E is detected by faster flip-flop rate of vitamin E observed in the SDPC bilayers. These results showed that the high disorder of polyunsaturated docosahexaenoic acid (DHA) chains allows vitamin E to easily tunnel down into the bilayer and often brings the PUFA chains up to the surface of the bilayer, improving the likelihood that the reactive (hydroxyl) group on vitamin E would encounter a lipid peroxyl radical and terminate the oxidation process. Thus, the simulations indicate that the molecular structure of vitamin E supports its role as an antioxidant in a PUFA-containing membrane. A subsequent study on the partitioning of vitamin E into PUFA-containing lipids was done by analyzing the binding energy of vitamin E in the corresponding lipid bilayer. The binding energy is obtained from the potential of mean force (PMF) profile of vitamin E alone the membrane normal direction (z), which is calculated from umbrella sampling MD simulations. We found the binding in SDPC is smaller in SOPC, indicating that vitamin E does not prefer PUFA-containing phospholipids. The flip-flop rate was also estimated from the PMF profile, confirming that vitamin E flip-flops across the SDPC bilayer more easily than the SOPC bilayer. From the simulations it was noted that the membrane deforms as vitamin E is pulled out, which suggests interactions between the phospholipids contribute to the binding energy of the vitamin E. In a final study, a comparison was made between the effect on membrane organization of the three types of long chain omega-3 (n-3) PUFA found in fish oils: eicosapentaenoic acid (EPA, 20:5), DHA (22:6) and docosapentaenoic acid (DPA, 22:5). MD simulations were run on lipid bilayers composed of 1-stearoyl-2-eicosapentaenoylphosphatidylcholine (EPA-PC, 18:0-20:5PC), 1-stearoyl-2-docosapentaenoylphosphatidylcholine (DPA-PC, 18:0-22:5PC), SDPC (DHA-PC, 18:0-22:6PC) and, as a monounsaturated control, SOPC (OA-PC, 18:0-18:1PC) in the absence and presence of cholesterol. By analyzing the physical properties such as membrane order and thickness, we found all three n-3 PUFAs disorder the membrane. The disordering is greatest with EPA and least with DPA. Unique among the n-3 PUFA-containing membranes, there is region of high order in the upper portion of the DPA chain. The PUFA-containing lipids were found to less favorably interact with cholesterol compared to the OA-containing lipid, which is caused by their disorder. We speculate that differences between DPA, DHA and EPA might potentially modulate their effect on lipid domain formation

Present Developments in Image Analysis

The paper discusses novel methods of analysing oriented textures, explains some of the problems which had to be overcome to make these methods work, and indicates where future developments might be expected. This has required improvements in the methods of intensity gradient analysis, the development of large filter methods for mapping features defined in terms of their texture, and the introduction of ideas from the theory of regionalised variables

Analysis of Some Textured Images by Transputer

Texture, as a visual perception, can be easily seen by eye and often described without much difficulty. However, textural recognition and measurement by machine is a very different issue and has only recently been developed. In this thesis, a whole set of new algorithms have been developed to analyse textured images with particular reference to the requirements of soil microstructural applications. The new technology of parallel processing is used to implement and improve the complicated computations

Membrane Disordering by Eicosapentaenoic Acid in B Lymphomas Is Reduced by Elongation to Docosapentaenoic Acid as Revealed with Solid-State Nuclear Magnetic Resonance Spectroscopy of Model Membranes

BACKGROUND: Plasma membrane organization is a mechanistic target of n-3 (ω-3) polyunsaturated fatty acids. Previous studies show that eicosapentaenoic acid (EPA; 20:5n-3) and docosahexaenoic acid (DHA; 22:6n-3) differentially disrupt plasma membrane molecular order to enhance the frequency and function of B lymphocytes. However, it is not known whether EPA and DHA affect the plasma membrane organization of B lymphomas differently to influence their function. OBJECTIVE: We tested whether EPA and DHA had different effects on membrane order in B lymphomas and liposomes and studied their effects on B-lymphoma growth. METHODS: B lymphomas were treated with 25 μmol EPA, DHA, or serum albumin control/L for 24 h. Membrane order was measured with fluorescence polarization, and cellular fatty acids (FAs) were analyzed with GC. Growth was quantified with a viability assay. (2)H nuclear magnetic resonance (NMR) studies were conducted on deuterated phospholipid bilayers. RESULTS: Treating Raji, Ramos, and RPMI lymphomas for 24 h with 25 μmol EPA or DHA/L lowered plasma membrane order by 10-40% relative to the control. There were no differences between EPA and DHA on membrane order for the 3 cell lines. FA analyses revealed complex changes in response to EPA or DHA treatment and a large fraction of EPA was converted to docosapentaenoic acid (DPA; 22:5n-3). NMR studies, which were used to understand why EPA and DHA had similiar membrane effects, showed that phospholipids containing DPA, similar to DHA, were more ordered than those containing EPA. Finally, treating B lymphomas with 25 μmol EPA or DHA/L did not increase the frequency of B lymphomas compared with controls. CONCLUSIONS: The results establish that 25 μmol EPA and DHA/L equally disrupt membrane order and do not promote B lymphoma growth. The data open a new area of investigation, which is how EPA's conversion to DPA substantially moderates its influence on membrane properties

