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I'm learning about how fatty acid names are formed. In the process, I noticed a molecule called alpha-linolenic acid. There's a similar molecule called linoleic acid. Both have 18 carbons, but alpha-linolenic acid has three double bonds, one more than linoleic acid.

Interestingly, linoleic acid doesn't have alpha, beta, or gamma subtypes. It's simply called "linoleic acid." My research revealed the existence of alpha-linolenic acid and gamma-linolenic acid, but there's no apparent "beta-linolenic acid."

What determines the alpha and gamma designations in alpha-linolenic acid and gamma-linolenic acid, respectively?


Linoleic acid:
linoleic acid

γ-Linolenic acid:
γ-Linolenic acid

α-Linolenic acid
α-Linolenic acid

Note: I came across this figure on the internet but I'm not sure it has anything to do with what alpha and gamma means in "alpha-/gamma- linolenic acids".

text in image: Use of Greek letters to designate carbons. The carbon next to the -COOH group is designated α; the next one is β, and so forth. The most distant carbon is designated ω. Sometimes carbon atoms close to the carbon are designated in relation to it. Eg, the third from the end is ω 3 (omega minus 3).

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The nomenclature of these acids seems to be a historical convention.

Alpha-linolenic acid was first isolated in pure form in 1909 by Ernst Erdmann and F. Bedford (Ref.1). The following passage from their paper (translated from German, emphasis added) explains the nomenclature:

We designate this linolenic acid, which the solid hexabromide [of linolenic acid] provides, as α-linolenic acid in order to distinguish [it] from an isomer [that will be] mentioned later.

In the very next paper in the same journal, Erdmann and coworkers have also elucidated the structure of $\alpha$-linolenic acid (Ref.2). Also in Ref.1, they have identified an isomer of above compound as beta-linolenic acid, which they described as follows:

The linolenic acid obtained by reduction represents a mixture of two isomeric acids, namely alpha-linolenic acid, which is present in linseed oil and yields the solid hexabromide with a melting point of 179°, and beta-linolenic acid, which adds only 4 bromine atoms smoothly and forms a liquid tetrabromide $\ce{C18H14Br4O2}$.

However this turned out to be an artifact of the original analytical process. In 1919, Heiduschka and Lüft identified gamma-linolenic acid (Ref.3).

Citations:

  1. E. Erdmann and F. Bedford (in German), "Über die im Leinöl enthaltene Linolensäure (On linolenic acid [that's] contained in flax oil)." Berichte der Deutschen Chemischen Gesellschaft 1909, 42(1), 1324–1333 (DOI: https://doi.org/10.1002/cber.190904201217).
  2. E. Erdmann, F. Bedford, and F. Raspe (in German), "Konstitution der Linolensäure (Structure of linolenic acid)," Berichte der Deutschen Chemischen Gesellschaft 1909, 42(1), 1334–1346 (DOI: doi:10.1002/cber.190904201218).
  3. A. Heiduschka and K. Lüft (in German), “Das fette Oel der Samen der Nachtkerze (Oenothera biennis) und über eine neue Linolensäure,” Archiv der Pharmazie 1919, 257(1-4), 33–69 (DOI: https://doi.org/10.1002/ardp.19192570105).
  4. Gamma-linolenic acid on Wikipedia
  5. Alpha-linolenic acid on Wikipedia
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