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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Figure 4 The cis <strong>and</strong> trans configuration of double bonds in fatty acids. Naturally occurring<br />

fatty acids are predominantly of the cis isomer.<br />

packing plays a key role in membrane structure. The placement of unsaturated fatty<br />

acids into membranes is confined, with few exceptions, to the sn-2 position of phospholipids;<br />

as a result, the unsaturated fatty acid content of membranes rarely if ever<br />

exceeds 50 mol % unsaturated acyl chains. An advantage of including both an unsaturated<br />

<strong>and</strong> a saturated fatty acid on the same phospholipid molecule is that the<br />

two types of acyl chains cannot demix in the membrane. This ensures that slight<br />

depressions in the energy of association, such as those achieved between the spontaneously<br />

formed sphingomyelin-cholesterol in raft complexes, are not achieved with<br />

individual fatty acid types. The association between adjacent phospholipid molecules<br />

is entirely dependent on the van der Waals forces that attract their fatty acid components.<br />

These effects can be attributed to two primary structural properties of the<br />

fatty acid: (1) the length of the acyl chain <strong>and</strong> (2) the degree of unsaturation. These<br />

two structural elements can significantly affect both the relative volume the fatty<br />

acid occupies <strong>and</strong> the distance between a fatty acid <strong>and</strong> its neighboring acyl chain.<br />

The degree of unsaturation is particularly important for membrane properties because<br />

van der Waals forces are acutely sensitive to the distance between the interacting<br />

acyl chains [50].<br />

Estimates of the spatial widths of fatty acids show significant differences between<br />

cis <strong>and</strong> trans double bonds. Whereas stearic acid is estimated to be 0.25 nm<br />

in diameter, �9-cis-octadecanoic acid (oleic) <strong>and</strong> �9-trans-octadecanoic acids have<br />

spatial widths of 0.72 <strong>and</strong> 0.31 nm, respectively [50]. Additionally, due to the fact<br />

that the acyl chain continues on the same side of the double bond, the cis configuration<br />

imparts a 30� bend in the fatty acid that is not relievable through any rotation<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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