Which stereochemical designation is assigned to naturally occurring $asparticacid$?

• A. $R$-aspartic acid
• B. $S$-aspartic acid
• C. $D$-aspartic acid
• D. $L$-aspartic acid

Answer: (D)

Verified step by step guidance

1. Understand that the question asks for the stereochemical designation of naturally occurring aspartic acid, which involves distinguishing between the R/S and D/L systems.
2. Recall that the R/S system is based on the Cahn-Ingold-Prelog priority rules, assigning absolute configuration to chiral centers by ranking substituents and determining the spatial arrangement.
3. Recognize that the D/L system is a relative configuration system based on the molecule's relation to D- or L-glyceraldehyde, commonly used for amino acids and sugars.
4. Know that naturally occurring amino acids, including aspartic acid, are typically in the L-configuration in the D/L system, which corresponds to the stereochemistry found in proteins.
5. Note that for aspartic acid, the L-configuration corresponds to the S absolute configuration at the alpha carbon, except for cysteine where priorities differ; thus, naturally occurring aspartic acid is L- and S-aspartic acid.