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| [A liquid crystal retains at least one dimension of order relative to the] solid state. Under usual physiological conditions, the lipid bilayers of biological membranes most closely resemble the lamellar liquid crystalline phase.
Hexagonal and cubic phases. For some phospholipid systems, the gel phase melts directly to a fluid non-lamellar phase with a hexagonal or cubic symmetry. In the hexagonal I phase HI, the phospholipids are organized in the form of cylinders with the polar headgroups facing out and the hydrocarbon chains facing the interior. The cylinders are packed onto a hexagonal lattice and the volume between the cylinders is filled with a continuous water phase. These long tubes can be thought of as many micelles fused together. This phase structure is only formed under specific conditions and is not particularly relevant to biological membranes. In the inverse hegagonal [sic] II phase HII, the phospholipids also aggregate into cylinders, but with the headgroups facing the inside and the hydrocarbon chains the outside. The polar groups contact water and surround an aqueous channel located at the center of the cylinder. These tubes also form a hexagonal array in cross section. The HII phase is very common in phospholipids such as PE having small weakly hydrated headgroups and attractive headgroup-headgroup interactions [12]. Phospholipids can also exhibit cubic phases, which almost behave as isotropic phases. The cubic phase structure consists of short tubes connected in a hexagonal array. In this structure, the phospholipid molecules experience all possible orientations, which accounts for its isotropic character. Cubic-like phases are observed in mitochondrial and ER membranes [13,14]. The reasons why biological membranes incorporate into their structure lipids that destabilize the bilayers organization can be found in many physiological processes. For instance, such lipids facilitate membrane fusion and stabilize regions of high curvature. The fluid phases are schematically represented in Figure 1.4. 12. Seddon, J. M. Biochim. Biophys. Acta 1990, 1031, 1. 13. Landh, T. FEBS Lett. 1995, 369, 13. 14. Luzzati, V. Curr. Opin. Struct. Biol. 1997, 7, 661. |
A liquid crystal retains at least one dimension of order relative to the solid state. Under usual physiological conditions, the lipid bilayer of biological membranes most closely resembles the lamellar liquid crystalline phase.
Hexagonal and cubic phases For some phospholipid systems, the gel phase melts directly to a fluid non-lamellar phase with a hexagonal or cubic symmetry. In the hexagonal I phase HI, the phospholipids are organized in the form of cylinders with the polar headgroups facing out and the hydrocarbon chains facing the interior. The cylinders are packed onto a hexagonal lattice and the volume between the cylinders is filled with a continuous water phase. These long tubes can be thought of as many micelles fused together. This phase structure is only formed under specific conditions and is not particularly relevant to biological membranes. In the inverse hexagonal II phase HII , the phospholipids also aggregate into cylinders, but with the headgroups facing the inside and the hydrocarbon chains the outside. The polar groups contact water and surround an aqueous channel located at the center of the cylinder. These tubes also form a hexagonal array in cross section. The HII phase is very common in phospholipids such as PE having small weakly hydrated headgroups and attractive headgroup-headgroup interactions [21]. Phospholipids can also exhibit cubic phases, which almost behave as isotropic phases. The cubic phase structure consists of short tubes connected in a hexagonal array. In this structure, the phospholipid molecules experience all possible orientations, which accounts for its isotropic character. Cubic-like phases are observed in mitochondrial and ER membranes [22,23]. The reasons why biological membranes incorporate into their structure lipids that destabilize the bilayer organization can be found in many physiological processes. For instance, such lipids facilitate membrane fusion and stabilize regions of high curvature. The fluid phases described above are schematically represented in Figure 1.10. [21] J. M. Seddon. Biochim. Biophys. Acta, 1031:1–69, 1990. [22] T. Landh. FEBS Lett., 369:13–17, 1995. [23] V. Luzzati. Curr. Opin. Struct. Biol., 7:661–668, 1997. |
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