Orientation of lutein in a lipid bilayer : revisited

Lutein is present in the human retina and lens, where it plays a protective role. As lutein is associated with the lipid matrix of biomembranes, the role depends on its membrane location. Experimental studies predicted two orientations of lutein in a phosphatidylcholine (PC) bi-layer: vertical and horizontal. Using a molecular dynamics simulation, we observed, in two different PC bilayers, both orientations of lutein, and in each bilayer, a single change from vertical to horizontal orientation or vice ver-sa. Both orientations were stabilized by hydrogen bonding of lutein OH groups with mainly carbonyl but also phosphate oxygen atoms of PC

Properties of water hydrating the galactolipid and phospholipid bilayers : a molecular dynamics simulation study

Molecular dynamics simulations of 1,2-di-O-acyl-3-O-β-D-galactopyranosyl-sn-glycerol (MGDG) and 1,2-dioleoyl-sn-glycero-3-phosphatidylcholine (DOPC) bilayers were carried out to compare the effect of the lipid head group's chemical structure on the dynamics and orientational order of the water molecules hydrating the bilayer. The effect of the bilayers on the diffusion of water is strong for the neighbouring water molecules i.e., those located not further than 4 Å from any bilayer atom. This is because the neighbouring water molecules are predominantly hydrogen bonded to the lipid oxygen atoms and their mobility is limited to a confined spatial volume. The choline group of DOPC and the galactose group of MGDG affect water diffusion less than the polar groups located deeper in the bilayer interface, and similarly. The latter is an unexpected result since interactions of water with these groups have a vastly different origin. The least affected by the bilayer lipids is the lateral diffusion of unbound water in the bilayer plane (x,y-plane) - it is because the diffusion is not confined by the periodic boundary conditions, whereas that perpendicular to the plane is. Interactions of water molecules with lipid groups also enforce certain orientations of water dipole moments. The profile of an average water orientation along the bilayer normal for the MGDG bilayer differs from that for the DOPC bilayer. In the DOPC bilayer, the ordering effect of the lipid head groups extends further into the water phase than in the MGDG bilayer, whereas inside the bilayer/water interface, ordering of the water dipoles in the MGDG bilayer is higher. It is possible that differences in the profiles of an average water orientation across the bilayer in the DOPC and MGDG bilayers are responsible for differences in the lateral pressure profiles of these bilayers

Network of lipid interconnections at the interfaces of galactolipid and phospholipid bilayers

Interactions among lipid head groups at the bilayer/water interface do, to a large extent, determine membrane properties. In this study graph theory is employed to objectively describe and compare the pattern of the interactions at the interfaces of computer models of 128- and 512-lipid monogalactolipid (MGDG) and phosphatidylcholine (DOPC) bilayers. Both MGDG and DOPC have polar head groups but of different chemical structures so at the bilayer interfaces they participate in different types of interaction. Nevertheless, at both interfaces these interactions and the lipid molecules they link make networks. In graph theory, a network of interconnected objects (nodes) is described by well-defined quantities which define its topology and can be used to assess inner properties of the network, its strength and density, etc. In this study, several topological properties of the networks in the DOPC and MGDG bilayers are determined. A comparison of these properties indicates that the topologies of both networks differ significantly but are stable during the simulation time. The networks in the MGDG bilayers are more extended, branched, stable, and stronger than those in the DOPC bilayers. This is consistent with the smaller surface area per lipid and higher rigidity of the MGDG than the DOPC bilayers as well as the tendency of MGDG to form an inverse hexagonal phase in water. The scale of the systems is an important factor when assessing the properties of the network; the system scaling is more evident in the DOPC bilayers where several quantities increase directly proportional to the increasing size of the system than in the MGDG bilayers where this is rarely the case

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On a generalization of multiplicative Sincov's equation for fuzzy implication functions

Characterizations of families of fuzzy implication functions are a necessary step to fully understand the behaviour of the members of the family, their potential applicability and their relations with other families. These characterizations are based on additional algebraical properties that completely define the family. Recently, the class of power based implications was characterized through, among others, the property I(x, y) ·I(y, z) =I(x, z)in a concrete sub-domain. This property, called in the literature also as multiplicative Sincov’s equation, was uncommon to other families, and it was studied in-depth in a previous article. This equality is generalized in this paper by understanding the internal product as the product t-norm and changing it to a general arbitrary continuous Archimedean t-norm. This additional property is analyzed jointly with the weak ordering property or the ordering property, leading to a characterization of those fuzzy implication functions satisfying both additional properties