L-Arginine (HN=C(NH₂)NH(CH₂)₃CH(NH₂)COOH) - R or S Configuration
Is L-Arginine R or S?
L-Arginine (HN=C(NH₂)NH(CH₂)₃CH(NH₂)COOH) has 1 chiral center. Has a guanidinium group at the end of a three-carbon chain. The guanidinium has three nitrogens but is too far from the α-carbon to beat COOH at the second-level CIP comparison.. The configuration is S based on CIP priority rules.
| Formula | HN=C(NH₂)NH(CH₂)₃CH(NH₂)COOH |
| Name | L-Arginine |
| Number of Chiral Centers | 1 |
| Configuration(s) | S |
| Difficulty | Advanced |
Chiral Center Analysis
L-Arginine (HN=C(NH₂)NH(CH₂)₃CH(NH₂)COOH) contains 1 chiral center. Has a guanidinium group at the end of a three-carbon chain. The guanidinium has three nitrogens but is too far from the α-carbon to beat COOH at the second-level CIP comparison.. 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)₃-guanidinium - Guanidinopropyl: C bonded to C, H, H at second level - guanidinium is 4+ atoms away. 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.
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