Final answer:
Most natural chiral amino acids possess an L-configuration and are S-configured, except for cysteine, which is R-configured, and glycine, which is non-chiral. The L-amino acids, despite their designation, are dextrorotary (rotate light to the right) and are the ones used by cells in protein synthesis.
Step-by-step explanation:
Most chiral natural amino acids have an L-configuration, which correlates with the S-absolute configuration according to the Cahn-Ingold-Prelog priority rules. However, there is a notable exception with the amino acid cysteine, which has an R-configuration. With the exception of glycine, which is not chiral, the naturally occurring chiral amino acids can exist as either D- or L-enantiomers, but only the L-amino acids are used by cells to build polypeptides and proteins.
Enantiomers are mirror images of a molecule that cannot be superimposed on one another, and the presence of a chiral carbon is what allows for these two forms. For the amino acids, the chiral carbon has an amine (-NH2) and a carboxylic acid (-COOH) attached to it. Interestingly, L-amino acids are dextrorotary, meaning they rotate plane-polarized light to the right, which is denoted as 'd' in the context of optical activity.
The S and R designations define the absolute configuration of a chiral molecule, and a separate designation of dextrorotatory (+) or levorotatory (-) is used to describe its optical activity. Therefore, it is possible for a compound with an S configuration to be dextrorotatory, as in the case of (S)-lactic acid, or levorotatory, as in its salt (S)-sodium lactate. This detail underscores the distinction between absolute configuration and optical activity.