L-Histidine (C₃H₃N₂CH₂CH(NH₂)COOH) - R or S Configuration
Is L-Histidine R or S?
L-Histidine (C₃H₃N₂CH₂CH(NH₂)COOH) has 1 chiral center. An aromatic amino acid with an imidazole ring containing two nitrogens. The ring nitrogens are at the third atom level and beyond, so COOH's oxygens win at the second level.. The configuration is S based on CIP priority rules.
| Formula | C₃H₃N₂CH₂CH(NH₂)COOH |
| Name | L-Histidine |
| Number of Chiral Centers | 1 |
| Configuration(s) | S |
| Difficulty | Advanced |
Chiral Center Analysis
L-Histidine (C₃H₃N₂CH₂CH(NH₂)COOH) contains 1 chiral center. An aromatic amino acid with an imidazole ring containing two nitrogens. The ring nitrogens are at the third atom level and beyond, so COOH's oxygens win at the second level.. 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: -COOH - Carboxyl: C bonded to O, O, O at second level - highest among carbon substituents
- Priority 3: -CH₂(imidazole) - Imidazolylmethyl: C bonded to C, H, H (the CH₂) - ring nitrogens are at 3rd level. COOH's O, O, O wins
- 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 counterclockwise path from priority 1 to 2 to 3. This gives the S configuration.
Interactive stereochemistry explorer with guided walkthroughs and practice quizzes.
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