Question

The configuration of the chiral center in \(\alpha\)-amino acids is most commonly specified using the \(\mathrm{D}, \mathrm{L}\) convention. It can also be identified using the \(R, S\) convention (Section 3.3). Does the chiral center in L-serine have the \(R\) or \(S\) configuration?

Short answer

Answer: The chiral center in L-serine has the S configuration.

Step-by-step solution

Understand the concepts of chiral centers and D,L and R,S conventions

A chiral center, usually denoted as a carbon atom, is a central point in a molecule that has four different atoms or groups attached to it, making the molecule non-superposable onto its mirror image. The D,L convention is used to represent the configuration of chiral centers in amino acids, considering the position of the amino group (-NH2) relative to the '-OH' of the reference carbohydrate, where the hydroxyl group is placed to the right (D-configuration) or to the left (L-configuration). The R,S convention, also known as Cahn-Ingold-Prelog rules, assigns priorities to the atoms/groups attached to the chiral center based on their atomic/molecular number. Following the priorities, you can determine the R (clockwise) or S (counterclockwise) configuration of the chiral center.

Determine the structure of L-serine

Structurally, L-serine (also called Ser or S) has the following groups attached to its chiral center (the \(\alpha\)-carbon): an amino group (-NH2), a carboxyl group (-COOH), a hydrogen atom (-H), and a hydroxymethyl group (-CH2OH).

Assign priority using the Cahn-Ingold-Prelog rules

According to the Cahn-Ingold-Prelog rules, we assign priority numbers to the four atoms/groups directly attached to the chiral center: 1. Amino group: Nitrogen (atomic number 7) has the highest atomic number, so it is assigned priority 1. 2. Carboxyl group: The second highest priority goes to the oxygen atom (atomic number 8) of the carboxyl group, so the carboxyl group is assigned priority 2. 3. Hydroxymethyl group: Carbon (atomic number 6) takes the third highest priority, so the hydroxymethyl group is assigned priority 3. 4. Hydrogen atom: Hydrogen (atomic number 1) has the lowest atomic number, so it has the lowest priority and is assigned priority 4.

Determine the configuration of the chiral center in L-serine

Now that we have assigned the priorities, we can determine the configuration of the chiral center in L-serine. Let's draw a circle from the amino group (priority 1) to the carboxyl group (priority 2) and then to the hydroxymethyl group (priority 3). Since we can draw a counterclockwise circle, the chiral center in L-serine has an S configuration. Therefore, the chiral center in L-serine has the S configuration.