What is the difference between 1-letter and 3-letter codes?

Both name the same amino acid. The 3-letter code (e.g. Ala) is readable and used in text, while the 1-letter code (e.g. A) is compact and used in long sequences and bioinformatics file formats like FASTA. This chart lists both for every residue.

Why does the molecular weight differ from the free amino acid?

The weights shown are residue masses, which is the free amino acid minus one water molecule (18.02 Da) lost when it forms a peptide bond. Residue masses are what you sum to estimate a protein's mass from its sequence.

Which amino acids are charged at physiological pH?

At pH 7.4, aspartate and glutamate carry a negative charge, while lysine and arginine carry a positive charge. Histidine is near its pKa and can be partially positive. These charged residues drive salt bridges, catalysis, and protein solubility.

What makes an amino acid hydrophobic?

Amino acids with nonpolar side chains made mostly of carbon and hydrogen, such as leucine, isoleucine, valine, and phenylalanine, are hydrophobic. They avoid water and cluster in a protein's core, providing much of the folding stability.

Are these the only amino acids in proteins?

These 20 are the standard genetically encoded amino acids. Two rare extras, selenocysteine and pyrrolysine, are inserted in special cases, and many residues are chemically modified after translation. This chart covers the 20 standard proteinogenic set.

Amino Acid Reference Chart

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Proteins are built from 20 standard amino acids, each with a distinct side chain that determines its chemistry. This reference lists all 20 with their codes, residue molecular weights, and key properties so you can look any of them up quickly.

How it works

Every amino acid shares a backbone (an amino group, a carboxyl group, and an alpha carbon) and differs only in its side chain (R group). The side chain sets the residue’s properties:

Nonpolar side chains are hydrophobic and bury in the protein core.
Polar uncharged side chains form hydrogen bonds.
Acidic residues (Asp, Glu) are negatively charged at pH 7.4.
Basic residues (Lys, Arg, His) can be positively charged.

The molecular weights shown are residue masses — the free amino acid minus a water molecule (−18.02 Da) lost when forming a peptide bond. Summing residue masses plus one water estimates a protein’s mass.

Tips and example

A FASTA sequence like MKV... uses the 1-letter codes listed here.
The smallest residue is glycine (Gly, G); the largest is tryptophan (Trp, W).
Cysteine’s thiol side chain forms disulfide bridges that stabilize folded proteins.