Reference
Amino acid properties
A reference table of the twenty amino acids: one- and three-letter codes, class, average residue mass, Kyte–Doolittle hydropathy, side-chain pKa for the ionisable residues, and the codons that encode each one.
| Code | 3-letter | Class | Residue mass (Da) | Hydropathy | Side-chain pKa | Codons |
|---|---|---|---|---|---|---|
| A | Ala | non-polar | 71.08 | +1.8 | — | GCT GCC GCA GCG |
| C | Cys | polar | 103.14 | +2.5 | 8.33 | TGT TGC |
| D | Asp | acidic | 115.09 | -3.5 | 3.65 | GAT GAC |
| E | Glu | acidic | 129.12 | -3.5 | 4.25 | GAA GAG |
| F | Phe | non-polar | 147.18 | +2.8 | — | TTT TTC |
| G | Gly | non-polar | 57.05 | -0.4 | — | GGT GGC GGA GGG |
| H | His | basic | 137.14 | -3.2 | 6.00 | CAT CAC |
| I | Ile | non-polar | 113.16 | +4.5 | — | ATT ATC ATA |
| K | Lys | basic | 128.17 | -3.9 | 10.53 | AAA AAG |
| L | Leu | non-polar | 113.16 | +3.8 | — | TTA TTG CTT CTC CTA CTG |
| M | Met | non-polar | 131.19 | +1.9 | — | ATG |
| N | Asn | polar | 114.10 | -3.5 | — | AAT AAC |
| P | Pro | non-polar | 97.12 | -1.6 | — | CCT CCC CCA CCG |
| Q | Gln | polar | 128.13 | -3.5 | — | CAA CAG |
| R | Arg | basic | 156.19 | -4.5 | 12.48 | CGT CGC CGA CGG AGA AGG |
| S | Ser | polar | 87.08 | -0.8 | — | TCT TCC TCA TCG AGT AGC |
| T | Thr | polar | 101.11 | -0.7 | — | ACT ACC ACA ACG |
| V | Val | non-polar | 99.13 | +4.2 | — | GTT GTC GTA GTG |
| W | Trp | non-polar | 186.21 | -0.9 | — | TGG |
| Y | Tyr | polar | 163.18 | -1.3 | 10.07 | TAT TAC |
Residue mass is the average monoisotopic-free mass of the residue in a chain, that is the free amino acid minus one water. A whole protein’s mass is the sum of its residues plus one water.
Hydropathy is the Kyte–Doolittle value; positive is hydrophobic, negative is hydrophilic. Side-chain pKa is the typical value for the ionisable residues and shifts with local environment.
These are standard reference constants. The protein parameters tool uses the same values to compute mass, charge and hydropathy for a whole sequence.
Frequently asked
Residue mass or free amino acid mass?
The table lists residue mass — the free amino acid minus one water — because that is what adds up along a chain. A whole protein is the sum of its residues plus one water for the free ends.
Why do pKa values vary between sources?
Side-chain pKa depends on the local environment in a folded protein, so published sets differ by a few tenths. These are typical model values for the ionisable residues; treat charge predictions as approximate.
Which hydropathy scale is this?
Kyte–Doolittle, the most widely used scale. Positive is hydrophobic, negative hydrophilic. Other scales exist and rank the residues slightly differently.