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 center configurations for each amino acid are L-Phenylalanine (S), L-Glutamic acid (S), and L-Methionine (S).

Step-by-step solution

Determine the chiral centers

In each amino acid, the chiral center is the α-carbon, which is the carbon atom attached to the amino group (NH2), carboxyl group (COOH), and the side chain (R group).

Assign priorities to the groups around the chiral center

Using the Cahn-Ingold-Prelog priority rules, assign priorities to the groups attached to the chiral center. Number the atoms in decreasing order of priority (1 for the highest priority, 4 for the lowest priority). The priority is usually determined by atomic number, with higher atomic numbers having higher priority.

Arrange the lowest priority group away from the observer

Position the molecule in space so that the lowest-priority group (4) is pointing away from the observer.

Determine the orientation of the three remaining groups

Observe the orientation of the remaining three groups around the chiral center. If the order of priority from 1 to 3 is clockwise, the configuration is R (rectus). If the order of priority from 1 to 3 is counterclockwise, the configuration is S (sinister).

Assign configuration to each amino acid

(a) L-Phenylalanine: 1. Chiral center: α-carbon 2. Priorities: COOH (1) > NH2 (2) > Phenyl group (3) > H (4) 3. Arrange H away from the observer 4. Orientation: counterclockwise (sinister) Configuration: S (b) L-Glutamic acid: 1. Chiral center: α-carbon 2. Priorities: COOH (1) > NH2 (2) > CH2-COOH (3) > H (4) 3. Arrange H away from the observer 4. Orientation: counterclockwise (sinister) Configuration: S (c) L-Methionine: 1. Chiral center: α-carbon 2. Priorities: COOH (1) > NH2 (2) > CH2-CH2-S-CH3 (3) > H (4) 3. Arrange H away from the observer 4. Orientation: counterclockwise (sinister) Configuration: S In conclusion, the chiral center configurations for each amino acid are L-Phenylalanine (S), L-Glutamic acid (S), and L-Methionine (S).

Key concepts

Cahn-Ingold-Prelog priority rules

The Cahn-Ingold-Prelog (CIP) priority rules are a set of guidelines used for determining the priority of substituents around a chiral center. These rules play a critical role in stereochemistry, helping us assign the correct configuration to molecules. Here's how these rules work:

• Atomic Number: The first criterion is the atomic number. Atoms with higher atomic numbers receive higher priority. For instance, iodine would receive a higher priority than chlorine due to its higher atomic number.
• Immediate Substituents: If two atoms attached to the chiral center have the same atomic number, look at the atoms they are bonded to. Compare their atomic numbers to establish priority.
• Double and Triple Bonds: Double or triple bonds require special consideration. A double bond is treated as two single bonds to the same atom, while a triple bond as three.
• Lowest Priority in the Back: When analyzing orientation, remember to position the lowest priority group facing away from you. It helps in correctly assigning R or S configuration.

By following these rules, you can systematically determine the priority of attached groups, which is essential for determining the stereochemical configuration of chiral centers in molecules.

Chiral center

A chiral center, often referred to as a stereocenter, is a carbon atom bonded to four distinct groups. This unique arrangement allows for two non-superimposable mirror images, or enantiomers, often referred to as "left-handed" and "right-handed" configurations.

• Importance of Chiral Centers: Chiral centers are crucial in biology and chemistry because they impart distinct three-dimensional configurations that can drastically affect a molecule's behavior and interactions.
• Determining Chiral Centers: To find a chiral center, look for carbons attached to four different atoms or groups. In amino acids, the α-carbon usually acts as the chiral center because it’s bonded to an amino group (NH₂), a carboxyl group (COOH), a hydrogen atom, and a variable R group.
• Chirality and Properties: The presence of a chiral center can impact physical properties, reactivity, and biological activity. For example, the difference in the arrangement of atoms at the chiral center can lead to molecules having different smells or tastes.

Understanding chiral centers helps in predicting the stereochemistry and potential reactivity of amino acids and other organic molecules.

Amino acid configuration

The configuration of an amino acid at its chiral center is significant as it influences the molecule's properties and its interaction within biological systems. In the step-by-step solution, amino acids like L-phenylalanine, L-glutamic acid, and L-methionine were analyzed, revealing their S configurations.

• Alpha Carbon Configuration: Each amino acid's configuration is determined at the α-carbon, which is the central point connecting an amino group, a carboxyl group, hydrogen, and a distinct side chain (R group).
• Assigning R or S Configuration: Using the Cahn-Ingold-Prelog priority rules, you assign priorities to four substituents, placing the lowest priority group backward. Then, determine if the sequence from highest to third-highest priority is clockwise (R) or counterclockwise (S).
• Biological Significance: Amino acids naturally exist as L-isomers, encompassing both R and S configurations due to nature’s preference. Incorrect configuration can alter protein structure and function, emphasizing the importance of precise stereochemical configuration.

Understanding amino acid configuration is crucial for studying protein function and the molecular mechanisms of life, enabling deeper insight into biochemical processes and drug design.