All n-3 PUFA are not the same: MD simulations reveal differences in membrane organization for EPA, DHA and DPA

Eicosapentaenoic (EPA, 20:5), docosahexaenoic (DHA, 22:6) and docosapentaenoic (DPA, 22:5) acids are omega-3 polyunsaturated fatty acids (n-3 PUFA) obtained from dietary consumption of fish oils that potentially alleviate the symptoms of a range of chronic diseases. We focus here on the plasma membrane as a site of action and investigate how they affect molecular organization when taken up into a phospholipid. All atom MD simulations were performed to compare 1-stearoyl-2-eicosapentaenoylphosphatylcholine (EPA-PC, 18:0–20:5PC), 1-stearoyl-2-docosahexaenoylphosphatylcholine (DHA-PC, 18:0–22:6PC), 1-stearoyl-2-docosapentaenoylphosphatylcholine (DPA-PC, 18:0–22:5PC) and, as a monounsaturated control, 1-stearoyl-2-oleoylphosphatidylcholine (OA-PC, 18:0–18:1PC) bilayers. They were run in the absence and presence of 20 mol% cholesterol. Multiple double bonds confer high disorder on all three n-3 PUFA. The different number of double bonds and chain length for each n-3 PUFA moderates the reduction in membrane order exerted (compared to OA-PC, ̅ = 0.152). EPA-PC (̅ = 0.131) is most disordered, while DPA-PC ( ̅ = 0.140) is least disordered. DHA-PC (̅ = 0.139) is, within uncertainty, the same as DPA-PC. Following the addition of cholesterol, order in EPA-PC (̅ = 0.169), DHA-PC (̅ = 0.178) and DPA-PC (̅ = 0.182) is increased less than in OA-PC (̅ = 0.214). The high disorder of n-3 PUFA is responsible, preventing the n-3 PUFA-containing phospholipids from packing as close to the rigid sterol as the monounsaturated control. Our findings establish that EPA, DHA and DPA are not equivalent in their interactions within membranes, which possibly contributes to differences in clinical efficacy

Interaction of α-tocopherol with a polyunsaturated lipid studied by MD simulations

poster abstractPolyunsaturated phospholipids are essential components of neural membranes and their effect on membrane architecture is proposed to be the molecular origin of a myriad of health benefits. A downside of polyunsaturated phospholipids is that they are highly susceptible to oxidation due to the presence of multiple double bonds. α-Tocopherol is the most biologically active component in a family of phenolic compounds that comprise vitamin E, which is the major lipid soluble antioxidant in cell membranes. To investigate whether α-tocopherol preferentially interacts with polyunsaturated phospholipids to optimize protection against oxidation, we performed MD simulations on 1-stearoyl-2-docosahexaenoylphosphatiylcholine (SDPC, 18:0-22:6PC) and 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18:0-18:1PC) bilayers containing α-tocopherol. SDPC with a docosahexaenoyl sn-2 chain is polyunsaturated, while SOPC with an oleoyl sn-2 chain serves as a monounsaturated control. The simulations were run under constant pressure for 200 ns on a system that comprised 80 phospholipid molecules, 20 α-tocopherol molecules and 2165 water molecules. We discovered significant differences between the two systems. Analysis of the simulations indicates that the α-tocopherol has a strong interaction with the polyunsaturated fatty acid. The flip-flop of α-tocopherol across the bilayer is also much faster in SDPC than in SOPC. Solid state NMR, neutron scattering and complementary experiments are now underway to test the predictions from the MD simulations

α-Tocopherol is well designed to protect polyunsaturated fatty acids

poster abstractPolyunsaturated fatty acids (PUFA) are an influential constituent in cell membranes, but are extremely vulnerable to oxidation. The presumptive role for α-tocopherol (α-toc), the molecular form of vitamin E retained by the human body, is to protect PUFA-containing lipids from oxidation. To investigate whether α-toc preferentially interacts with PUFA in support of this function, we performed MD simulations on lipid bilayers composed of 1-stearoyl-2-docosahexaenoylphosphatidylcholine (SDPC, 18:0-22-6PC) and 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18:0-18:1PC) in the presence of α-toc. SDPC with docosahexaenoic acid (DHA) for the sn-2 chain is polyunsaturated, while SOPC with oleic acid (OA) for the sn-2 chain serves as a monounsaturated control. The simulations were run at 37 °C under constant pressure for 200 ns on a system that comprised 80 phospholipid molecules, 20 α-toc molecules and 2165 water molecules. In qualitative agreement with our results from solid state 2H NMR and neutron scattering experiments, the simulations show that α-toc increases order inside the bilayer and that the chromanol headgroup sits near the surface in both SDPC and SOPC. Analyses of the density distribution of the lipid chains relative to α-toc show that the α-toc’s chromanol headgroup, the part of the molecule that protects against oxidation, would have more chance to interact with PUFA chains than saturated chains. A major prediction from our simulations is that α-toc undergoes flip-flop across the bilayer and that the rate is an order of magnitude greater in SDPC than SOPC. This is a remarkable finding that reveals a possible mechanism by which the chromanol group would not only wait at the membrane surface but would also patrol the membrane interior to meet lipid radicals and terminate the chain reaction by which lipid peroxidation proceeds

