Question

Assign an \(R\) or \(S\) configuration to the chiral center in each amino acid. (a) L-Phenylalanine (b) L-Glutamic acid (c) L-Methionine

Short answer

Answer: The chiral centers of L-Phenylalanine, L-Glutamic acid, and L-Methionine all have S configuration.

Step-by-step solution

Draw the Fischer projection and identify the chiral center

Draw the Fischer projection of each amino acid. The chiral center is the alpha carbon atom connected to the amino group (NH2), carboxyl group (COOH), side chain group (R), and a hydrogen atom (H). (a) L-Phenylalanine: H | - NH2 - C - COOH | CH2 | C6H5 (b) L-Glutamic acid: H | - NH2 - C - COOH | CH2 | CH2 | COOH (c) L-Methionine: H | - NH2 - C - COOH | CH2 | CH2 | S | CH3

Apply the Cahn-Ingold-Prelog (CIP) priority rules

Assign priorities to the four groups connected to the chiral center according to their atomic numbers. The group with the lowest priority (usually hydrogen) should be in the horizontal position (going into the plane) of the Fischer projection. (a) L-Phenylalanine: Priority order: NH2 (1) > COOH (2) > CH2-C6H5 (3) > H (4) (b) L-Glutamic acid: Priority order: NH2 (1) > COOH (2) > CH2-CH2-COOH (3) > H (4) (c) L-Methionine: Priority order: NH2 (1) > COOH (2) > CH2-CH2-S-CH3 (3) > H (4)

Determine the R or S configuration

Observe the order of the groups, as determined in Step 2. If the order of the highest three priority groups goes from 1 to 2 to 3 in a counterclockwise direction, the configuration is S. If it goes in a clockwise direction, the configuration is R. (a) L-Phenylalanine: Counterclockwise (S configuration) (b) L-Glutamic acid: Counterclockwise (S configuration) (c) L-Methionine: Counterclockwise (S configuration) So, L-Phenylalanine, L-Glutamic acid, and L-Methionine all have S configuration at their chiral centers.