Polysialate
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Recrystallization of racemic phenylalanine, some observations and questions
I found some very old racemic phenylalanine among some old nutritional supplements. At least I think it was racemic phenylalanine; the label had worn
off so I'm just going by vague recollection, appearance, smell, and it having water solubility consistent with phenylalanine. Rather than just
throwing it out and buying some new, I thought I'd try to recrystallize it.
The first thing I found is that it forms a polyhydrate, such that leaving an 85°C saturated solution (~7g/100mL water) to cool to 20°C overnight
just results in the whole thing solidifying. The "crystals" grow as (hemi)spherical solids from nucleation points until the whole thing solidifies.
It's interesting to watch, but not particularly useful, since the perfectly round shape implies an amorphous precipitate. Plus the lack of supernatant
liquid to filter off means all impurities are still present in the solid.
I found a somewhat recent paper about crystallization of L-phenylalanine:
https://www.mdpi.com/2073-4352/10/2/60/htm
It confirms that L-phenylalanine forms a hydrate, though the only hydrate it describes is a crystalline monohydrate (confirmed by X-ray diffraction).
I'm guessing that since I have racemic phenylalanine, an amorphous polyhydrate solid phase is precipitating instead. Either that or I have some other
other old nutritional supplement powder which I've misidentified.
Taking some ideas from that paper, I re-heated it to ~85°C and added ethanol (such that it was now ~15% ethanol). Upon cooling to room temp, the
crystals that formed actually looked crystalline, albeit like tiny white sea urchins. IE, a lot of needle-like crystals growing outward from each
nucleation point. This seems to be a viable means of purifying racemic phenylalanine.
Some questions: if anyone here has recrystallized racemic phenylalanine, does this match your experience? On a more theoretical note, are the
individual "growths" of crystals enantiomerically resolved? Or does racemic phenylalanine crystallize racemically?
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zed
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Ummm. I would reflux it all with acetophenone. Soon enough you will know what it is. And your unknown, if it is Phenylalanine... Will be converted
to Phenethylamine.
Confusing Zwitterions and Racemic mixtures.... Gone!
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Polysialate
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Quote: Originally posted by zed  | Ummm. I would reflux it all with acetophenone. Soon enough you will know what it is. And your unknown, if it is Phenylalanine... Will be converted
to Phenethylamine.
Confusing Zwitterions and Racemic mixtures.... Gone! |
Alternatively, just adding a bit of bleach should result in Strecker Degradation to phenylacetaldehyde, which has a very potent and characteristic
smell:
http://www.sciencemadness.org/talk/viewthread.php?tid=66331
Will update if I pick some up.
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Polysialate
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I found something interesting in my attempts at recrystallization which might generalize to other compounds. By briefly super-cooling only the bottom
of the container (while the bulk of the liquid was still ~80°C), I could nucleate a different crystal structure instead of the needle-like
fuzzballs that I was getting before. Basically by placing it on a sheet of ice for only a few minutes, the resulting nuclei would be more regular
polyhedra instead.
If over-cooled, the needle-like crystals would grow off of them, but by immediately re-heating the whole thing to ~60°C after the nucleation, then
slow cooling it over 24 hours, I got much nicer, larger crystals which are easily washed and filtered.
I'm not sure if this is a different crystal structure of phenylalanine, or if the locally lower temperatures drove the precipitation of a particular
hydrate which was different than the needle-like crystals I was getting before.
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