Heat transfer performance of a device integrating thermosyphon with form-stable phase change materials

peer reviewedPhase change materials (PCM) are applied worldwide as a thermal energy storage technology to reduce energy demands in buildings and solve environmental contamination issues. Form-stable phase change materials (FSPCM), as one branch of PCMs, can be improved by embedding them with thermosyphon, resulting in a better thermal performance. In this paper, a novel thermoplastic elastomer-based FSPCM was developed and tested. A device integrating FSPCM with thermosyphon was created, and the heat transfer mechanism of the unit was studied. The numerical model was established, and experiments were conducted accordingly. The average relative error between the experimental data and the model predictions was <3 %. Furthermore, a parameter study was conducted to investigate the effects of several essential factors. As a result, the evaporator length and PCM thermal conductivity were found to significantly influence the unit's heat transfer rate and overall thermal performance. However, the impact of the latent heat of the FSPCM on the heat transfer rate was negligible, for which it takes 1170 s, 1230 s, and 1394 s to finish changing phase with 136 kJ/kg, 160 kJ/kg, and 200 kJ/kg, respectively. This paper provides insights on the performance of thermosyphon integrated form-stable phase change materials and discusses its relevance for thermal energy storage applications.11. Sustainable cities and communitie

Docosahexaenoic acid regulates the formation of lipid rafts: A unified view from experiment and simulation

Docosahexaenoic acid (DHA, 22:6) is an n-3 polyunsaturated fatty acid (n-3 PUFA) that influences immunological, metabolic, and neurological responses through complex mechanisms. One structural mechanism by which DHA exerts its biological effects is through its ability to modify the physical organization of plasma membrane signaling assemblies known as sphingomyelin/cholesterol (SM/chol)-enriched lipid rafts. Here we studied how DHA acyl chains esterified in the sn-2 position of phosphatidylcholine (PC) regulate the formation of raft and non-raft domains in mixtures with SM and chol on differing size scales. Coarse grained molecular dynamics simulations showed that 1-palmitoyl-2-docosahexaenoylphosphatylcholine (PDPC) enhances segregation into domains more than the monounsaturated control, 1-palmitoyl-2-oleoyl-phosphatidylcholine (POPC). Solid state 2H NMR and neutron scattering experiments provided direct experimental evidence that substituting PDPC for POPC increases the size of raft-like domains on the nanoscale. Confocal imaging of giant unilamellar vesicles with a non-raft fluorescent probe revealed that POPC had no influence on phase separation in the presence of SM/chol whereas PDPC drove strong domain segregation. Finally, monolayer compression studies suggest that PDPC increases lipid-lipid immiscibility in the presence of SM/chol compared to POPC. Collectively, the data across model systems provide compelling support for the emerging model that DHA acyl chains of PC lipids tune the size of lipid rafts, which has potential implications for signaling networks that rely on the compartmentalization of proteins within and outside of rafts

Vitamin E Has Reduced Affinity for a Polyunsaturated Phospholipid: An Umbrella Sampling Molecular Dynamics Simulations Study

Vitamin E is an essential micronutrient. The primary function of this lipid-soluble antioxidant is to protect membrane phospholipids from oxidation. Whether vitamin E preferentially interacts with polyunsaturated phospholipids to optimize protection of the lipid species most vulnerable to oxidative attack has been an unanswered question for a long time. In this work, we compared the binding of α-tocopherol (αtoc), the form of vitamin E retained by the human body, in bilayers composed of polyunsaturated 1-stearoyl-2-docosahexaenoylphosphatidylcholine (SDPC, 18:0-22:6PC) and, as a control, monounsaturated 1-stearoyl-2-oleoylphosphatidylcholine (SOPC, 18:0-18:1PC) by umbrella sampling molecular dynamics simulations. From the potential of mean force as a function depth within the bilayer, we find that the binding energy of αtoc is less in SDPC (ΔGbind = 16.7 ± 0.3 kcal/mol) than that in SOPC (ΔGbind = 18.3 ± 0.4 kcal/mol). The lower value in SDPC is ascribed to the high disorder of polyunsaturated fatty acids that produces a less tightly packed arrangement. Deformation of the bilayer is observed during desorption, indicating that phosphatidylcholine (PC)–PC and αtoc–PC interactions contribute to the binding energy. Our results do not support the proposal that vitamin E interacts more favorably with polyunsaturated phospholipids