sandozscientist8
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Botanicals ~ understanding control factors in Acid / Base rxn extractions
I have been developing methods for the isolation of phyto-constituents from natural products for a number of years.
I would like to better understand the control factors that govern the outcomes of acid base reactions preformed on extracts of natural products.
To give an example:
The Chinese yew presents an interesting target for exploration, containing a number of unique photo-chemicals.
One would initially preform a single solvent extraction with some type of generic primary alcohol such as methanol or ethanol. The extraction may or
may not be heated. Afterwards would be removed of the spent plant matter. The extract would then be concentrated to a liquor, aprx. 20% of initial
volume. This is the substance then subjected to acid : base reaction.
This is my primary focus in this line of questioning; When I go into method development to determine the best combination of acid and base to use I
am able to achieve a whole range of different outcomes resulting in the extraction of different products with varying acids and bases. Of course I
always try to maintain strict PH guidelines so that this is not a variable ( usually acidify to PH 2.8 and basify to PH 11) sometimes varies based on
the pKa of the photo-constituent. I always take into consideration the pKa of the desired photo-constituents. However, still, so many different
outcomes result from different acid : base combinations. These combinations of acids/bases are only further divided by the solvent then used to
extract from the reaction mixture. So really the variables are 1.Initial extraction solvent, 2. Acid, 3. Base, 4. Final Extraction Solvent.
I am just trying to better understand the process/control factors that govern the selectivity of these reactions and their ability to result in the
"extractability" of a particular compound/set of compounds of interest.
Does anyone out there have a good protocol they use for method development in this sense?
It would save a great deal of time and laborious experimentation if I were able to circumvent so much of this R&D part of the process.
My understanding of acid base chemistry in this area though is lacking. Even if anyone has a good reference for something like this it would be most
appreciated!
Thanks everyone!
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JJay
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I'm not sure if this is the right section for this question, but the best single source of information on that topic would be a long-running series
called The Alkaloids. These days it is published by Elsevier with Hans-Joachim Knölker as editor. I am pretty sure they have at least one entry on
yews, which are a source of taxol and for that reason heavily researched.
One problem with using an alcohol for the initial extraction is that they tend to dissolve a lot of undesired substances and can complicate the
workup.
Acid-base extractions work very differently with nitrogen-containing compounds and carboxylic acids, and many things you would be extracting are both.
Often, hydrochloride salts of nitrogen-containing compounds are soluble in water. The same could be said for sodium or potassium salts of carboxylic
acids. And often, the free bases and free acids are not water soluble but are soluble in nonpolar solvents. Sulfates and phosphates are usually less
soluble in water than hydrochlorides. Salts of organic acids can convey a range of physical properties, such as solubility of one chirality and not
another or solubility in a particular organic solvent and not another. For this reason, they can be useful for purification, and I think some organic
acid salts, particularly citrates, tend to be more stable than the corresponding mineral acid salts when composed of an easily-oxidized alkaloid.
There aren't a lot of hard-and-fast rules, and you'll want to do a lot of experimentation to try to optimize the process anyway. So good luck with it.
[Edited on 30-1-2017 by JJay]
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Melgar
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I read a paper a few years ago, where they extracted the alkaloids from a plant by crushing it up mechanically, adding HCl until the pH dropped to the
right level, then running the filtered aqueous extract through an acidified ion exchange resin. It was essentially the same type of stuff that they
use in water softeners, ie polystyrene sulfonate/divinylbenzene crosslinked copolymer. The anions were eluted with dilute HCl. Workup is easy
enough, since you can just evaporate, then selectively dissolve the organic salts (which should all be amine HCl salts) in methanol or ethanol.
I've actually used this a few times myself, to isolate amines that are prone to dissolving in water with a basic pH. It's used a lot for isolating
proteins and amino acids I know, via ion-exchange chromatography.
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Nicodem
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Thread Moved
30-1-2017 at 11:44 |
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