Question

2. The common naturally occurring form of cysteine has a chirality center that is named (R), however;

(a) What is the relationship between (R)-cysteine and (S)-alanine?
(b) Do they have the opposite three-dimensional configuration (as the names might suggest) or the same configuration?
(c) Is (R)-cysteine a D-amino acid or an L-amino acid?

Answer 1

2. A) Cysteine (R) have Sulfur atom as a second Neighbour to the asymmetric carbon based on the atomic number of atoms placed near to the asymmetric carbon and cysteine possess R chirality and so these remaining chiral amino acids, will be substituted as S chirality that will be having lighter atoms in that position.

B) In (R,S) they do not have same configuration. Due to the Sulfur atom presence in the side chain of Cysteine , the side chain has a greater priority than the Carbonyl group, which leads to the S configuration than R configuration. Hence, they have opposite Three-dimensional Configuration.

C) Cysteine (R) is L-amino acid.
- Since Cysteine is found in High Protein foods, like other amino acids it is Monomeric and has its amphoteric character. Cysteine has L chirality in the older D/L notation based on homology to D and L-glyceraldehyde. Below the image of an cysteine having L-amino acid: