Most naturally occurring amino acids have chirality centers (the asymmetric carbon atoms) that are named (S) by the Cahn-Ingold-Prelog convention (Section 5-3). 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? 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?