L amino acid and D carbohydrates

[drdrdrdrdrdrdr]

Hi everyone,

Is there a reason why L amino acids are R vs. D sugars are R?

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[milski]

L/D depends very much on the shape of the molecule, R/S is a very arbitrary designation based on the order of elements in the periodic table. You'll be hard pressed to find any useful correlation between the two.

 

[drdrdrdrdrdrdr]

So am I correct in understanding that we can only use the D form of carbohydrates (which happens to be R) and L forms of amino acids (which happen to be S)?

 

[milski]

Yes, that's a reasonable way to look at things. D/L is something that is related to properties of the compound, R/S is just nomenclature which happens to be whatever it is.

 

[loveoforganic]

So am I correct in understanding that we can only use the D form of carbohydrates (which happens to be R) and L forms of amino acids (which happen to be S)?

IIRC this is almost always true, almost. D and L refer to stereocenter orientation, but they base it on stereochemistry of a particular side group, regardless of that group's priority. So an oddball high priority group could flip R/S around while retaining D/L.

Been a long time though, def wait for someone to verify that before taking it as fact

 

[FIREitUP]

D (dextrorotary) and L (levorotary) is a designation that tells you how the molecule (enantiomer) rotates plane polarized light. D rotates it to the right while L rotates it to the left. R and S as others have said is an arbitrary designation.

 

[loveoforganic]

D (dextrorotary) and L (levorotary) is a designation that tells you how the molecule (enantiomer) rotates plane polarized light. D rotates it to the right while L rotates it to the left. R and S as others have said is an arbitrary designation.

I know D and L refer to that, but I don't believe that sugar naming is based on that. Iirc, the first pure isomer of glucose was dextrorotary, and they then named other sugars D based on having the same skeletal configuration as glucose. I could be completely misremembering though, would love for someone to clear it up.

What do you mean by R and S being arbitrary designations?

 

[ljc]

What do you mean by R and S being arbitrary designations?

What (I think) he means is that L and D are observable properties of the molecule. No one can just look at a chemical formula and determine L and D, it must be determined experimentally.

R and S, on the other hand, are just conventions. We could have just as easily switched them and the molecule would never know the difference. R and S (as far as I know) do not correlate to any experimentally observable characteristic.