Double chain amphiphiles form bilayers instead of micelles. Note: single and double chain amphiphiles can form other multimolecular aggregate structures as well, but these are the most common and are the only ones we will consider. Figure: Structures of single and double chain amphipiles in water - Micelles and Bilayers.
The micelle interior is completely nonpolar. Spherical bilayers that enclose an aqueous compartment are called vesicles or liposomes. Micelles and bilayers, formed from single and double-chain amphiphiles, respectively, represent noncovalent aggregates and hence are formed by an entirely physical process.
No covalent steps are required. The formation of these structures can be understood from the study of the intermolecular forces IMFs involved as well as thermodyamics. First we will review the IMFs involved. Consider the attractive forces. The buried acyl chains can interact and be stabilized by London forces. They are sequestered from water.
This view fits our simple axiom of "like-dissolves like". The polar head groups can be stabilized by ion-dipole bonds between charged head groups and water.
Likewise H-bonds between water and the head group stabilizes the exposed head groups in water. Repulsive forces may also be involved. Head groups can repel each other through steric factors, or ion-ion repulsion from like-charged head groups.
The attractive forces must be greater than the repulsive forces, which lead to these molecular aggregates. In this arrangement of molecules, the outer layer hydrophilic heads are arranged where they are pointed outwards exposing to the outer environment. The inner hydrophilic core is formed by the hydrophilic heads of the inner layer. The hydrophobic tails of both layers are arranged between the two concentric rings. The formation of a liposome occurs by a process where the dry lipid molecules are hydrated through a nonpolar solvent that is followed an agitation process mechanical induction.
The major sources for liposome formation are phospholipid molecules along with cholesterol. The types of liposomes vary according to how they are formed. This criterion of liposome classification depends on the extent of mechanical agitation and the use of a polar solvent in some instances. In the human body, the liposomes are taken up by organs that are rich in the reticuloendothelial system.
Therefore the main objective of liposomes is drug delivery, which is targeted to these organs. In order to target specific tumor cells, the liposomes are coated with special polymers. The relative liposomes production process is costly. Therefore, these liposomes are used only during viral infection treatment and tumor cell killing. Drug administration is achieved via the parenteral route. Micelle is defined as a lipid molecule that is arranged in a spherical form in aqueous solution.
Micelles are formed in response to the amphipathic nature of fatty acids. Micelles are composed of both hydrophilic regions and hydrophobic regions. The hydrophilic regions are polar head groups whilst the hydrophobic regions are the long hydrophobic chains tails. The polar head groups usually involve in the formation of the outside layer of the micelles since they have the ability to interact with water due to their polar nature.
The hydrophobic tails are present inside of the structure to prevent the interaction with water due to their nonpolar nature. Fatty acids that are produced from micelles contain a single hydrocarbon chain in opposite direction to two hydrocarbon chains. This structure enables the fatty acids to develop a spherical shape and thereby it lessens the steric hindrance that occurs within the fatty acid molecules themselves.
Fatty acids from Glycolipids and phospholipids, on the other hand, have two hydrophobic chains that are too bulky to fit into the a spherical shape as micelles do. Thus, they preferred to form glycolipids and phospholipids as "lipid bilayers", which are discussed in the next section. Micelles form spontaneously in water, as stated above this spontaneous arrangement is due to the amphipatic nature of the molecule.
The driving force for this arrangement is the hydrophobic interactions the molecules experience. When the hydrophobic tails are not sequestered from water this results in in the water forming an organized cage around the hydrophobic tail and this entropy is unfavorable.
However, when the lipids form micelles the hydrophobic tails interact with each other, and this interaction releases water from the hydrophobic tail and this increases the disorder of the system, and this increase in entropy is favorable. The preferred structure of lipids in aqueous solutions are usually a bilayer sheet of lipids rather than spherical micelles. This is because the two fatty acid chains are too big and bulky to fit into the interior of a micelle.
Therefore, micelles usually have one hydrocarbon chain instead of two. Lipid bilayers" form rapidly and spontaneously in an aqueous media and are stabilized by hydrophobic interactions, Van der Waals attractive forces, and electrostatic interactions. The function of the lipid bilayer is to form a barrier between the two sides of the membrane. Due to the fact that the lipid bilayer consists of hydrophobic fatty acid chains, ions and most polar molecules have trouble passing through the bilayer.
The one exception to this rule is water because water has a high concentration, small size, and a lack of a complete charge. In order for a molecule to pass through the lipid bilayer it must move from an aqueous environment to a hydrophobic environment and then back into an aqueous environment. Therefore the permeability of small molecules is related to the solubility of said molecule in a nonpolar solvent versus the solubility of the molecule in water. Micelles can also have a structure that is inside out of its normal structure.
Instead of having the hydrocarbon chains inside, they can face outside and while the polar heads are arranged inside the sphere.
This happens in a "water in oil" situation because there is so much oil surrounding the drop of water that the hydrocarbon chains face outside instead of inside. Sizes of micelles range from 2 nm 20 A to 20 nm A , depending on composition and concentration. The size of a micelle is more limited than that of a lipid bilayer. A lipid bilayer can span up to 10 7 A or 10 6 nm. The lipid bilayer is not a rigid structures, rather they are quite fluid. The individual lipid molecules are able to move or diffuse laterally across the membrane quite easily, this process is called lateral diffusion.
However, lipids have much more trouble flipping from one side of the membrane to the other, this process is called traverse diffusion or flip, because this would involve the polar head traveling through the hydrophobic core, and this interaction between polar and hydrophobic regions is unfavorable. So the lipid can move around laterally at a rate of about 2 micrometers per second, while it takes a much longer amount of time to flip flop.
As the temperature is increased the fluidity of the lipid bilayer increases as well. Also the more cis double bonds the hydrocarbon tail has the more fluid the structure becomes. This is because when the hydrocarbon tail has cis double bonds it can no longer pack as well as the saturated hydrocarbon tail, so it becomes more fluid.
Also the longer the hydrocarbon tail, the higher the transition temperature, which is the temperature at which the bilayer goes from rigid to fluid, this is because longer hydrocarbon tails can interact more strongly than shorter chains.
0コメント