How can I draw the structures of the two enantiomers of cysteine?
1 Answer
The easiest way is to draw them first as Fischer projections.
The general formula for an amino acid is RCH(NH₂)COOH. In cysteine, R = CH₂SH.
You draw the Fischer projection for an amino acid in the same way as for a carbohydrate.
The main chain is vertical, with C-1 at the top. If the NH₂ group is on the left, you have an L-amino acid. If the NH₂ group is on the right, you have an R-amino acid.
Once you have the Fischer projections, you can turn them on their side and convert them into wedge-dash formulas.
If you draw the NH₂-C-COOH bonds in the plane of the paper, with NH₂ on the left and the "V" pointing down, the L-amino acid has the bond to the R group as a dashed line.
If the "V" points up, the L-amino acid has the bond to the R group as a wedge.
So the two structures of cysteine are those above, with R = CH₂SH.
Note that L-cysteine has the (R) configuration. In contrast, L-serine has the (S) configuration.
