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 H 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.

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 20mol% 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, S¯=0.152). EPA-PC (S¯=0.131) is most disordered, while DPA-PC (S¯=0.140) is least disordered. DHA-PC (S¯=0.139) is, within uncertainty, the same as DPA-PC. Following the addition of cholesterol, order in EPA-PC (S¯=0.169), DHA-PC (S¯=0.178) and DPA-PC (S¯=0.182) is increased less than in OA-PC (S¯=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.

Vitamin E (α-tocopherol) and a range of other biological compounds have long been known to promote the H II (inverted hexagonal) phase in lipids. Now, it has been well established that purely hydrophobic lipids such as dodecane promote the H II phase by relieving extensive packing stress. They do so by residing deep within the hydrocarbon core. However, we argue from X-ray diffraction data obtained with 1-palmitoyl-2-oleoylphosphatidylcholine (POPE) and 1,2-dioleoylphosphatidylcholine (DOPE) that α-tocopherol promotes the H II phase by a different mechanism. The OH group on the chromanol moiety of α-tocopherol anchors it near the aqueous interface. This restriction combined with the relatively short length of α-tocopherol (compared to DOPE and POPE), means that α-tocopherol promotes the H II phase by relieving compressive packing stress. This observation offers new insight into the nature of packing stress and lipid biophysics. With the deeper understanding of packing stress offered by our results, we also explore the role molecular structure plays in the primary function of vitamin E, which is to prevent the oxidation of polyunsaturated membrane lipids.

The detailed lipid organization of cellular membranes remains elusive. A typical plasma membrane contains hundreds of different lipid species that are actively regulated by the cell. Currently over 40,000 biologically relevant lipids have been identified and specific organisms often synthesize thousands of different lipid types. This is far greater diversity than is needed to maintain bilayer barrier properties and to solvate membrane proteins. Why do organisms go through the costly process of creating and maintaining such a large diversity of lipids? What is the individual role of these lipids, and how do they interact and organize in the membrane plane? To start to address these questions we model biologically realistic membranes using coarse-grained Martini molecular dynamics simulations. We optimized and developed the Martini lipidome and systematically explored physiochemical properties of >100 different Martini lipid types. Bulk properties of each type (e.g. bilayer thickness, area per lipid, diffusion, order parameter and area compressibility) were analyzed and overall trends compared to experimental values. Biologically realistic idealized membrane compositions were constructed and simulated, such as in (Ingólfsson, et al. Lipid organization of the plasma membrane. JACS, 136:14554-14559, 2014). These large-scale simulations (~70 by 70 nm and multi microsecond long) are in terms of lipid composition by an order of magnitude the most complex simulations to date. They provide a high-resolution view of the lipid organization of biologically relevant membranes; revealing a complex global non-ideal lipid mixing of different species at different spatiotemporal scales. We analyze a variety of membrane physicochemical properties, including: lipid-lipid interactions, bilayer bulk material properties, domain formation and coupling between the bilayer leaflets, for a number of lipid mixtures and conditions.