The difference between a peptide vs amino acid is one bond. An amino acid is a single small molecule with an amine group, a carboxyl group and a side chain. A peptide is two or more amino acids joined by amide bonds called peptide bonds. In plain terms, amino acids are beads and a peptide is beads on a string.
- An amino acid is a single molecule; a peptide is two or more amino acids joined by amide bonds.
- IUPAC defines the amide bonds in peptides as peptide bonds.
- Each peptide bond releases one water, about 18.015 g/mol.
- A peptide has direction, from its free amine end (N-terminus) to its free carboxyl end (C-terminus).
- Peptides under about 10 to 20 residues may be called oligopeptides; beyond about 50 residues the term protein is usual.
Is a single amino acid a peptide?
Is glycine a peptide? No. Is glycylglycine a peptide? Yes. The only thing separating the two is a single bond between two glycine molecules, and that bond is the whole subject of this page.
An amino acid is a single molecule. The twenty that are coded by genes all share one plan: a central carbon carrying an amine group, a carboxyl group, a hydrogen and a side chain. The IUPAC-IUB nomenclature tables list each one with its formula; glycine, the smallest, is CH2(NH2)-COOH.
A peptide is what you get when amino acids link up. The IUPAC definition is exact: a peptide is any compound produced by amide formation between a carboxyl group of one amino acid and an amino group of another, and those amide bonds may be called peptide bonds. Two amino acids joined this way make the smallest peptide, a dipeptide.
So the difference is one bond, or more than one. A free amino acid has none. A peptide has at least one, and everything else on this page follows from that.
- Two free amino acids
- Carboxyl meets amine
- One water is released
- A peptide bond joins them
- The chain now has two ends
What changes when a peptide bond forms?
When the carboxyl group of one amino acid meets the amine group of the next, a hydroxyl leaves one and a hydrogen leaves the other. They leave together as water. What remains of each amino acid is no longer called an amino acid at all; the IUPAC rules call it an amino acid residue.
Four practical changes come with that step:
- Mass drops. Each bond removes one water, which PubChem lists at 18.015 grams per mole on average. Two glycines weigh 150.14 as free amino acids, and glycylglycine weighs 18.015 less.
- Groups disappear from the middle. The amine and carboxyl groups that formed the bond are gone, replaced by an amide. Only the two ends of the chain keep a free amine and a free carboxyl.
- Direction appears. A lone amino acid has no front or back. A peptide has an N-terminus and a C-terminus, and Gly-Ala is a different molecule from Ala-Gly.
- Side chains stay put. The R groups are untouched by an ordinary peptide bond, so a serine residue still has its hydroxyl.
That last point is why a peptide keeps the chemical personality of its amino acids. The backbone becomes a repeating chain of amide links, and the side chains hang off it unchanged.
The formula tells the same story in atoms. Glycine is C2H5NO2. Two of them hold C4H10N2O4 between them, and taking away one H2O leaves C4H8N2O3 for glycylglycine. Carbon and nitrogen counts never change when a peptide bond forms; only hydrogen and oxygen drop, two and one at a time. If a formula on a label or certificate does not fit that pattern for the stated sequence, something is off, whether a typo, a salt counted into the formula, or a modification that was never mentioned.
Side by side
| Feature | Amino acid | Peptide |
|---|---|---|
| What it is | One molecule | Two or more residues joined in a chain |
| Peptide bonds | None | One fewer than the number of residues in a linear chain |
| Free amine and carboxyl | One of each on the same carbon | One free amine at the N-terminus, one free carboxyl at the C-terminus |
| Mass | Its own formula weight | Sum of the amino acids minus one water per bond |
| Name | Trivial name, such as glycine | Residue names with -yl endings, such as glycylalanine |
| Order | No sequence | Read from N-terminus to C-terminus |
The table also answers a question people ask the other way round: can a peptide ever count as an amino acid? No. Even the smallest peptide contains an amide bond between two residues, and a single amino acid contains none. A compound can be built from amino acids and still not be a peptide, however, if the pieces are joined by something other than an amide bond.
