Arginine (Arg, R) - Structure, pKa, pI, and Codons
What is arginine and what is its side chain?
Arginine (Arg, R) has the side chain -(CH₂)₃NHC(NH₂)=NH - 3-guanidinopropyl; a strongly basic guanidinium group. It is classified as basic (positive at ph 7), carries a positive (+1) charge at pH 7, and has an isoelectric point of 10.76.
| Three-letter code | Arg |
| One-letter code | R |
| Molecular formula | C₆H₁₄N₄O₂ |
| Molecular weight | 174.20 g/mol |
| Residue mass | 156.19 Da |
| Side chain | -(CH₂)₃NHC(NH₂)=NH - 3-guanidinopropyl; a strongly basic guanidinium group. |
| Classification | Basic (positive at pH 7) |
| pKa (alpha-COOH) | 2.17 |
| pKa (alpha-NH3+) | 9.04 |
| pKa (side chain guanidinium) | 12.48 |
| Isoelectric point (pI) | 10.76 |
| Charge at pH 7 | positive (+1) |
| Essentiality | Conditionally essential |
| Hydropathy (Kyte-Doolittle) | -4.5 |
| Configuration | L-form is (S) |
| Codons | CGU, CGC, CGA, CGG, AGA, AGG (6) |
Side chain
The R group of arginine is -(CH₂)₃NHC(NH₂)=NH. 3-guanidinopropyl; a strongly basic guanidinium group. This is what distinguishes it from the other 19 amino acids, since all of them share the same backbone: an alpha carbon bonded to an amino group, a carboxyl group, and a hydrogen.
Ionization and isoelectric point
Arginine has 3 ionizable groups: alpha-COOH (pKa 2.17), alpha-NH3+ (pKa 9.04), side chain guanidinium (pKa 12.48). The isoelectric point is the pH at which the molecule carries no net charge, found by averaging the two pKa values that bracket the neutral species.
pI = (9.04 + 12.48) / 2 = 10.76
Note that this uses the side chain pKa rather than both alpha group values, because the side chain ionizes within the range that brackets the neutral form.
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-arginine is (S).
Genetic code
6 codons encode arginine: CGU, CGC, CGA, CGG, AGA, AGG. This is the maximum degeneracy in the genetic code.
MCAT notes
- The most basic side chain (pKa 12.48) and the highest pI of the 20 (10.76).
- The most hydrophilic residue on the Kyte-Doolittle scale (-4.5).
- Its guanidinium group is resonance-stabilized across three nitrogens, so it stays protonated at essentially any physiological pH.
- The substrate for nitric oxide synthase (giving NO and citrulline) and a urea cycle intermediate, cleaved by arginase to urea and ornithine.
- One of three amino acids with six codons, and its codon set is not contiguous (CGN plus AGA/AGG).
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View arginine in 3D