Short Chain Fatty Acids?

I came across this story in the Sydney Morning Herald this morning, "A make-or-break moment for what may be a new molecule". Sounds exciting doesn't it. A hint of uncertainty, is it or isn't it a new molecule? What will it "make or break"?

It appears that Charles Mackay, a Professor of Microbiology at Monash Biomedicine Discovery Institute has, "demonstrated that dietary fibre and its breakdown fermented products, the short chain fatty acids (SCFAs) influence gut homeostasis, the composition of the gut microbiota, immune tolerance, and inflammatory responses."  I must apologize, I've never been particularly interested in biology, so I actually have no idea what any of that means, but from the story in the newspaper it appears that Professor Mackay thinks we should be eating more "short chain fatty acids" as are present in fermented foods such as vinegars and hence vegetables pickled in vinegars, and this will help regulate our blood pressure and immune system, keeping us healthy. Good news for me, I love pickled onions and gherkins, and have been known to indulge in sauerkraut and kimchi.

But what on earth is a "short chain fatty acid"?

Now, a fatty acid is a long chain carboxylic acid.
A carboxylic acid is a molecule with a carboxylic acid functional group (COOH) at the end of a hydrocarbon chain.
For example, formic acid is the carboxylic acid which is present in an ant's sting. It has only one carbon atom in the chain, its formula is HCOOH (CH₂O₂)
Acetic acid is the carboxylic acid that gives vinegar it's tang. It has 2 carbon atoms in its chain,

CH₃-COOH (C₂H₄O₂)

But fatty acids have lots of carbon atoms in the chain, common fatty acids have 12 or 14 or 16, or 18 carbon atoms in the chain. These fatty acids (long chain carboxylic acids) are found naturally in the oils and fats of plants and animals. If the long hydrocarbon chain is saturated (contains only single bonds between the carbon atoms in the chain) it is called a saturated fatty acid. If there is one, or more, double bonds between carbon atoms in the long chain then it is referred to as an unsaturated fatty acid.

So a "short chain fatty acid" would be what? A short long chain carboxylic acid? Sounds like nonsense doesn't it? Maybe it's a medium length chain carboxylic acid?

According to wikipedia (which may or may not be a reliable source of information),  a "short chain fatty acid" is a fatty acid with 2 to 6 carbon atoms. Included in wikipedia's list of "short chain fatty acids" are the two carboxylic acids we used as examples above, formic acid (which has only 1 carbon atom in its chain and hence does not actually fit within wikipedia's own definition and hence my concern about the reliability of the information it provides), and, acetic acid. So, a "short chain fatty acid" is in fact NOT a short chain "fatty acid", it is simply a short chain carboxylic acid!

Is the "molecule" new, as claimed by the headline? Most unlikely because:

• "short chain fatty acids" is just a poor description of a group of molecules sharing a carboxylic acid functional group, not just 1 molecule
• we've known about these short chain carboxylic acids for a very, very long time

References:
http://www.smh.com.au/national/a-make-or-break-moment-for-what-may-be-a-new-miracle-molecule-20180216-p4z0ky.html
https://research.monash.edu/en/persons/charles-mackay 
https://en.wikipedia.org/wiki/Short-chain_fatty_acid 

Suggested Study Questions:

1.  Draw the 2-dimensional structure formula for each of the following carboxylic acids:
   • formic acid
   • acetic acid
   • propanoic acid
   • butanoic acid
   • pentanoic acid
   • hexanoic acid
2. Draw a condensed structural formula for each of the carboxylic acids in question 1.
3. Draw a skeletal structural formula for each of the carboxylic acids in question 1.
4. Give the molecular formula for each of the carboxylic acids in question 1.
5. Give the empirical formula for each of the carboxylic acids in question 1
6. Are the carboxylic acids in question best described as saturated or unsaturated? Explain your answer.
7. On each 2-dimensional structural formula, identify, circle and name the functional group common to all the molecules.
8. Draw at least 2 structural isomers with the molecular formula C₅H₁₀O₂
9. Circle the functional groups in the molecules you draw in question 8. Name these functional groups.
10. Choose 2 of the structural isomers drawn in question 8. Would you expect these 2 molecules to have very similar, or very different, physical and chemical properties. Explain your answer.