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Lipids and Detergents
Lipids are molecules that contain both a hydrophilic head group and a hydrophobic fatty acid chain as shown in the scheme and space-filling model (of phosphatidyl choline) below. The polar head group is usually a common alcohol such as serine, ethanolamine, choline, or glycerol with a phosphate group. The fatty acid chains present in cells have a range of lengths with C16 to C18 being common.
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The actual packing of the lipids will be determined by geometric aspects of the lipid and interactions between the head groups. For lipids the formation of the bilayers (see Figure) is a rapid and spontaneous process once the concentration reaches a critical point. Other types of arrays of lipids are also possible. At an air-water interface the lipids will form monolayers with the polar head groups directed towards the water the fatty acid chains exposed to the air as seen in the figure below.
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Detergent molecules have the same overall properties of lipids, namely a polar head group and hydrocarbon chain. However, detergents usually have only one carbon chain and can pack into other arrays, micelles and liposomes, as shown below. In a micelle, the hydrophobic chains and in the interior and the polar head groups are exposed to the water while liposomes are bilayers arranged with a central polar environment as shown in the figures below.
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The proteins present in the membranes can be purified by the use of detergents.
The lipid environment surrounding the hydrophobic surface of the protein
is replaced by detergents allowing extraction from the cell as seen in the
figure below. The proteins can then be further purified by conventional techniques
provided the presence of detergents is maintained in the solutions.
Figures are from Stryer, Biochemistry.
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Photosynthesis Center Arizona State University Box 871604 Room PSD 209 Tempe, AZ 85287-1604
13 February 2006 |
phone: (480) 965-1963 fax: (480) 965-2747 |