I'm finding difficulties in drawing the D and L configurations of these two aminoacids, because, differently from the others, they have two chiral carbons (specifically α and β). In particoular, in the slides of my professor (I'll share the picture), in the representation of L-isoleucine both the amino group and the methyl group are on the left side. Instead, in the representation of L-threonine only the amino group is on the left, while the alcohol group is on the right (I expected it to be on the left side too). Why L-threonine doens't have the OH on the left side? enter image description here

  • $\begingroup$ Some reading here. You are correct. Threonine was one of the last amino acids discovered. $\endgroup$ Commented Dec 18, 2022 at 15:04
  • $\begingroup$ Yes. You get one chiral center "for free", but you have to memorize which one, and what the rule is, so it doesn't really help. See masterorganicchemistry.com/2017/05/24/d-and-l-sugars $\endgroup$
    – Karsten
    Commented Dec 20, 2022 at 20:24
  • $\begingroup$ @Karsten Thanks again for your help 😊 $\endgroup$ Commented Dec 20, 2022 at 21:49

1 Answer 1


There are four stereoisomers of D-threonine. One is called L-threonine, and the other two are not called threonine (because they are diastereomers of threonine, so they have different physical properties).

So the D/L nomenclature is a way to distinguish enantiomers. For a history of how D or L were assigned, see https://chemistry.stackexchange.com/a/50561/72973.

I'm finding difficulties in drawing the D and L configurations of these two aminoacids

You can't draw these from first principles, you have to look up the stereochemistry. This is different from the R/S nomenclature (where the complete stereochemical information is available from the R or S assigned to the "chiral" carbons).

This is illustrated nicely for the sugars glucose and galactose. Both of them have a D and an L enantiomer, but that does not tell you the configuration of the other 4 stereocenters.

  • $\begingroup$ Thanks for your answer. In practice, I just have to learn the structure by heart. $\endgroup$ Commented Dec 20, 2022 at 19:12

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