Amino Acid Chart
All 20 standard amino acids with their one- and three-letter codes, side-chain class, molecular weight, pKa and hydropathy.
| Name | 3-letter | 1-letter | Side-chain class | Free MW (g/mol) | Residue MW | Side-chain pKa | Hydropathy |
|---|---|---|---|---|---|---|---|
| Alanine | Ala | A | Nonpolar | 89.09 | 71.08 | — | 1.8 |
| Arginine | Arg | R | Positive (basic) | 174.20 | 156.19 | 12.5 | -4.5 |
| Asparagine | Asn | N | Polar, uncharged | 132.12 | 114.10 | — | -3.5 |
| Aspartic acid | Asp | D | Negative (acidic) | 133.10 | 115.09 | 3.9 | -3.5 |
| Cysteine | Cys | C | Polar, uncharged | 121.15 | 103.14 | 8.5 | 2.5 |
| Glutamine | Gln | Q | Polar, uncharged | 146.15 | 128.13 | — | -3.5 |
| Glutamic acid | Glu | E | Negative (acidic) | 147.13 | 129.12 | 4.1 | -3.5 |
| Glycine | Gly | G | Nonpolar | 75.07 | 57.05 | — | -0.4 |
| Histidine * | His | H | Positive (basic) | 155.16 | 137.14 | 6.5 | -3.2 |
| Isoleucine * | Ile | I | Nonpolar | 131.17 | 113.16 | — | 4.5 |
| Leucine * | Leu | L | Nonpolar | 131.17 | 113.16 | — | 3.8 |
| Lysine * | Lys | K | Positive (basic) | 146.19 | 128.17 | 10.8 | -3.9 |
| Methionine * | Met | M | Nonpolar | 149.21 | 131.19 | — | 1.9 |
| Phenylalanine * | Phe | F | Nonpolar (aromatic) | 165.19 | 147.18 | — | 2.8 |
| Proline | Pro | P | Nonpolar | 115.13 | 97.12 | — | -1.6 |
| Serine | Ser | S | Polar, uncharged | 105.09 | 87.08 | — | -0.8 |
| Threonine * | Thr | T | Polar, uncharged | 119.12 | 101.11 | — | -0.7 |
| Tryptophan * | Trp | W | Nonpolar (aromatic) | 204.23 | 186.21 | — | -0.9 |
| Tyrosine | Tyr | Y | Polar (aromatic) | 181.19 | 163.18 | 10.1 | -1.3 |
| Valine * | Val | V | Nonpolar | 117.15 | 99.13 | — | 4.2 |
* Essential amino acid for adults. Residue MW = free MW − water (18.015). pKa values: EMBOSS set. Hydropathy: Kyte–Doolittle scale.
Amino acids grouped by side chain
- Nonpolar: Gly (G), Ala (A), Val (V), Leu (L), Ile (I), Met (M), Pro (P), Phe (F), Trp (W)
- Polar, uncharged: Ser (S), Thr (T), Cys (C), Asn (N), Gln (Q), Tyr (Y)
- Acidic (negative): Asp (D), Glu (E)
- Basic (positive): Lys (K), Arg (R), His (H)
Easily confused one-letter codes
Several codes are not the first letter of the name: R = Arginine, N = Asparagine, D = Aspartic acid, Q = Glutamine, E = Glutamic acid, K = Lysine, F = Phenylalanine, W = Tryptophan, Y = Tyrosine.
Add up a whole sequence with the peptide molecular weight calculator, or see which codons encode each amino acid in the codon chart.
FAQ
How many amino acids are there?
There are 20 standard amino acids encoded by the genetic code. Selenocysteine (U) and pyrrolysine (O) are sometimes counted as the 21st and 22nd.
What is the difference between free and residue mass?
When amino acids join by a peptide bond, one water molecule (18.015 Da) is released. The residue mass is the free amino acid mass minus water, and it is what you add up to get a peptide’s molecular weight.
Which amino acids are essential?
Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine are the nine essential amino acids for adults.
Which amino acids are charged at pH 7?
Aspartic acid and glutamic acid are negatively charged; lysine and arginine are positively charged. Histidine (pKa ≈ 6.5) is only partly protonated at pH 7.
What is a peptide bond?
A peptide bond is the amide link (–CO–NH–) formed between the carboxyl group of one amino acid and the amino group of the next, releasing one molecule of water.
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