Question

Eighteen of the nineteen 1 -amino acids have the \(S\) configuration at the \(\alpha\) carbon (the first carbon after the carboxyl carbon). Cysteine is the only 1 -amino acid that has an \(R\) configuration. Explain.

Short answer

Cysteine is the only 1 -amino acid with an R configuration because of its molecular structure. The alpha carbon of Cysteine is bound to a -NH2, a -SH group, a -COOH group, and a hydrogen atom. According to the Cahn-Ingold-Prelog priority rules for deciding R/S configuration, sulfur (S) in the -SH group gets the highest priority due to its highest atomic number. Hence, when the lowest priority - hydrogen is oriented away from the observer, the rest of the groups follow a clockwise path (R configuration)

Step-by-step solution

Understand the structure of Cysteine

The molecular structure of Cysteine is H2N-CH(COOH)-SH. From this, it can be seen that the alpha carbon (the carbon next to the carboxyl group) is attached to a -NH2 group, a -SH group, a -COOH group, and a Hydrogen atom.

Apply the Cahn-Ingold-Prelog rules

Cahn-Ingold-Prelog rules are used to assign priorities to the groups attached to the chiral carbon. The atom with the highest atomic number gets the highest priority. If there is a tie, then we move to the next atom in each group. In Cysteine, sulphur (S) has the highest atomic number, followed by oxygen (O) in the carboxyl group, then nitrogen (N) in the amine group, and at last hydrogen (H) which has the lowest atomic number. Therefore, the -SH group has the highest priority, followed by the -COOH group, then the -NH2 group and then the hydrogen atom.

Determine the configuration

Using the right-hand rule or left-hand rule, depending on priorities and orientation of groups, one can determine the configuration. Here, the lowest priority group - Hydrogen is away from the observer. The rest of the groups follow in the order of clock (from -SH to -COOH to -NH2), indicating an R configuration. Hence, unlike other 1 amino acids, Cysteine has an R configuration due to the presence of sulphur atom in the place of usual carbon, which gives it the highest priority.

Key concepts

Cahn-Ingold-Prelog Rules

When you come across a chiral molecule, understanding its spatial arrangement is key. The Cahn-Ingold-Prelog rules help assign priorities to the groups attached to a chiral center, which is important for determining the molecule's configuration. These rules are based on the atomic number of the atoms directly attached to the chiral center. Here’s how it works:

• Atoms with higher atomic numbers get higher priority. For example, sulfur (S) has a higher priority than carbon (C) or oxygen (O).
• If two atoms directly attached to the chiral carbon are the same, you look at the next set of atoms in the chain.
• Once all groups are prioritized, you observe the molecule from the direction where the lowest priority group is the farthest away.

With these rules, you can identify the order in which higher priority groups encircle the chiral center. If the sequence is clockwise, the configuration is labeled as "R" (rectus, Latin for "right"). If it's counterclockwise, the configuration is "S" (sinister, Latin for "left"). This method helps you figure out the specific spatial arrangement of atoms in chiral molecules.

Alpha Carbon

The alpha carbon is central to understanding amino acid structure. It's the first carbon atom attached to a functional group—in this case, the carboxyl group in amino acids. In chiral amino acids, this carbon is crucial because it connects to four different groups. These groups often define the properties and reactivity of the amino acid.

• The alpha carbon connects to an amino group \(-NH_2\), a carboxyl group \(-COOH\), and a hydrogen atom.
• In most amino acids, the fourth group varies, contributing to the unique characteristics of each specific amino acid.
• This setup makes the alpha carbon a chiral center, except in glycine where the fourth group is another hydrogen atom.

Due to these varied attachments, the alpha carbon plays a vital role in the chiral nature and functionality of amino acids, affecting protein folding and activity.

Cysteine Configuration

Cysteine is unique among amino acids for its configuration. While most amino acids with chiral alpha carbons have "S" configurations, Cysteine is an exception with an "R" configuration.

Here's how this works:

• Cysteine's structure includes a sulfhydryl group \(-SH\) attached to the alpha carbon.
• The sulfur atom in this group has a higher atomic number than the oxygen in the carboxyl group, giving it the highest priority according to the Cahn-Ingold-Prelog rules.
• When the priorities are considered, the hydrogen atom, being the lowest priority, is oriented away from the observer.
• The remaining groups are arranged in a clockwise order from \(-SH\) to \(-COOH\) to \(-NH_2\).

This sequence of priorities results in the R configuration for Cysteine. This unique positioning of the sulfur atom differentiates Cysteine from other amino acids, impacting its chemical behavior and importance in protein structures.