Question

a. Which isomer—(R)-alanine or (S)-alanine—is d-alanine?

b. Which isomer—(R)-aspartate or (S)-aspartate—is d-aspartate?

c. Can a general statement be made relating R and S to d and l?

Short answer

a. Isomer—(R)-alanine is D-alanine.

b. Isomer—(R)-aspartate is D-aspartate

c. The α-carbon of all the naturally occurring D-amino acids except cysteine have the R-configuration. Similarly, the α-carbon of all the naturally occurring L-amino acids except cysteine has the S-configuration.

Step-by-step solution

Configuration of Amino acids

Amino acid is an organic molecule having carboxyl and amino group that combines to form proteins.The α-carbon of all the naturally occurring amino acids (except glycine) is an asymmetric center, therefore, 19 of the 20 amino acids can exist as enantiomers (having mirror images of each other).

The D and L notation used for monosaccharides, is also used for amino acids. Unlike monosaccharides, where the D-isomer is the one found in nature, most amino acids found in nature have the L configuration

An amino acid drawn in a Fischer projection with the carboxyl group at the top and the R group at the bottom of the vertical axis refers to a D-amino acid if the amino group is on the right.

An amino acid drawn in a Fischer projection with the carboxyl group at the top and the R group at the bottom of the vertical axis refers to an L-amino acid if the amino group is on the left in its Fischer projection.

Identify (a) and (b)

In a Fischer projection, horizontal lines project out of the plane of the paper toward the viewer, and vertical lines extend back from the plane of the paper away from the viewer.

a. Isomer—(R)-alanine is D-alanine. In, (R)-alanine, counterclockwise signifies R because the lowest priority substituent (H) is on a horizontal bond. Therefore, we change the configuration from S to R.

b. Isomer—(R)-aspartate is D-aspartate. In, (R)-aspartate, counterclockwise signifiesRbecause the lowest priority substituent (H) is on a horizontal bond. Therefore, we change the configuration from S to R.

General statement relating R and S to D and L

D and L is a relative configuration that can be measured by determining whether the substance rotates the plane-polarized light counterclockwise or clockwise, whereas R and S (absolute configuration) refers to the structural difference between stereoisomers.

The α-carbon of all the naturally occurring D-amino acids except cysteine has the R-configuration. Similarly, the α-carbon of all the naturally occurring L-amino acids except cysteine has the S-configuration.

The amino group has the highest priority and the carboxyl group has the second-highest priority in all the amino acids except cysteine. Because in cysteine, the thiomethyl group has the highest priority than the carboxylate group, due to the greater atomic number of sulfur as compared to oxygen, causing the counter clock-wise arrow to be clock-wise.