What is the stereochemistry of isoleucine

What is the stereochemistry of isoleucine?
C2 is (5) and C3 is (R)
C2 is (R) and C3 is (R)
C2 is (R) and C3 is (S)
C2 is (S) and C3 is (S)

The correct answer and explanation is :

The correct stereochemistry of isoleucine is:
C2 is (S) and C3 is (S).

Explanation:

Isoleucine (Ile) is one of the 20 standard amino acids, with the molecular formula C6H13NO2. It is unique because it has two chiral centers, making it one of the two proteinogenic amino acids (along with threonine) that contain multiple stereocenters.

Step 1: Identifying the Chiral Centers

Isoleucine has chiral centers at C2 (the α-carbon) and C3 (the β-carbon).

• C2 (α-carbon): This carbon is attached to four different groups:

1. Carboxyl (-COOH)
2. Amino (-NH2)
3. Hydrogen (-H)
4. The side chain (-CH(CH3)CH2CH3)

• C3 (β-carbon): This carbon is part of the side chain and is also attached to four different groups:

1. -CH3 (methyl group)
2. -CH2CH3 (ethyl group)
3. Hydrogen (-H)
4. The remainder of the amino acid backbone

Step 2: Assigning Configuration (Cahn-Ingold-Prelog Rules)

To determine R or S configuration, we follow the Cahn-Ingold-Prelog priority rules:

1. Assign priorities to the four groups based on atomic number.
2. Arrange the molecule so the lowest priority group (usually hydrogen) is in the back.
3. Determine the direction of priority movement (clockwise = R, counterclockwise = S).

• C2 (α-carbon): The correct assignment follows standard L-amino acid stereochemistry, meaning it is (S).
• C3 (β-carbon): When applying the priority rules, the configuration is also (S).

Thus, the correct stereochemistry of isoleucine is (2S, 3S).