Question

Most naturally occurring amino acids have chiral centers (the asymmetric α carbon atoms) that are named (S) by the Cahn-Ingold-Prelog convention (Section 5-3). The common naturally occurring form of cysteine has a chiral center that is named (R), however.

(a) What is the relationship between (R)-Cysteine and (S)-alanine? Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?

(b) (S)-Alanine is an L-amino acid (Figure 24-2). Is (R)-cysteine a D-amino acid or an L-amino acid?

Short answer

(S)-alanine (R)-cysteine

(a) The configurations around the asymmetric or chiral carbons of (R)-cysteine and (S)-alanine are same. The designation of configuration is different due to sulfur present in cysteine.

(S)-alanine (R)-cysteine

L-alanine L-cysteine

(b) Fischer projections of (S)-alanine and (R)-cysteine indicates that both are L-amino acids.

Step-by-step solution

Step-1. R and S configurations:

Absolute configurations describe the positions of each bond around the chiral center atom. R and S are used for labelling the chiral center which in Latin is right and left respectively. The designation of configuration is different in (S)-alanine and (R)-cysteine due to different priorities of groups attached to the chiral center atom. The groups are numbered according to the priority which is given by CIP rules in which atom in the group having higher atomic number is given first priority followed by second highest atom with atomic number. If first atom fails to tell the priority then we move onto the second atom of that group. In (R)-cysteine, sulfur has higher atomic number than oxygen, thus given more priority. “R” designation is given when motion is clockwise with respect to the groups and provided fourth-priority group should be at dashed position. If fourth priority group is at wedged position then “R” designation changes to “S” or vice-versa.

(S)-alanine (R)-cysteine

Step-2. Fischer projection of (S)-alanine and (R)-cysteine:

Fischer projections is a two-dimensional representation of a three-dimensional molecule. In the Fischer projection, horizontal lines represent bonds which project out of plane of paper towards the viewer whereas, the vertical lines represent bonds which project away from the viewer. In this, numbering is also given on priority basis, atom of the group having higher atomic number will have more priority. Amino group if lies at left at the chiral center, then amino acid will have designation “L”. Based on this rule, Fischer projections of (S)-alanine and (R)-cysteine indicates that both are L-amino acids.

(S)-alanine (R)-cysteine

L-alanine L-cysteine