Community hub
Recent from talks
Knowledge base stats:
Talk channels stats:
Members stats:
Lipid polymorphism
In biophysics and colloidal chemistry, polymorphism is the ability of lipids to aggregate in a variety of ways, giving rise to structures of different shapes, known as "phases". This can be in the form of spheres of lipid molecules (micelles), pairs of layers that face one another (lamellar phase, observed in biological systems as a lipid bilayer), a tubular arrangement (hexagonal), or various cubic phases (Fd3m, Im3m, Ia3m, Pn3m, and Pm3m being those discovered so far). More complicated aggregations have also been observed, such as rhombohedral, tetragonal and orthorhombic phases.
It forms an important part of current academic research in the fields of membrane biophysics (polymorphism), biochemistry (biological impact) and organic chemistry (synthesis).
Determination of the topology of a lipid system is possible by a number of methods, the most reliable of which is x-ray diffraction. This uses a beam of x-rays that are scattered by the sample, giving a diffraction pattern as a set of rings. The ratio of the distances of these rings from the central point indicates which phase(s) are present.
The structural phase of the aggregation is influenced by the ratio of lipids present, temperature, hydration, pressure and ionic strength (and type).
In lipid polymorphism, if the packing ratio[clarification needed] of lipids is greater or less than one, lipid membranes can form two separate hexagonal phases, or nonlamellar phases, in which long, tubular aggregates form according to the environment in which the lipid is introduced.
This phase is favored in detergent-in-water solutions and has a packing ratio of less than one. The micellar population in a detergent/water mixture cannot increase without limit as the detergent to water ratio increases. In the presence of low amounts of water, lipids that would normally form micelles will form larger aggregates in the form of micellar tubules in order to satisfy the requirements of the hydrophobic effect. These aggregates can be thought of as micelles that are fused together. These tubes have the polar head groups facing out, and the hydrophobic hydrocarbon chains facing the interior. This phase is only seen under unique, specialized conditions, and most likely is not relevant for biological membranes.
Lipid molecules in the HII phase pack inversely to the packing observed in the hexagonal I phase described above. This phase has the polar head groups on the inside and the hydrophobic, hydrocarbon tails on the outside in solution. The packing ratio for this phase is larger than one, which is synonymous with an inverse cone packing.
Extended arrays of long tubes will form (as in the hexagonal I phase), but because of the way the polar head groups pack, the tubes take the shape of aqueous channels. These arrays can stack together like pipes. This way of packing may leave a finite hydrophobic surface in contact with water on the outside of the array. However, the otherwise energetically favorable packing apparently stabilizes this phase as a whole. It is also possible that an outer monolayer of lipid coats the surface of the collection of tubes to protect the hydrophobic surface from interaction with the aqueous phase.
Hub AI
Lipid polymorphism AI simulator
(@Lipid polymorphism_simulator)
Lipid polymorphism
In biophysics and colloidal chemistry, polymorphism is the ability of lipids to aggregate in a variety of ways, giving rise to structures of different shapes, known as "phases". This can be in the form of spheres of lipid molecules (micelles), pairs of layers that face one another (lamellar phase, observed in biological systems as a lipid bilayer), a tubular arrangement (hexagonal), or various cubic phases (Fd3m, Im3m, Ia3m, Pn3m, and Pm3m being those discovered so far). More complicated aggregations have also been observed, such as rhombohedral, tetragonal and orthorhombic phases.
It forms an important part of current academic research in the fields of membrane biophysics (polymorphism), biochemistry (biological impact) and organic chemistry (synthesis).
Determination of the topology of a lipid system is possible by a number of methods, the most reliable of which is x-ray diffraction. This uses a beam of x-rays that are scattered by the sample, giving a diffraction pattern as a set of rings. The ratio of the distances of these rings from the central point indicates which phase(s) are present.
The structural phase of the aggregation is influenced by the ratio of lipids present, temperature, hydration, pressure and ionic strength (and type).
In lipid polymorphism, if the packing ratio[clarification needed] of lipids is greater or less than one, lipid membranes can form two separate hexagonal phases, or nonlamellar phases, in which long, tubular aggregates form according to the environment in which the lipid is introduced.
This phase is favored in detergent-in-water solutions and has a packing ratio of less than one. The micellar population in a detergent/water mixture cannot increase without limit as the detergent to water ratio increases. In the presence of low amounts of water, lipids that would normally form micelles will form larger aggregates in the form of micellar tubules in order to satisfy the requirements of the hydrophobic effect. These aggregates can be thought of as micelles that are fused together. These tubes have the polar head groups facing out, and the hydrophobic hydrocarbon chains facing the interior. This phase is only seen under unique, specialized conditions, and most likely is not relevant for biological membranes.
Lipid molecules in the HII phase pack inversely to the packing observed in the hexagonal I phase described above. This phase has the polar head groups on the inside and the hydrophobic, hydrocarbon tails on the outside in solution. The packing ratio for this phase is larger than one, which is synonymous with an inverse cone packing.
Extended arrays of long tubes will form (as in the hexagonal I phase), but because of the way the polar head groups pack, the tubes take the shape of aqueous channels. These arrays can stack together like pipes. This way of packing may leave a finite hydrophobic surface in contact with water on the outside of the array. However, the otherwise energetically favorable packing apparently stabilizes this phase as a whole. It is also possible that an outer monolayer of lipid coats the surface of the collection of tubes to protect the hydrophobic surface from interaction with the aqueous phase.