Arginine (Arg, R)
Positively charged side chain, conditionally essential in the human diet
What is the charge of arginine at pH 7.4?
At pH 7.4, the pH of blood plasma, arginine carries a net charge of +0.98, and 98% of its molecules are the cation, charge +1. Its guanidinium side chain, pKR 12.48, carries a proton in over 99.9% of molecules. Its isoelectric point is 10.76, from Lehninger’s pKa values of 2.17, 9.04 and 12.48.
Properties of arginine
| Three-letter code | Arg |
|---|---|
| One-letter code | R |
| Side-chain class (Lehninger) | Positively charged |
| Side-chain group | guanidinium |
| Formula (uncharged form) | C₆H₁₄N₄O₂ |
| Molar mass | 174.20 g/mol |
| pK₁ (α-carboxyl) | 2.17 |
| pK₂ (α-amino) | 9.04 |
| pKR (side chain) | 12.48 |
| Isoelectric point (pI) | 10.76 |
| Configuration at Cα (L form) | S |
| Hydropathy (Kyte and Doolittle) | −4.5 |
| mRNA codons | CGU, CGC, CGA, CGG, AGA, AGG |
| Human diet | Conditionally essential |
pKa values and pI from Nelson and Cox, Lehninger Principles of Biochemistry, 7th edition, Table 3-1. Formula and molar mass from the wwPDB Chemical Component Dictionary and the IUPAC standard atomic weights; codons from NCBI translation table 1; hydropathy from Kyte and Doolittle (1982); diet from the Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005), chapter 10.
The values are facts and free to use; this page’s selection and presentation are © 2026 ScienceQuest.
Charge of arginine at common pH values
| pH | Net charge | Most common form | Share of molecules |
|---|---|---|---|
| 1 | +1.94 | dication (+2) | 94% |
| 3 | +1.13 | cation (+1) | 87% |
| 5 | +1.00 | cation (+1) | 99.8% |
| 7 | +0.99 | cation (+1) | 99% |
| 7.4 (blood plasma) | +0.98 | cation (+1) | 98% |
| 9 | +0.52 | cation (+1) | 52% |
| 11 | −0.02 | zwitterion (0) | 96% |
| 13 | −0.77 | anion (−1) | 77% |
The net charge is an average over all the molecules, so it can be a fraction; each molecule carries a whole number of charges, and the most common form is the one protonation state with the largest share.
Which form dominates between the pKas
- Below pH 2.17: the dication, net charge +2.
- pH 2.17 to 9.04: the cation, net charge +1.
- pH 9.04 to 12.48: the zwitterion, net charge 0.
- Above pH 12.48: the anion, net charge −1.
Open arginine in the Amino Acid 3D Explorer and drag the pH to watch each group gain and lose its proton, with every pKa and its pI marked on the pH slider.
How the charge at pH 7.4 is calculated
Each group is treated on its own with the Henderson-Hasselbalch equation. An acidic group adds
−1/(1 + 10^(pKa − pH)), the share of molecules in which it has lost its proton, and a basic group
adds +1/(1 + 10^(pH − pKa)), the share in which it holds one.
Net charge of arginine at pH 7.4
- α-carboxyl (pK₁ = 2.17): −1/(1 + 10^(2.17 − 7.4)) = −1.0000
- α-amino (pK₂ = 9.04): +1/(1 + 10^(7.4 − 9.04)) = +0.9776
- guanidinium (pKR = 12.48): +1/(1 + 10^(7.4 − 12.48)) = +1.0000
- q = −1.0000 + 0.9776 + 1.0000 = +0.9776
- q ≈ +0.98
How to calculate the pI of arginine
Arginine has three ionizable groups. Taken from the lowest pKa up, its neutral form exists between pK₂ (9.04, the α-amino) and pKR (12.48, the guanidinium), so the pI is the mean of those two.
Isoelectric point of arginine
- pI = (pK₂ + pKR)/2
- = (9.04 + 12.48)/2
- = 10.76
Averaging all three pKas, a common mistake, gives 7.90, 2.86 pH units below the pI.
Stereochemistry
L-arginine is S at its α-carbon, as 18 of the 19 chiral amino acids are: its amino group outranks its carboxyl, which outranks its side chain. The R and S Configuration Explorer ranks the groups on a stereocenter step by step.
How arginine compares with the other 19
- Arginine is one of 3 amino acids in the class Lehninger calls positively charged; the others are lysine and histidine.
- Its molar mass, 174.20 g/mol, makes it the 3rd heaviest of the 20.
- Its pI, 10.76, is the highest of the 20.
- On Kyte and Doolittle’s hydropathy scale it scores −4.5, the most hydrophilic of the 20.
- It has 6 codons, as do leucine and serine, the most of any amino acid.
Is arginine an essential amino acid?
Only conditionally. Arginine is one of 6 conditionally essential amino acids: the body makes it, but needs it from food when its own synthesis cannot keep up with demand. The other 5 are cysteine, glutamine, glycine, proline and tyrosine.
Classification from the Institute of Medicine, Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids (2005), chapter 10, which calls the three lists indispensable, conditionally indispensable and dispensable.
Codons for arginine
Arginine is encoded by 6 of the 61 sense codons in the standard genetic code: CGU, CGC, CGA, CGG, AGA and AGG in mRNA, written CGT, CGC, CGA, CGG, AGA and AGG on the DNA coding strand.
The DNA to Protein Translator reads codons in a sequence of your own, and the Protein Synthesis Simulator builds the chain on a ribosome, one amino acid per codon.
Work with arginine
- Amino Acid 3D Explorer: arginine in 3D, with its charge at any pH
- Molar mass of C₆H₁₄N₄O₂, worked out atom by atom in the Molar Mass Calculator
- Henderson-Hasselbalch Calculator: the acid to base ratio of one group at any pH
- Titration Curve Calculator: the curve for a weak acid titrated with a strong base
- All 20 amino acids: pKa values, pI, charge and codons in one table
- Arginine on Wikipedia