The 20 amino acids
Every standard amino acid with its ionization constants from Nelson and Cox, Lehninger Principles of Biochemistry, 7th edition, Table 3-1, its isoelectric point, its charge at pH 7.4, its codes, codons and molar mass, and whether the human diet has to supply it. Each name opens a page with the charge worked out group by group.
Amino acid pKa, pI and charge table
| Amino acid | Codes | Side-chain class | pK₁ | pK₂ | pKR | pI | Charge at pH 7.4 | Molar mass (g/mol) | Codons | Human diet |
|---|---|---|---|---|---|---|---|---|---|---|
| Glycine | Gly, G | Nonpolar, aliphatic | 2.34 | 9.60 | none | 5.97 | −0.01 | 75.07 | 4 | Conditionally essential |
| Alanine | Ala, A | Nonpolar, aliphatic | 2.34 | 9.69 | none | 6.01 | −0.01 | 89.09 | 4 | Nonessential |
| Proline | Pro, P | Nonpolar, aliphatic | 1.99 | 10.96 | none | 6.48 | 0.00 | 115.13 | 4 | Conditionally essential |
| Valine | Val, V | Nonpolar, aliphatic | 2.32 | 9.62 | none | 5.97 | −0.01 | 117.15 | 4 | Essential |
| Leucine | Leu, L | Nonpolar, aliphatic | 2.36 | 9.60 | none | 5.98 | −0.01 | 131.18 | 6 | Essential |
| Isoleucine | Ile, I | Nonpolar, aliphatic | 2.36 | 9.68 | none | 6.02 | −0.01 | 131.18 | 3 | Essential |
| Methionine | Met, M | Nonpolar, aliphatic | 2.28 | 9.21 | none | 5.74 | −0.02 | 149.21 | 1 | Essential |
| Phenylalanine | Phe, F | Aromatic | 1.83 | 9.13 | none | 5.48 | −0.02 | 165.19 | 2 | Essential |
| Tyrosine | Tyr, Y | Aromatic | 2.20 | 9.11 | 10.07 | 5.66 | −0.02 | 181.19 | 2 | Conditionally essential |
| Tryptophan | Trp, W | Aromatic | 2.38 | 9.39 | none | 5.89 | −0.01 | 204.23 | 1 | Essential |
| Serine | Ser, S | Polar, uncharged | 2.21 | 9.15 | none | 5.68 | −0.02 | 105.09 | 6 | Nonessential |
| Threonine | Thr, T | Polar, uncharged | 2.11 | 9.62 | none | 5.87 | −0.01 | 119.12 | 4 | Essential |
| Cysteine | Cys, C | Polar, uncharged | 1.96 | 10.28 | 8.18 | 5.07 | −0.14 | 121.15 | 2 | Conditionally essential |
| Asparagine | Asn, N | Polar, uncharged | 2.02 | 8.80 | none | 5.41 | −0.04 | 132.12 | 2 | Nonessential |
| Glutamine | Gln, Q | Polar, uncharged | 2.17 | 9.13 | none | 5.65 | −0.02 | 146.15 | 2 | Conditionally essential |
| Lysine | Lys, K | Positively charged | 2.18 | 8.95 | 10.53 | 9.74 | +0.97 | 146.19 | 2 | Essential |
| Arginine | Arg, R | Positively charged | 2.17 | 9.04 | 12.48 | 10.76 | +0.98 | 174.20 | 6 | Conditionally essential |
| Histidine | His, H | Positively charged | 1.82 | 9.17 | 6.00 | 7.59 | +0.02 | 155.16 | 2 | Essential |
| Aspartic acid | Asp, D | Negatively charged | 1.88 | 9.60 | 3.65 | 2.77 | −1.01 | 133.10 | 2 | Nonessential |
| Glutamic acid | Glu, E | Negatively charged | 2.19 | 9.67 | 4.25 | 3.22 | −1.00 | 147.13 | 2 | Nonessential |
pK₁ belongs to the α-carboxyl group, pK₂ to the α-amino group and pKR to the side chain, from Nelson and Cox, Lehninger Principles of Biochemistry, 7th edition, Table 3-1. Charges are computed group by group with the Henderson-Hasselbalch equation. Molar masses are for the uncharged formula.
