L-Cysteine (HSCH₂CH(NH₂)COOH) - R or S Configuration
Is L-Cysteine R or S?
L-Cysteine (HSCH₂CH(NH₂)COOH) has 1 chiral center. The only standard amino acid where L = R. Sulfur (atomic number 16) in the side chain outranks oxygen (8) in the carboxyl group at the second atom level, flipping the usual priority order.. The configuration is R based on CIP priority rules.
| Formula | HSCH₂CH(NH₂)COOH |
| Name | L-Cysteine |
| Number of Chiral Centers | 1 |
| Configuration(s) | R |
| Difficulty | Intermediate |
Chiral Center Analysis
L-Cysteine (HSCH₂CH(NH₂)COOH) contains 1 chiral center. The only standard amino acid where L = R. Sulfur (atomic number 16) in the side chain outranks oxygen (8) in the carboxyl group at the second atom level, flipping the usual priority order.. Each chiral center is analyzed below using CIP (Cahn-Ingold-Prelog) priority rules to determine R or S configuration.
CIP Priority Assignment
The four substituents on the chiral center are ranked by atomic number of the directly attached atom, with ties broken by exploring outward along each branch.
- Priority 1: -NH₂ - Nitrogen directly bonded (atomic number 7) - highest first-atom priority
- Priority 2: -CH₂SH - Thiol side chain: C bonded to S (atomic number 16) at second level - S(16) beats O(8)
- Priority 3: -COOH - Carboxyl: C bonded to O, O, O - but oxygen (8) loses to sulfur (16) at second atom level
- Priority 4: H - Hydrogen always lowest (atomic number 1)
R/S Configuration
With the lowest priority group (priority 4) oriented away from the viewer, the remaining three substituents trace a clockwise path from priority 1 to 2 to 3. This gives the R configuration.
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