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It’s been said over and over in my biochemistry books that the cis-double bonds between the Cs in the hydrocarbon tails in triglycerides causes a kink. This kink disappears when the bond is isomerized into its trans conformation. What is it about the cis bond that causes the kink?

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  • $\begingroup$ Do you know what a kink is? And what the shape of a cis double bond is? $\endgroup$ – bon Sep 27 '15 at 9:24
  • $\begingroup$ Yes. But why is the cis double bond shaped like that? $\endgroup$ – lightweaver Sep 27 '15 at 9:48
  • $\begingroup$ Does @BenNorris's answer solve your problem, or are you asking about the reason for cis/trans isomerism and the shapes of the respective isomers? $\endgroup$ – bon Sep 27 '15 at 12:19
  • $\begingroup$ "But why is the cis double bond shaped like that?" - The cis isomer, by definition, is the one in which the substituents on the double bond are on the same side. This leads to the kink that you see in the pictures that @BenNorris uploaded in his answer. Rotation of the C-C single bonds adjacent to the double bond is restricted, so once an alkene is in the cis configuration, it will stay as a cis alkene unless something helps it isomerise (in a biochemistry context, probably an enzyme, but you may not come across that yet so don't worry). $\endgroup$ – orthocresol Sep 27 '15 at 15:49
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    $\begingroup$ You'll have to go and read up about electron pair repulsion and the concept of hybridisation in organic chemistry. The bond angles of the sp2 carbon (i.e. the ones in the double bond) are 120 degrees. How much chemistry background do you have? I don't think you should be reading biochemistry without having taken some organic chemistry in the past. $\endgroup$ – orthocresol Sep 28 '15 at 4:53
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The best way to demonstrate this kink is with pictures. Usually we have some motivation to try to represent the molecule on a single line. So for oleic acid (cis-octadec-9-en-1-oic acid), we often get pictures like the following:

enter image description here

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Elaidic acid, the trans-isomer is also sometimes depicted unhelpfully to save space:

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enter image description here

The second structure for elaidic acid is not as misleading as the corresponding structure for oleic acid. A better bond-line structure for oleic acid is:

enter image description here

Better yet, here are some space-filling structures from Wikipedia based on crystal structures of both molecules.

Oleic

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Elaidic

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And here's $\gamma$-linolenic acid, which has three cis alkenes in it:

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