How to draw L and D configuration for isoleucine and threonine?

Question

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?

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.