How can I draw the structures of the two enantiomers of cysteine?

Answer 1

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.

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Answer 2

Christian Anderson

To draw the structures of the two enantiomers of cysteine, start with the chemical structure of cysteine and then assign the stereochemistry of the chiral center (the alpha carbon). You'll need to depict the arrangement of the substituents (amino group, carboxyl group, hydrogen, and R-group) around the chiral center to represent each enantiomer.

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Answer from HIX Tutor

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