No, not all amino acids have an S configuration.
While it's common to think that all natural amino acids are S configuration, it is not always the case. Most amino acids found in proteins have an L-configuration, which corresponds to the S configuration, except for two special cases: glycine and cysteine.
Chirality of Amino Acids
Most amino acids, with the exception of glycine, are chiral due to the presence of four different substituents attached to the alpha carbon. This chirality is what gives rise to the L and D configurations as well as the R and S configurations.
The S Configuration
- The S configuration refers to the absolute configuration of a chiral molecule, determined using the Cahn–Ingold–Prelog (CIP) priority rules.
- L-Glyceraldehyde and the natural amino acids all have the S absolute configuration, according to our reference material. This is the case for the majority of amino acids.
Exceptions to the Rule
Despite the trend of L-amino acids being in the S configuration, there are two key exceptions:
- Glycine: Glycine is not chiral because its alpha carbon is attached to two hydrogen atoms. Since it lacks a chiral center, it does not have an R or S configuration.
- Cysteine: Cysteine, although an L-amino acid, has an R configuration due to the sulfur atom in its side chain taking precedence over the carboxyl group in terms of the CIP priority rules.
Summary Table
| Amino Acid | Chirality | Configuration |
|---|---|---|
| Most Amino Acids | Chiral | S |
| Glycine | Achiral | N/A |
| Cysteine | Chiral | R |
Understanding the Exceptions
- Glycine's achirality is a direct result of the identical nature of its two hydrogen atoms attached to the alpha carbon.
- Cysteine's reversed configuration (R) is due to the sulfur atom of its side chain having a higher atomic number than the oxygen in the carboxyl group. Based on the CIP rules, the higher the atomic number, the higher the priority. This results in the rotation direction being reversed when assigning the absolute configuration.
In conclusion, while most natural L-amino acids are in the S configuration, glycine has no chiral center, and cysteine is an exception, having an R configuration.
