Most naturally occurring unsaturated fatty acids have cis double bonds
The carbon chains of saturated fatty acids form a zigzag pattern when extended, as at low temperatures. A higher temperatures, some bonds rotate, causing chainshortening, which explains why biomembranes become thinner with increases in temperature. A type of geometric isomerism occurs in unsaturated fatty acids, depending on the orientation of atoms or groups around the axes of double bonds, which do not allow rotation. If the acyl chains are on the same side of the bond, it is cis-, as in oleic acid; if on opposite sides, it is trans-, as in elaidic acid, the trans isomer of oleic acid. Naturally occurring unsaturated long-chain fatty acids are nearly all of cis configuration. Thus, oleic acid has an L shape, whereas elaidic acid remains “straight.” Increase in the number of cis double bonds in a fatty acid leads to a variety of possible spatial configurations of the molecule – e.g., arachidonic acid, with four cis double bonds, has “kinks” or a U shape. This has profound significance on molecular packing in membranes and on the positions occupied by fatty acids in more complex molecules such as phospholipids. Trans double bonds alter these spatial relationships. Trans fatty acids are present in certain foods, arising as a by-product of the saturation of fatty acids during hydrogenation, or “hardening,” of natural oils in the manufacture of margarine. An additional small contribution comes from the ingestion of ruminant fat that contains trans fatty acids arising from the action of microorganisms in the rumen.
