Conversion of Fatty Acid Methyl Esters by Saponification for U^k'₃₇ Paleothermometry

Saponification is a technique commonly used to remove fatty acid methyl esters from a complex organic mixture.

Complex organic samples are often analyzed by gas chromatography, which is used to determine the relative concentrations of individual components.

However, compounds that are similar in size and structure cannot be distinguished by the instrument, skewing the results. Therefore, unwanted compounds that produce overlapping signals must be removed in order to obtain accurate results.

This video covers the use of saponification to purify alkenones for paleoclimatology. It is the first in a series detailing the purification of complex biomarker samples. It will cover the procedure, as well as some other uses of the technique.

Polyunsaturated long chain alkyl ketones, called alkenones, have been shown to provide useful information about past sea surface temperature.

However, the organisms that produce alkenones often create fatty acid methyl esters that are similar in size and chemical structure, called alkenoates. Because of these similarities, alkenoates must be removed before an accurate analysis can be obtained.

Saponification is a common technique used to prevent co-elution of these molecules. Saponification utilizes water to split the molecular bond of an ester. A base attaches to the carbon at the heart of the alkenoate. This addition reaction creates an unstable intermediate, and the alkoxide is expelled.

The hydrogen from the newly formed acid moves to the expelled alkoxide, and the resulting carboxylate anion forms an ionic bond with the cation from the base. The result is an alcohol and a fatty acid salt. Adding a strong acid will re-generate the carboxylic acid. At this point, the offending alkenoate has been converted to a form that no longer co-elutes with the alkenone of interest.

Now that you understand how saponification can be used to purify an organic mixture, you are ready to begin the procedure.

First, acquire a dried total lipid extract - or TLE - that was obtained using a solvent extraction method. Next, prepare the saponification solutions as outlined in the text protocol. Ensure that all components are pure and free of hydrocarbons. Once the solutions are prepared, add the dried TLE to a 60-mL borosilicate glass vial. Add 2 mL of 2 normal potassium hydroxide, and seal the vial. Next, heat the vial to 60 ?C for 2.5 h to cleave the ester bond. When this is complete, allow the sample to cool to room temperature. Once the sample has cooled, add 2 mL of 5% sodium chloride solution to the vial. Cap the vial, and shake briefly. Add 6 N hydrochloric acid dropwise until a pH of 2 is reached - using pH paper to test. The addition of this acid will protonate the carboxylate anion to form the final product - the stable carboxylic acid.

Now that the acidified solution is ester-free, add 5 mL of hexane. Cap and shake vigorously for 5 seconds to extract the organic compounds from the water. Allow the solution to rest until the organic and aqueous phases separate completely. Salts, ions, and unreacted hydrochloric acid will remain in the aqueous phase, while organic compounds will separate into the hexane. Once the phases have completely separated, remove approximately 75% of the hexane using a pipette, and dispense into another 40-mL vial. Repeat this extraction process two additional times, adding 5 mL of hexane each time. Once this is complete, discard the leftover aqueous solution into a proper waste container. Label the vial containing the freshly saponified organic phase. The carboxylic acids produced cannot be injected into the instruments commonly used for alkenone analysis without further purification. Carboxylic acids injected into a gas chromatograph quickly accumulate and ruin inlets, inlet liners, and the front end of the columns.?To remove these acids, the sample first needs to undergo another purification technique: separation via column chromatography.

Saponification has several applications in the extraction and purification of organic molecules.

Saponification can be used not only to separate biomarkers, but also to extract individual components for use in commercial products. In this example, compounds from the tobacco tree -?Nicotiana glauca?- were isolated and analyzed to investigate their potential as feedstock for a wide variety of bio-based products, such as fuel, heat, and an array of chemical compounds.

The leaves were first homogenized, then centrifuged, to concentrate the molecules of interest. The concentrated plant material was then saponified. The extracted material was analyzed with liquid chromatography-mass spectrometry, to determine the concentration of tocopherol - a family of vitamin E compounds typically found in plants.

The extracted material can also be reconstituted?in vitro?to its original composition. In this example, saponification was used to extract carotenoids from spinach plants, to be later reconstituted?in vitro. The spinach was first homogenized, then centrifuged, to harvest the pigment molecules. These molecules were then suspended in a solution of potassium hydroxide in a separatory funnel, initiating saponification. The saponified carotenoids separated into an ether layer, which was collected and dried. Using a series of solution buffers, the carotenoids - and other pigmentation molecules - were later reconstituted?in vitro. This purification allowed for analysis of these pigments without the interference of similarly structured organic compounds.

Because of its ability to hydrolyze esters, saponification can be used to "free" compounds that are otherwise bound to macromolecules. In this example, an anhydrous saponification step is used to convert ethyl 4-fluorobenzoate to the carboxylic acid salt of potassium4-fluorobenzoate. This deprotection through saponification allows for the production of crude SFB - a reaction that wouldn't be possible had the molecule remained "stuck".

You've just watched JoVE's introduction to the purification of Uk'37?samples through saponification. You should now understand how saponification works, and how to use it to purify alkenones in a total lipid extract. Further purification processes will be demonstrated in subsequent videos.

Thanks for watching!