Amino acid chart

Names and abbreviations

Protein sequences usually use one-letter amino-acid codes. Three-letter abbreviations are useful when discussing individual residues: alanine is Ala or A, lysine is Lys or K, and tryptophan is Trp or W. The chart above shows each of the 20 standard amino acids with its structure, its three-letter and one-letter code, its side-chain class and the codons that specify it in the selected genetic code. Search by a full name, three-letter abbreviation or one-letter code to see its name, symbols and matching codons immediately. Select a result to inspect the assignment.

Choose Codes & codons above the chart (Codons on phones) for a compact list of names, codes and codons, or Structures to see the drawings. Options includes a side-chain group filter. A new search looks across all 20 amino acids. Downloads and printing always include all 20; the fixed printable charts include their structures.

Choose Properties on a card or lookup result to jump to that amino acid’s complete property row. Select its name in the row to return to its structure and codons.

Search treats Arg, Asn, Cys and Tyr as amino-acid abbreviations first. Because these are also valid strings of IUPAC nucleotide letters, the answer offers an explicit option to interpret them as codons instead.

These single letters describe amino acids, not nucleotide ambiguity codes. For example, R in a protein means arginine; R in a DNA sequence means either A or G. The alphabet and the kind of sequence matter. Use the DNA-to-protein tool when converting nucleotide letters into a protein sequence.

For a desk reference, download the one-page amino acid chart: all 20 structures with names, codes, side-chain classes, molecular weights, pI and either RNA or DNA codons, as a PDF, SVG or PNG. These fixed sheets use the standard genetic code.

Convert one-letter and three-letter amino acid codes

Choose the notation you are entering: Met is one amino acid in three-letter notation, while MET in one-letter notation means methionine, glutamic acid and threonine. Case alone cannot distinguish them. The converter keeps the order of your sequence and provides both notations for copying or saving.

If a code is unrecognized, the converter identifies its position. Choose Select problem in input to highlight the text to correct, then convert again. Invalid input produces no sequence; unsupported symbols are never silently removed.

The symbols follow the IUPAC-IUB amino acid code table. This converter covers the 20 standard amino acids shown above. For a sequence’s mass, pI and composition, copy its one-letter output into the protein calculator.

Polar, nonpolar, acidic and basic amino acids

The chart groups the 20 amino acids by side chain, the part that differs from one amino acid to the next:

• Nonpolar (9): glycine, alanine, valine, leucine, isoleucine, proline, methionine, phenylalanine and tryptophan.
• Polar, uncharged (6): serine, threonine, cysteine, tyrosine, asparagine and glutamine.
• Acidic (2): aspartic acid and glutamic acid. Their side chains carry a negative charge at physiological pH.
• Basic (3): lysine, arginine and histidine. Lysine and arginine carry a positive charge at physiological pH; histidine is mostly uncharged there.

This is a broad teaching classification, not a complete prediction of a residue's behavior in a folded protein. Other charts place glycine, cysteine, tyrosine or tryptophan differently, or give aromatic and sulfur-containing residues groups of their own.

Smaller groups cut across those four:

• Aromatic: phenylalanine, tyrosine and tryptophan each have a benzene ring. Histidine's imidazole ring is aromatic too.
• Sulfur-containing: cysteine and methionine.
• With a hydroxyl group: serine, threonine and tyrosine.
• Amides: asparagine and glutamine.
• Branched-chain: valine, leucine and isoleucine.

Proline is the only one whose side chain joins its own amino group in a ring, which makes it a secondary amine. Glycine, with a single hydrogen as its side chain, is the only one that is not chiral.

pKa values, charge and the isoelectric point

Every amino acid has an α-carboxyl group and an α-amino group. Seven also have a side chain that gains or loses a proton: aspartic acid (pKa 3.65), glutamic acid (4.25), histidine (6.00), cysteine (8.18), tyrosine (10.07), lysine (10.53) and arginine (12.48). The properties table lists all three values for each amino acid. Choose a property in Compare by to rank all 20 amino acids from lowest to highest, or reverse the order. Amino acids without a listed side-chain pKa stay at the end; a dash is not zero.

