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.

What is the Amino Acid Reference Chart?

Reference all 20 proteinogenic amino acids with their 1-letter and 3-letter codes, molecular weight, side-chain class, polarity, and charge at physiological pH. Search and filter by property. Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Amino Acid Reference Chart

Name: Amino Acid Reference Chart
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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 lost when forming a peptide bond. Summing residue masses plus one water estimates a protein’s mass from its sequence.

The 20 amino acids grouped by side-chain class

Knowing which group an amino acid belongs to immediately tells you a lot about its behaviour in a protein:

Nonpolar (hydrophobic)

Glycine (G), Alanine (A), Valine (V), Leucine (L), Isoleucine (I), Proline (P), Phenylalanine (F), Methionine (M), Tryptophan (W)

These pack into the protein interior away from water. Proline is unusual in its cyclic structure — it introduces a rigid kink into a peptide chain that can break or cap secondary structures.

Polar uncharged

Serine (S), Threonine (T), Cysteine (C), Tyrosine (Y), Asparagine (N), Glutamine (Q)

These are hydrophilic enough to sit on a protein surface or inside active sites. Cysteine is particularly notable: its thiol group can form disulfide bonds with another cysteine, creating a covalent crosslink that stabilises the folded protein or tethers two chains together.

Acidic (negatively charged at pH 7.4)

Aspartate (D), Glutamate (E)

Both carry a carboxylate side chain that loses a proton at physiological pH. They form salt bridges with basic residues and are common in enzyme active sites where a negative charge is catalytically useful.

Basic (positively charged at pH 7.4)

Lysine (K), Arginine (R), Histidine (H)

Lysine and arginine are fully protonated at pH 7.4. Histidine’s pKa near 6.0 means it can switch between protonated and neutral forms under physiological conditions, making it exceptionally useful as an acid–base catalyst — it appears in the active sites of many proteases, kinases, and phosphatases.

Practical uses for this reference

Reading FASTA sequences: The 1-letter codes (A, C, D, E, F, G, H, I, K, L, M, N, P, Q, R, S, T, V, W, Y) are what you see in sequence databases like UniProt and NCBI. This chart decodes each instantly.
Estimating protein mass: Sum the residue weights for each amino acid in the sequence, then add 18.02 Da for the terminal water. Most protein analysis software does this automatically, but knowing the residue masses helps you sanity-check results.
Interpreting mass spectra: In proteomics, peptide fragments are identified by their mass differences. The residue masses here are the values added to a fragment ion when extending by one amino acid.
Understanding mutations: A conservative mutation replaces an amino acid with one of similar class (e.g. Ile to Val, both nonpolar). A radical mutation crosses class lines and is more likely to disrupt function.