Phenylalanine (Phe, F)
Aromatic side chain, essential in the human diet
What is the charge of phenylalanine at pH 7.4?
At pH 7.4, the pH of blood plasma, phenylalanine carries a net charge of −0.02, and 98% of its molecules are the zwitterion, charge 0. Its side chain does not ionize, so only the α-carboxyl and α-amino groups are charged, and their charges nearly cancel. Its isoelectric point is 5.48, from Lehninger’s pKa values of 1.83 and 9.13.
Properties of phenylalanine
| Three-letter code | Phe |
|---|---|
| One-letter code | F |
| Side-chain class (Lehninger) | Aromatic |
| Side-chain group | phenyl ring |
| Formula (uncharged form) | C₉H₁₁NO₂ |
| Molar mass | 165.19 g/mol |
| pK₁ (α-carboxyl) | 1.83 |
| pK₂ (α-amino) | 9.13 |
| pKR (side chain) | none, the side chain does not ionize |
| Isoelectric point (pI) | 5.48 |
| Configuration at Cα (L form) | S |
| Hydropathy (Kyte and Doolittle) | 2.8 |
| mRNA codons | UUU, UUC |
| Human diet | 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 phenylalanine at common pH values
| pH | Net charge | Most common form | Share of molecules |
|---|---|---|---|
| 1 | +0.87 | cation (+1) | 87% |
| 3 | +0.06 | zwitterion (0) | 94% |
| 5 | 0.00 | zwitterion (0) | 99.9% |
| 7 | −0.01 | zwitterion (0) | 99% |
| 7.4 (blood plasma) | −0.02 | zwitterion (0) | 98% |
| 9 | −0.43 | zwitterion (0) | 57% |
| 11 | −0.99 | anion (−1) | 99% |
| 13 | −1.00 | anion (−1) | over 99.9% |
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 1.83: the cation, net charge +1.
- pH 1.83 to 9.13: the zwitterion, net charge 0.
- Above pH 9.13: the anion, net charge −1.
Open phenylalanine 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 phenylalanine at pH 7.4
- α-carboxyl (pK₁ = 1.83): −1/(1 + 10^(1.83 − 7.4)) = −1.0000
- α-amino (pK₂ = 9.13): +1/(1 + 10^(7.4 − 9.13)) = +0.9817
- q = −1.0000 + 0.9817 = −0.0183
- q ≈ −0.02
How to calculate the pI of phenylalanine
Phenylalanine has two ionizable groups, so its neutral form, the zwitterion, lies between them and the pI is the mean of pK₁ and pK₂.
Isoelectric point of phenylalanine
- pI = (pK₁ + pK₂)/2
- = (1.83 + 9.13)/2
- = 5.48
Stereochemistry
L-phenylalanine 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 phenylalanine compares with the other 19
- Phenylalanine is one of 3 amino acids in the class Lehninger calls aromatic; the others are tyrosine and tryptophan.
- Its molar mass, 165.19 g/mol, makes it the 4th heaviest of the 20.
- Its pI, 5.48, is the 5th lowest of the 20.
- On Kyte and Doolittle’s hydropathy scale it scores 2.8, the 4th most hydrophobic of the 20.
- It has 2 codons, as do 8 of the other 19.
Is phenylalanine an essential amino acid?
Yes. Phenylalanine is one of the 9 essential amino acids, which the body cannot make from other molecules in the amounts it needs, so they have to come from food. The other 8 are histidine, isoleucine, leucine, lysine, methionine, threonine, tryptophan and valine.
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 phenylalanine
Phenylalanine is encoded by 2 of the 61 sense codons in the standard genetic code: UUU and UUC in mRNA, written TTT and TTC 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 phenylalanine
- Amino Acid 3D Explorer: phenylalanine in 3D, with its charge at any pH
- Molar mass of C₉H₁₁NO₂, 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
- Phenylalanine on Wikipedia