The isoelectric point (pI) is the pH at which an amino acid carries no net charge. It is the average of the two pKa values on either side of the neutral form: the two highest for a basic side chain, the two lowest for every other amino acid. Alanine's pI is (2.34 + 9.69) ÷ 2 = 6.01, aspartic acid's is (1.88 + 3.65) ÷ 2 = 2.77, and lysine's is (8.95 + 10.53) ÷ 2 = 9.74. OpenStax Organic Chemistry, section 26.2 works through the calculation.

These values describe free amino acids. In a protein, only the two ends keep a free α-amino or α-carboxyl group; the others are joined in peptide bonds.

Hydrophobic and hydrophilic amino acids

The hydropathy column in the properties table is the Kyte–Doolittle index, a scale made for plotting how hydrophobic a protein is along its sequence. The most hydrophobic side chains are isoleucine (4.5), valine (4.2) and leucine (3.8). The most hydrophilic are arginine (−4.5) and lysine (−3.9), followed by asparagine, aspartic acid, glutamic acid and glutamine at −3.5. Seven amino acids score above zero: isoleucine, valine, leucine, phenylalanine, cysteine, methionine and alanine.

In the properties comparison, choose Hydropathy to see all 20 ranked by this scale. The complete properties CSV includes every value, its source and the documented pKa corrections, independently of the current view or genetic code.

For a protein sequence, the protein calculator's hydropathy profile uses these same values to show moving-window averages, with exact residue positions and a complete CSV. Its whole-chain average is GRAVY; local windows can differ from that average.

A scale and a class answer different questions. Glycine, proline and tryptophan are in the nonpolar group above but score below zero on this scale, and cysteine is grouped as polar but scores 2.5. Other hydrophobicity scales rank some side chains differently.

Essential amino acids

Humans cannot make nine of the 20 and must get them from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. The other eleven are nonessential. Seven of those are conditionally essential, usually not essential except in times of illness and stress: arginine, cysteine, glutamine, glycine, proline, serine and tyrosine. The lists are from MedlinePlus.

One-letter protein codes versus DNA ambiguity

Several letters occur in both alphabets with different meanings. The NCBI nucleotide-code table defines the nucleotide symbols; the amino-acid names above use the protein alphabet.

In a translated protein, X means an unresolved residue here, while * marks a definite stop. See the translator's output explanation before treating either symbol as an ordinary amino acid.

Why some amino acids have several codons

There are 64 possible three-base combinations, but only 20 standard amino acids. Multiple codons therefore map to the same residue in the standard code. Organisms do not use those synonymous codons equally: E. coli genes use CTG for 49.9% of their leucine codons. The codon optimization tool shows the codon usage of E. coli, yeast and human genes. The assignments are not identical in every genetic code: changing the NCBI table in Options updates the codons on each card. A dagger marks context-dependent termination. Read the genetic-code guide before interpreting a sequence from an unfamiliar organism or organelle.

Each card also shows its codons in compact IUPAC notation, the smallest set of ambiguity triplets that covers exactly those codons. Leucine's six codons become CUN and UUR, and alanine's four become GCN; N means any base and R means A or G. This form is useful when writing degenerate primers or reading a compressed codon table. It never includes a codon for a different assignment.

Stop signals are not amino acids, so they have no card in this chart. You can inspect them on the complete codon chart or the codon wheel. The reference covers the usual 20-residue alphabet, rather than recoding rules for selenocysteine or pyrrolysine.

Inspect ambiguous codons and code differences

An amino acid can have several definite codons, while one ambiguous triplet can describe several possible codons. Those are different ideas. In the standard code, GCN covers GCA, GCC, GCG and GCT; all four encode alanine. TAN instead mixes tyrosine codons with stop signals. The genetic-code ambiguity examples show why only the first set resolves to one residue.

Choose a codon on an amino acid's card to inspect its assignment. The panel below the chart lists all possible initiation codons separately from their internal amino-acid assignments. For a nonstandard table, it compares both the start list and internal assignments with table 1, preserving context-dependent stops. Code CSV in Options saves all 64 selected-table codons, both alphabets, initiation/stop flags and source details, independently of the amino-acid filter. SVG and PNG exports from Options follow the selected table and alphabet as a plain table of names, codes and codons, with the full genetic-code name, NCBI table number and alphabet printed on the image. The permanent printable amino acid charts stay on NCBI table 1.