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Distinguishing Between Alanine and Glycine

Alanine is chiral so would rotate plane polarised light but Glycine isn't so wouldn't rotate plane polarised light. That means you could distinguish the above amino acids using this idea. However, isn't that assuming we don't have a racemic mixture of alanine?
Original post by KaziMahathir
Alanine is chiral so would rotate plane polarised light but Glycine isn't so wouldn't rotate plane polarised light. That means you could distinguish the above amino acids using this idea. However, isn't that assuming we don't have a racemic mixture of alanine?

correct
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Pigster
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Original post by KaziMahathir
Alanine is chiral so would rotate plane polarised light but Glycine isn't so wouldn't rotate plane polarised light. That means you could distinguish the above amino acids using this idea. However, isn't that assuming we don't have a racemic mixture of alanine?


I'm guessing the nice people in the spectroscopy department have banned you again?
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BDavies1
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Original post by Pigster
I'm guessing the nice people in the spectroscopy department have banned you again?

Naturally occurring alanine (or any amino acid) is always just one optical isomer, never a racemic mixture. Alanine is synthesised by enzymes which are always stereospecific i.e. they only work with one optical isomer and not the other. So the alanine would rotate the plain of polarisation of light and the glycine wouldn't.

You could run a thinlayer chromatogram, develop with Ninhydrin and then compare the rf value to known samples of glycine and alanine. However, this would require you to have a reference sample of both amino acids.
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Pigster
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Original post by BDavies1
Naturally occurring alanine (or any amino acid) is always just one optical isomer, never a racemic mixture. Alanine is synthesised by enzymes which are always stereospecific i.e. they only work with one optical isomer and not the other. So the alanine would rotate the plain of polarisation of light and the glycine wouldn't.

You could run a thinlayer chromatogram, develop with Ninhydrin and then compare the rf value to known samples of glycine and alanine. However, this would require you to have a reference sample of both amino acids.

Why tell me? I'd bet the OP would be more interested.

If you ran it in a standard solvent, you'd be able to look up the Rf values in a databook, i.e. no need for reference samples.
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BDavies1
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Original post by Pigster
Why tell me? I'd bet the OP would be more interested.

If you ran it in a standard solvent, you'd be able to look up the Rf values in a databook, i.e. no need for reference samples.


sorry for telling you and not the original poster, a mistake, I'm new to TSR
Original post by BDavies1
sorry for telling you and not the original poster, a mistake, I'm new to TSR


Out of curiosity, what actually makes chiral compounds rotate plane polarised light?
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BDavies1
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Original post by KaziMahathir
Out of curiosity, what actually makes chiral compounds rotate plane polarised light?

I believe all compounds absorb visible light and change the plane of polarisation. However, if they are symmetrical then they will change the plane of polarisation in different ways. The net effect of adding all these changes together means that they cancel each other out. However, if the molecule is asymmetrical, the changes will not cancel each other out.
Original post by BDavies1
I believe all compounds absorb visible light and change the plane of polarisation. However, if they are symmetrical then they will change the plane of polarisation in different ways. The net effect of adding all these changes together means that they cancel each other out. However, if the molecule is asymmetrical, the changes will not cancel each other out.


Ah, I see. Thanks!

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