An amino acid chart helps you identify the building blocks of peptides and proteins. It brings together their names, chemical features, abbreviations, and properties so that a sequence of letters becomes easier to understand.
The 20 standard amino acids share a common structural framework, but their side chains differ. Those differences influence how a peptide interacts with water, carries charge, and adopts a particular shape.
Below is a reference chart, followed by a simple explanation of the structures and classifications.
Complete Amino Acid Chart: The 20 Standard Amino Acids
Each amino acid has a three-letter code and a one-letter code. Scientists use these abbreviations to write peptide and protein sequences.
The structure column below describes the side chain, also called the R group. It does not show the entire amino acid molecule.
| Amino acid | Three-letter code | One-letter code | Side-chain structure or feature | Common classification |
|---|---|---|---|---|
| Alanine | Ala | A | –CH₃ | Nonpolar |
| Arginine | Arg | R | –(CH₂)₃–NH–C(=NH₂⁺)–NH₂ | Basic; positively charged |
| Asparagine | Asn | N | –CH₂–C(=O)–NH₂ | Polar; uncharged |
| Aspartic acid / Aspartate | Asp | D | –CH₂–COO⁻ | Acidic; negatively charged |
| Cysteine | Cys | C | –CH₂–SH | Polar; usually uncharged |
| Glutamic acid / Glutamate | Glu | E | –CH₂–CH₂–COO⁻ | Acidic; negatively charged |
| Glutamine | Gln | Q | –CH₂–CH₂–C(=O)–NH₂ | Polar; uncharged |
| Glycine | Gly | G | –H | Nonpolar; structurally distinctive |
| Histidine | His | H | –CH₂–imidazole ring | Basic; charge depends strongly on environment |
| Isoleucine | Ile | I | –CH(CH₃)–CH₂–CH₃ | Nonpolar |
| Leucine | Leu | L | –CH₂–CH(CH₃)₂ | Nonpolar |
| Lysine | Lys | K | –(CH₂)₄–NH₃⁺ | Basic; positively charged |
| Methionine | Met | M | –CH₂–CH₂–S–CH₃ | Nonpolar; sulfur-containing |
| Phenylalanine | Phe | F | –CH₂–phenyl ring | Nonpolar; aromatic |
| Proline | Pro | P | –(CH₂)₃– reconnects to the backbone nitrogen | Nonpolar; cyclic |
| Serine | Ser | S | –CH₂–OH | Polar; uncharged |
| Threonine | Thr | T | –CH(OH)–CH₃ | Polar; uncharged |
| Tryptophan | Trp | W | –CH₂–indole ring | Predominantly nonpolar; aromatic |
| Tyrosine | Tyr | Y | –CH₂–phenyl–OH, with OH at the para position | Polar aromatic; usually uncharged |
| Valine | Val | V | –CH(CH₃)₂ | Nonpolar |
How to interpret the chart: Charge descriptions refer to side chains under typical aqueous conditions near pH 7. They are not fixed under every condition. The condensed structures omit stereochemistry and, for some groups, resonance details.
Codes follow IUPAC amino acid nomenclature. Property groupings draw on Thermo Fisher’s amino acid reference and EMBL-EBI’s side-chain chemistry guide. iupac.qmul.ac.uk
How to Read Amino Acid Structures
For most standard amino acids, a simplified uncharged structure can be written as:
H₂N–CH(R)–COOH
The central carbon is called the alpha carbon. It connects to:
- An amino group.
- A carboxyl group.
- A hydrogen atom.
- A variable side chain, represented by R.
To understand alanine, for example, replace R with –CH₃. For serine, replace it with –CH₂–OH.
Proline needs special treatment because its side chain reconnects to the amino nitrogen, forming a ring.
In water near neutral pH, free amino acids usually have a positively charged amino group and a negatively charged carboxyl group. This form is called a zwitterion. A simplified representation is:
⁺H₃N–CH(R)–COO⁻
These charges belong to the shared framework. The chart’s charge classifications describe the additional contribution from the side chain. Thermo Fisher Scientific – US
That distinction explains how an amino acid with an “uncharged side chain” can still contain charged groups.
What Do Polar and Nonpolar Mean?
Polarity describes how electrical charge is distributed within a chemical group. It helps explain how amino acid side chains interact with their surroundings.
Nonpolar Amino Acids
Nonpolar side chains generally interact less favorably with water than charged or strongly polar groups.
Examples include alanine, valine, leucine, and isoleucine. Their side chains contain mostly carbon and hydrogen.
In peptide design, a high proportion of hydrophobic residues can make a sequence more difficult to dissolve in water. However, solubility depends on the entire sequence and the solution conditions, not one residue alone. Thermo Fisher’s peptide-design guide discusses this relationship. Thermo Fisher Scientific – US
Polar, Uncharged Amino Acids
These side chains can interact with water through polar groups without usually carrying a net charge near neutral pH.
Examples include:
- Serine and threonine: contain hydroxyl groups.
- Asparagine and glutamine: contain amide groups.
- Cysteine: contains a thiol group.
- Tyrosine: combines an aromatic ring with a hydroxyl group.
Why Do Some Charts Classify Amino Acids Differently?
Classification systems simplify a continuous range of chemical behavior.
