Phenylalanine (Phe, F) - Structure, pKa, pI, and Codons
What is phenylalanine and what is its side chain?
Phenylalanine (Phe, F) has the side chain -CH₂C₆H₅ - Benzyl; an unsubstituted aromatic ring. It is classified as aromatic, carries a neutral (0) charge at pH 7, and has an isoelectric point of 5.48.
| Three-letter code | Phe |
| One-letter code | F |
| Molecular formula | C₉H₁₁NO₂ |
| Molecular weight | 165.19 g/mol |
| Residue mass | 147.18 Da |
| Side chain | -CH₂C₆H₅ - Benzyl; an unsubstituted aromatic ring. |
| Classification | Aromatic |
| pKa (alpha-COOH) | 1.83 |
| pKa (alpha-NH3+) | 9.13 |
| Isoelectric point (pI) | 5.48 |
| Charge at pH 7 | neutral (0) |
| Essentiality | Essential |
| Hydropathy (Kyte-Doolittle) | +2.8 |
| Configuration | L-form is (S) |
| Codons | UUU, UUC (2) |
Side chain
The R group of phenylalanine is -CH₂C₆H₅. Benzyl; an unsubstituted aromatic ring. 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
Phenylalanine has 2 ionizable groups: alpha-COOH (pKa 1.83), alpha-NH3+ (pKa 9.13). 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 = (1.83 + 9.13) / 2 = 5.48
These values are for the free amino acid. Inside a folded protein, the local environment shifts them substantially.
Stereochemistry
The alpha carbon of L-phenylalanine is (S).
Genetic code
2 codons encode phenylalanine: UUU, UUC.
MCAT notes
- Aromatic and nonpolar, and essential.
- Hydroxylated to tyrosine by phenylalanine hydroxylase - the defective step in phenylketonuria (PKU), which makes tyrosine essential in PKU patients.
- Absorbs UV at about 257 nm, and is essentially invisible at 280 nm. Tryptophan, not phenylalanine, dominates A280.
- Chymotrypsin cleaves C-terminal to the bulky aromatics: Phe, Trp, and Tyr.
Interactive 3D structure with the R group isolated, a titration curve with a live pH cursor, and flashcards for all 20 amino acids.
View phenylalanine in 3D