LIPID MINITOPICS

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Micelles

Introduction:

A micelle is formed when a variety of molecules including soaps and detergents are added to water. The molecule may be a fatty acid, a salt of a fatty acid (soap), phospholipids, or other similar molecules.

The molecule must have a strongly polar "head" and a non-polar hydrocarbon chain "tail". When this type of molecule is added to water, the non-polar tails of the molecules clump into the center of a ball like structure, called a micelle, because they are hydrophobic or "water hating". The polar head of the molecule presents itself for interaction with the water molecules on the outside of the micelle.


Structure of Dodecylphosphocholine (DPC):

An example of a micelle uses DPC is a synthetic phosphodiester. The phosphoric acid group has ester bonds between 1) choline, (CH3)3N(CH2)2OH, and 2) dodecyl (or lauryl) alcohol, CH3(CH2)11OH.

The choline,which contains a quaternary amine with a positive charge, and the phosphate are ionic and polar. The dodecyl part is the non-polar hydrocarbon chain.

 Quiz: Which part of the molecule is soluble in water?  
Which part of the molecule is insoluble in water?  


Structure of a Micelle:

The theoretical model shows 54 molecules of dodecylphosphocholine (DPC) and about 1200 H2O molecules. Each lipid has a polar head group (phosphocholine) and a hydrophobic tail (dodecyl = C12).

The graphic on the left represents a cross section of a micelle.

The gray spheres on the interior represent the long hydrocarbon chains of the dodecyl groups which are massed together because they are non-polar.

The polar head groups of the phosphate are shown as red and orange spheres. The amine nitrogen is shown in blue surrounded by the gray methyl groups.

The water molecules are represented as red and white spheres surrounding the outside of the micelle and penetrates all of the spaces in the head group region.

Micelle - Chime in new window

The hydrophobic tails are shown Spacefill. H2O is excluded from this entire interior volume. The hydrocarbon chains vary in their individual conformations (e.g. trans/gauche configuration at each carbon-carbon bond), but adapt so as to fill all of the interior space.

 

Single DPC and Surrounding Molecules:

The close-up of a DPC molecule (spacefill) in the micelle is shown in the graphic on the left. Other DPC neighbor molecules are shown in thick wire form. The rest of the micelle is white sticks.

The DPC is in contact with 10-15 H2O's (red/white spheres) that make favorable H-bond or ion-dipole interactions (<3.5 Å).

Neighboring DPC molecules that are within 4.0 Å of each DPC are thicker Sticks; the atoms on each that can make favorable van der Waals interactions are colored yellow.

Note: In contrast to protein crystal structures where interior atoms are relatively fixed. ON the other hand, the micelle interior is highly dynamic, i.e. each lipid may have 4-8 contacting neighbor lipids at any instant, but these partners change several times every nanosecond on average.

Chime Credits: Dr. William McClure, Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213

Credits: D. P. Tieleman, D. van der Spoel, H.J.C. Berendsen (2000) "Molecular dynamics simulations of dodecyl phosphocholine micelles at three different aggregate sizes: micellar structure and lipid chain relaxation" J. Phys. Chem. B 104:6380-6388.
Dr. Tieleman's Project Page at the Univ. of Calgary describes micelle structures in more detail.