Omega three fatty acids are almost as well known as vitamins as important parts of the diet, but what are they?
As with all complex ideas, it has been reduced to its simplest form as “omega threes”, which as you will soon see, is not actually very descriptive at all. A slightly better name is poly-unsaturated fatty acids, but that still gets shortened by scientists to "PUFAs".
Fatty acids are integral components of all biological cells. They consist of a carboxyl group at one end (labelled the delta end, when used by chemists), and a carbon backbone extending for up as many as 50 carbon atoms or more. Most fatty acids of dietary concern are 8 to 24 carbons long, and usually an even number. The tail end (labelled the omega end) is of greater interest to physiologists and nutritionists, as this is where all the biochemical action takes place. Fatty acids are found in cells as free fatty acids, or as part of phospholipids or triglycerides, (2 or 3 acids bound to a phosphate or glycerol backbone).
A fatty acid is “saturated” if each carbon atom in the chain uses its 4 available electrons to bond to the 2 adjacent carbons and 2 hydrogen atoms. Saturated fats until recently, generally were regarded as the “bad” fats. (So it might help to ignore these when trying to solve the puzzles.)
A saturated fatty acid: C12:0. (NB the delta 1 carbon isn't shown in these diagrams.)
If a pair of carbons shares 2 electrons in a double bond, then the chain is un-saturated. Only 1 hydrogen atom can bond with each carbon atom on each side of the bond, and the chain develops a kink (which I haven't drawn, but you can look it up elsewhere if you like). If the hydrogens are on the same side the fatty acid is a “cis” acid, but if they are on opposite sides it is a “trans” acid or a “really bad” fat. When there is only one double bond, the acid is mono-unsaturated. If there is more than one double bond it is... anyone?... anyone?... a poly-unsaturated fat. Multiple double bonds are usually always 3 carbons apart due to the biochemical pathways involved. As mentioned above, the distance this last double bond is located from the omega end of the carbon chain gives it the "omega" value, usually written as n-3 or n-6, as the case may be.
Omega 3 and omega 6 fatty acids are mono- and poly-unsaturated fatty acids with their double bonds 3 or 6 carbon atoms from the end of their chains.
An omega 3 fatty acid: Docosahexanoic Acid (DHA) C22:6(n-3).
A polyunsaturated omega 6 fatty acid: Linoleic acid C18:2(n-6).
Physiologists have found many biological functions are improved by increasing the proportion of omega 3 fatty acids in the diet. Some nutritionists suggest that the modern Western diet has an imbalance of omega 6 to omega 3 acids and we should adjust it by eating more fish and other seafoods with higher omega 3 content than typically mammalian based diets provide. By labelling these fatty acids with their chain length, the number of double bonds (if any) and the distance from the omega carbon in the sequence, the composition of foods and oil mixtures is much easier to summarize. A typical acid is linoleic acid C18:2 (n-6). You can see the 6 double bonds with the crucial one between carbons 6 and 7 from the omega tail end, which is "delta12" carbon from the head end.
These compounds are usually measured by gas chromatography (known as GC by us scientist types). GC is a chemical analysis technique that has been around since the 1950s and can be used to analyse a wide variety of organic materials. It is a favourite tool of TV scriptwriters when you need forensic results... fast. It sounds cool, but its a whole lot more complex than most TV plotlines allow.
(Thanks to xkcd.com for the comics published under creative commons 2.5).
A gas chromatograph hard at work.
In short, (but longer than a scriptwriter's attention span) it requires extracting the sample of interest from the matrix (e.g. petrol contaminants from soil or water, fatty acids from blood or tissue samples etc) in a suitable solvent, then injecting about 1 microlitre into a long glass capillary (up to 120 m long, 250 micrometres internal diameter). The glass has a coating of waxes or silicone polymers on the interior surface (the stationary phase), that, if you choose the right material, the compound(s) you are trying to identify will “stick” to. The compounds can be persuaded to come off by heating the whole column in a very precisely controlled oven. Many different sensor types are available to detect the compounds leaving the column, and most can detect levels down to parts-per-billion level or even better.
The time each compound comes off the column (normally measured in decimal minutes) is related to the structure of the compounds, the stationary phase, and the temperature of the column. The peak height and area are also very closely related to the amount of the compound present in the sample. Calibrating the system requires a set of known compounds to be analysed so that the time of elution and the detector response can be checked for each run. As long as everything is kept the same each time, you will get highly reproducible results from sample to sample, making GC a very reliable method of identifying an unknown mixture of previously identified compounds. One feature of fatty acid analysis is that because many different combinations of PUFAs can exist as pairs and triplets as phospholipids and triglycerides, they have to be chemically broken apart and methylated, forming fatty acid methyl esters or FAMEs.
Analysing a FAME chromatogram can involve simply seeing what PUFAs are present in the samples, calculating the percentage of the omega 3 and omega 6 compounds as a fraction of the entire yield, or it might require more in depth quantification to determine the actual mass of each compound. Some scientists suggest that the molar ratios (ie the molecular mass) of the omega 3 and omega 6 acids is the most important factor in a dietary context.
So now, after that rather extensive introduction, you should be able to interpret the following chromatogram from a sample of red herring we processed in the laboratory recently. But there are several twists!
Click on the image to get a bigger view - you will need it.
I'm sorry, I couldn't get a bigger container.
The fish in the background picture are in the garden of the venue at the listed coordinates. Feel free to email me: as I tell the students, the only stupid questions are the ones you don't ask.
I found this coordinate checker when trying some puzzles from OS:
You can validate your puzzle solution with certitude
When you've hade enough of this one, try:Gas Chromatography II and Gas Chromatography III