There is one edge case. The IUPAC definition also covers isopeptide bonds, amides formed through a side-chain amine or carboxyl instead of the backbone. Those molecules are still peptides, just linked a different way.
Charge and behavior in water
Both amino acids and peptides carry charges in water, but in different places. The IUPAC rules note that the predominant form of a simple amino acid at pH 7 is R-CH(NH3+)-COO-, a zwitterion with a positive amine and a negative carboxyl on the same carbon. Structures are drawn in the uncharged form mostly for convenience.
In a peptide, the backbone amines and carboxyls that formed bonds are no longer free to take or lose protons. The charged groups left are the two chain ends and any ionizable side chains, such as lysine's second amine or aspartic acid's second carboxyl. So a peptide's net charge depends less on its backbone and more on its sequence. Two peptides with the same length can behave very differently in solution if one is rich in basic residues and the other in acidic ones.
This matters at the bench. Charge affects how a peptide dissolves, where it runs on a chromatography column and which charge states appear in a mass spectrum. A free amino acid is simpler to predict because it has only one set of ends and one side chain.
Size: from peptide to protein
Once you have a peptide, the next question is usually how long it is. The IUPAC definitions give rough bands. Peptides with fewer than about 10 to 20 residues may be called oligopeptides, and longer ones polypeptides. Polypeptides of specific sequence with more than about 50 residues are usually known as proteins, though the rules admit that authors differ greatly on where they start using that word.
The comparison with proteins is a separate topic, and our guide to amino acids versus protein covers it. This page stays at the first step: the jump from a single molecule to a chain of two or more. For how chains grow one residue at a time, see polypeptide formation.
Length also changes naming. Short peptides can be named in full, residue by residue, such as alanylleucyltryptophan. The IUPAC rules note that longer peptides usually get trivial names and are described by their sequence in symbols, because a residue-by-residue name becomes unreadable. A free amino acid never has this problem. Its name is fixed, its formula is short, and there is only one way to write it. The moment a second residue joins, order enters the picture, and with twenty coded amino acids the number of possible sequences multiplies by twenty for every residue added. A number can stand in for the prefix, so a peptide of 22 residues can simply be called a 22-peptide.
How the lab tells them apart
In practice nobody confuses a bottle of glycine with a peptide, but the distinction matters when you check what is in a vial. Mass is the first clue. A peptide's measured mass should equal the sum of its residue masses plus one water, and a free amino acid in the same sample shows up at a much lower mass. The LC-MS identity testing guide shows how that comparison is made.
The reverse direction is used too. Heating a peptide in strong acid breaks the peptide bonds and releases the amino acids again; the IUPAC notes on symbols mention this hydrolysis when they describe how glutamine and 5-oxoproline both yield glutamic acid. Counting the released amino acids gives a composition, though not the order they were in.
Purity is a different measurement from identity. A chromatogram shows how much of the detected material is the main peptide and how much is something else, including stray amino acids or shorter chains. Certificates for our own lots report those results once each certificate is published; the certificate index shows which are available.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION.
Common questions
Is a peptide made of amino acids?
What is the smallest peptide?
Why does a peptide weigh less than its amino acids added together?
Can a peptide be turned back into amino acids?
More science guides
Sources
- IUPAC-IUB JCBN, Nomenclature and Symbolism for Amino Acids and Peptides, 3AA-11 to 3AA-13. Definition of a peptide and peptide bond, amino-acid residues, N- and C-terminal residues, and how peptides are named.
- IUPAC-IUB JCBN, Nomenclature and Symbolism for Amino Acids and Peptides, 3AA-1 and 3AA-2. Table 1: names, symbols, systematic names and formulas of the twenty genetically coded amino acids.
- IUPAC-IUB JCBN, Nomenclature and Symbolism for Amino Acids and Peptides, 3AA-6 to 3AA-10. Ionization of functional groups: the predominant zwitterion form at pH 7.
- PubChem compound record: Glycine. Formula and molecular weight of the smallest amino acid.
- PubChem compound record: Water. Molecular weight 18.015 used for the mass lost per bond.