Tyrosine, for example, has both a hydrophobic aromatic ring and a polar hydroxyl group. A chart focused on aromatic structures may group it with phenylalanine and tryptophan. Another may place it among polar amino acids.
Neither category alone describes every aspect of the molecule. Read the classification alongside the structure.
Acidic and Basic Amino Acids
Acidic and basic side chains can gain or lose protons, changing their charge.
Aspartate and glutamate generally carry negative side-chain charges near neutral pH.
Lysine and arginine generally carry positive side-chain charges under the same conditions.
Histidine is more sensitive to its surroundings. Its side chain can occur in charged or uncharged forms, making it important in pH-dependent interactions.
EMBL-EBI’s side-chain chemistry resource explains these distinctions. Foundations of protein structure
An amino acid chart therefore provides a starting point for understanding charge. It does not replace a calculation that accounts for pH, terminal groups, and the molecule’s environment.
Three Amino Acids With Distinctive Structural Features
Glycine: The Smallest Side Chain
Glycine’s side chain is a hydrogen atom. Its alpha carbon therefore has two hydrogen atoms, making glycine achiral.
Its small size also gives it more freedom to adopt backbone conformations than most other amino acids.
Proline: A Ring Connected to the Backbone
Proline’s side chain connects back to the backbone nitrogen. This restricts its conformational freedom.
Consequently, proline can strongly influence the direction of a peptide chain. Its effect depends on where it occurs and the surrounding sequence.
The distinct conformational behavior of glycine and proline is reflected in the separate structural categories used in EMBL-EBI’s explanation of Ramachandran plots. Foundations of protein structure
Cysteine: A Potential Disulfide Connection
Cysteine contains a sulfur-bearing thiol group. Under suitable conditions, two cysteine residues can form a disulfide bond.
Such a connection can link different parts of a peptide or protein. However, a sequence containing cysteine does not automatically establish whether disulfide bonds are present or how they are arranged. Foundations of protein structure
How to Read Amino Acid Codes in a Peptide Sequence
Consider this example:
Ala–Gly–Ser–Lys
Using the one-letter amino acid codes, it becomes:
AGSK
Both forms describe the same residue order:
- Alanine.
- Glycine.
- Serine.
- Lysine.
Sequences are conventionally written from the N-terminus toward the C-terminus.
Order matters. AGSK and GASK contain the same types and numbers of amino acids, but they describe different sequences.
One-letter notation is compact and useful for comparing long sequences. It does not necessarily communicate every modification, unusual bond, or stereochemical detail. IUPAC’s guidance on the one-letter system discusses its uses and limitations. iupac.qmul.ac.uk
For an explanation of how researchers determine residue order experimentally, read Orion’s guide to peptide sequencing.
What an Amino Acid Chart Can—and Cannot—Tell You
An amino acid chart helps you:
- Translate sequence abbreviations.
- Recognize charged and hydrophobic groups.
- Identify sulfur-containing or aromatic residues.
- Understand why sequence changes may affect peptide behavior.
It cannot, by itself, establish a peptide’s exact solubility, three-dimensional structure, stability, or biological activity.
Those properties depend on how residues are arranged, which modifications are present, and the experimental environment. Peptide-design guidance therefore considers overall composition and sequence together. Thermo Fisher Scientific – US
Glutathione illustrates another limitation of simple sequence notation. Its glutamate, cysteine, and glycine residues include an unusual gamma-glutamyl connection. Researchers reviewing Orion’s glutathione compound information should therefore consider the complete chemical identity, not just three amino acid names.
Are There Only 20 Amino Acids?
No. This chart covers the 20 standard amino acids commonly introduced in protein chemistry.
Other amino acids exist. Two additional genetically encoded examples are:
| Amino acid | Three-letter code | One-letter code |
|---|---|---|
| Selenocysteine | Sec | U |
| Pyrrolysine | Pyl | O |
Their incorporation involves specialized biological mechanisms. EMBL-EBI’s protein-structure reference identifies these additional amino acids and their codes. Protein Data Bank in Europe
Research peptides may also contain nonstandard amino acids or chemical modifications. A standard chart is a foundation, rather than an exhaustive list of every possible peptide building block.
Frequently Asked Questions
What is the difference between an amino acid chart and a codon chart?
An amino acid chart describes molecules and their properties. A codon chart maps three-letter RNA sequences to amino acids or translation signals.
Which amino acids contain sulfur?
Among the 20 standard amino acids, cysteine and methionine contain sulfur. Their sulfur-containing groups are chemically different.
Which amino acids are aromatic?
Phenylalanine, tyrosine, and tryptophan are commonly grouped as aromatic amino acids. Histidine also contains an aromatic ring, although introductory charts usually classify it with the basic amino acids.
What is the difference between glutamate and glutamine?
Glutamate has a side-chain carboxyl group, which is usually negatively charged near neutral pH. Glutamine has a side-chain amide group, which is usually uncharged.
Their codes are Glu/E and Gln/Q, respectively.
Why is lysine represented by K instead of L?
The letter L is assigned to leucine. Standardized one-letter codes use different letters where amino acid names share the same initial.
Does an amino acid’s one-letter code show whether it is D or L?
Not by itself. Ordinary protein sequence notation generally assumes the standard L forms, except achiral glycine. D-amino acids and other stereochemical differences require additional notation.
This reference explains amino acid chemistry and peptide terminology for educational and laboratory research purposes.
