General Index
Amphipathic Structures
Amphipathic lipids may undergo Autostructuration. In aqueous media the
hydrophobic portion tend to be excluded from water, interacting among themselves (the Hydrophobic Effect) while
the polar portions interact with water. For this reason, amphipathic lipids are present at interphases,
and hence, in cell membranes.
In water, some amphipathic compounds, like Hexyl sulfate:
may form Micelles, spherical aggregates in which the hydrophobic portion of the lipid is directed towards the inside while the polar part (sulfate group) is at the outside in contact with water:
In a hydrophobic medium (benzene, hexane, chloroform) this same molecule would form Inverse Micelles:
With the hydrophobic groups in contact with the solvent and the polar sulfate groups around an internal cavity in which polar compounds like water could be trapped.
Autostructuration can also give laminar structures, that are the structural basis of the biological membranes.
NOTE: The following images have been produced by Drs.H.Heller, M.Schaefer y K. Schulten of the Ludwig Maximilian University of Munich, Germany
Consider a lecithin molecule (Hydrogens not represented):
if we extend a group of lecithin molecules on a interphase (air-water, or hydrophobic-polar), the hydrocarbon chains direct themselves towards the air, while the polar heads interact with the surface of water. This structure is known as Monolayer:
Biological membranes are formed by the superposition of two lipid monolayers, in such as way that the hydrophobic tails are directed towards the interior of the structure, and the polar heads towards the exterior, giving the structure called Bilayer:
In this image we can appreciate how the polar heads are solvated with water:
Depending on many factors, but mostly on temperature, bilayers can be in different states. We have seen the Liquid Crystal phase, that is the most ordered state. A phase much more disordered is the Fluid phase:
In general, membranes are lipid bilayers, but also containing proteins and a variety of amphipathic lipids.
Detergents are amphipathic molecules, and their mode action consist in the formation of micelles that trap in their interior hydrophobic molecules. Let's see the structures of Dodecyl sulfate (Lauryl sulfate):
And Cetyl trimethylammonium (CTAB, Cetavlon):