# Double bonds in polyunsaturated fatty acids

$\omega - 3$ fatty acids are very frequently discussed. $\omega - 6$ is also mentioned. However, I don't recall mention of $\omega - 1$, $\omega - 2$, or $\omega - 4$. Also, in polyunsaturated fatty acids, the double bonds are frequently all multiples of $3$ from the $\omega$ end.

Is it just some quirk of evolution that has led to this pattern or is there a chemical explanation? For example, are double bonds at these positions more favourable than other positions? If an $\omega - 2$ or $\omega - 4$ fatty acid was synthesised might it isomerise to $\omega - 3$?

In the cases with multiple double bonds 3 apart, is it really: pure single, pure single, pure double repeating? Or is there some degree of delocalisation and the "single" bonds are actually a bit more than single and the "double" bond a bit less than double? I guess that it does not go to the extreme of benzene in which all the bonds are equal or we would have developed more accurate names and notations to represent this.

• There is no delocalisation here. However, if the bonds were 2 apart, then there would be some, and that would change pretty much everything. You'd have to develop a whole new set of enzymes to work with those. – Ivan Neretin Jun 27 '18 at 11:17
• @IvanNeretin Thanks, so the standard notation is accurate and not like the old alternating single and double bond diagram for benzene. If delocalisation cannot occur at this level of separation then I guess that it partially answers my question. The double bonds could not be closer without a very significant change in properties. – badjohn Jun 27 '18 at 11:22
• That's exactly right. – Ivan Neretin Jun 27 '18 at 11:34