Final answer:
L- or D-amino acid designations are based on the spatial arrangement of the -NH₂ group in the Fischer projection. L-amino acids have the -NH₂ group on the left side, while D-amino acids have the -NH₂ group on the right side. This designation helps distinguish between enantiomers of amino acids.
Step-by-step explanation:
The designations of L- or D-amino acids refer to the spatial arrangement of the amino group (-NH₂) in relation to the carboxyl group (-COOH) in the Fischer projection. In the case of L-amino acids, the -NH₂ group is positioned on the left side of the Fischer projection, while in the case of D-amino acids, the -NH₂ group is positioned on the right side. These designations are used to distinguish between enantiomers, which are mirror image isomers of each other. To determine whether an amino acid is L or D, you can refer to the Fischer projection. If the -NH₂ group is on the left side, it is L. If it is on the right side, it is D. An example can be seen with alanine, where L-alanine has the -NH₂ group on the left side and D-alanine has the -NH₂ group on the right side in the Fischer projection.