The values are facts and free to use; this table’s selection and presentation are © 2026 ScienceQuest.
Which amino acids are charged at pH 7.4?
Four carry about one full charge: aspartic acid (−1.01) and glutamic acid (−1.00) are negative, and lysine (+0.97) and arginine (+0.98) are positive. Histidine is filed with the positive ones but carries only +0.02, because its side chain’s pKa is 6.00. Cysteine carries −0.14, and the other 14 lie within 0.04 of zero.
Net charge from pH 1 to 13
| Amino acid | pH 1 | pH 3 | pH 5 | pH 7 | pH 7.4 | pH 9 | pH 11 | pH 13 |
|---|---|---|---|---|---|---|---|---|
| Glycine | +0.96 | +0.18 | 0.00 | 0.00 | −0.01 | −0.20 | −0.96 | −1.00 |
| Alanine | +0.96 | +0.18 | 0.00 | 0.00 | −0.01 | −0.17 | −0.95 | −1.00 |
| Proline | +0.91 | +0.09 | 0.00 | 0.00 | 0.00 | −0.01 | −0.52 | −0.99 |
| Valine | +0.95 | +0.17 | 0.00 | 0.00 | −0.01 | −0.19 | −0.96 | −1.00 |
| Leucine | +0.96 | +0.19 | 0.00 | 0.00 | −0.01 | −0.20 | −0.96 | −1.00 |
| Isoleucine | +0.96 | +0.19 | 0.00 | 0.00 | −0.01 | −0.17 | −0.95 | −1.00 |
| Methionine | +0.95 | +0.16 | 0.00 | −0.01 | −0.02 | −0.38 | −0.98 | −1.00 |
| Phenylalanine | +0.87 | +0.06 | 0.00 | −0.01 | −0.02 | −0.43 | −0.99 | −1.00 |
| Tyrosine | +0.94 | +0.14 | 0.00 | −0.01 | −0.02 | −0.52 | −1.88 | −2.00 |
| Tryptophan | +0.96 | +0.19 | 0.00 | 0.00 | −0.01 | −0.29 | −0.98 | −1.00 |
| Serine | +0.94 | +0.14 | 0.00 | −0.01 | −0.02 | −0.41 | −0.99 | −1.00 |
| Threonine | +0.93 | +0.11 | 0.00 | 0.00 | −0.01 | −0.19 | −0.96 | −1.00 |
| Cysteine | +0.90 | +0.08 | 0.00 | −0.06 | −0.14 | −0.92 | −1.84 | −2.00 |
| Asparagine | +0.91 | +0.09 | 0.00 | −0.02 | −0.04 | −0.61 | −0.99 | −1.00 |
| Glutamine | +0.94 | +0.13 | 0.00 | −0.01 | −0.02 | −0.43 | −0.99 | −1.00 |
| Lysine | +1.94 | +1.13 | +1.00 | +0.99 | +0.97 | +0.44 | −0.74 | −1.00 |
| Arginine | +1.94 | +1.13 | +1.00 | +0.99 | +0.98 | +0.52 | −0.02 | −0.77 |
| Histidine | +1.87 | +1.06 | +0.91 | +0.08 | +0.02 | −0.40 | −0.99 | −1.00 |
| Aspartic acid | +0.88 | −0.11 | −0.96 | −1.00 | −1.01 | −1.20 | −1.96 | −2.00 |
| Glutamic acid | +0.94 | +0.08 | −0.85 | −1.00 | −1.00 | −1.18 | −1.96 | −2.00 |
In gel electrophoresis at a given pH, an amino acid with a positive net charge moves toward the cathode and one with a negative charge toward the anode; at its pI it does not move.
Which amino acids are essential?
The 9 essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The body cannot make enough of them, so they must come from food. Arginine, cysteine, glutamine, glycine, proline and tyrosine are conditionally essential, made in the body but needed from food when synthesis cannot keep up, and the other 5 are nonessential.
Essential 9
Conditionally essential 6
Nonessential 5
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.
To see any of them in 3D, with its charge changing as the pH moves, open the Amino Acid 3D Explorer. The DNA to Protein Translator reads their codons in a sequence, and the Henderson-Hasselbalch Calculator gives the ratio of acid to base for one group at any pH.