Is it a peptide? Retinol, collagen, creatine, glutathione and others
Is retinol a peptide? Is collagen or creatine? How to tell from the structure, with retinol, collagen, creatine, glutathione, carnosine and NAD+ sorted.
By Zachery Casey · Published · Reviewed · Editorial policy
"Peptide" is used loosely on labels and in conversation, so it is often attached to molecules that aren't peptides at all. The question has a plain chemical answer. A peptide is made of amino acids joined by peptide bonds, so you can tell from the structure alone. This guide gives a simple test, then applies it to some common names: retinol, collagen, creatine, glutathione, carnosine, keratin and NAD+.
Terms are defined in the glossary.
The test: is it a peptide?
Ask three questions about the structure:
- Is it built from amino acids? An amino acid has an amino group (–NH₂) and a carboxylic acid group (–COOH) on the same molecule. See the 20 amino acids.
- Are at least two of them joined? One amino acid on its own is not a peptide.
- Is the link a peptide bond? That is an amide bond (–CO–NH–) formed when the acid group of one amino acid reacts with the amino group of the next, releasing water. See what is a peptide.
If the answer to all three is yes, it is a peptide. A quick shortcut works in one direction: a molecule with no nitrogen atoms in its formula can't be a peptide, because every amino acid contains nitrogen.
How long is a peptide?
Length decides the name, not whether something is a peptide:
| Name | Length |
|---|---|
| Dipeptide, tripeptide | 2 or 3 residues |
| Oligopeptide | Up to about 10–20 residues |
| Polypeptide | Longer chains |
| Protein | Usually more than about 50 residues, with a defined shape |
A protein is, chemically, a long polypeptide. The boundary is a convention, and authors draw it in different places (IUPAC-IUB, 3AA-11).
The common names, sorted
| Compound | Formula | Made of amino acids? | Peptide bonds | Verdict |
|---|---|---|---|---|
| Retinol | C₂₀H₃₀O | No | None | Not a peptide |
| Creatine | C₄H₉N₃O₂ | One modified amino-acid unit | None | Not a peptide |
| Carnosine | C₉H₁₄N₄O₃ | β-Alanine and histidine | 1 | Dipeptide |
| Glutathione | C₁₀H₁₇N₃O₆S | Glutamic acid, cysteine, glycine | 2 (one through a side chain) | Tripeptide |
| GHK | C₁₄H₂₄N₆O₄ | Glycine, histidine, lysine | 2 | Tripeptide |
| NAD+ | C₂₁H₂₇N₇O₁₄P₂ | No | None | Not a peptide |
| Collagen | Very large | Yes | Hundreds per chain | Protein |
| Keratin | Very large | Yes | Hundreds per chain | Protein |
Formulas are from PubChem: retinol, creatine, carnosine, glutathione, GHK and NAD+.
Is retinol a peptide?
No. Retinol is a long hydrocarbon chain with a six-membered ring at one end and an alcohol group (–OH) at the other. Its formula, C₂₀H₃₀O, has no nitrogen at all, so it fails the first test straight away (PubChem). Chemically it is an alcohol, one of the group of compounds called retinoids.
Is collagen a peptide?
Collagen is a protein, so in chemical terms it is a very long polypeptide. Each collagen molecule is made of three chains wound into a triple helix (Shoulders and Raines, 2009). One chain of type I collagen is listed at 1,464 residues before processing (UniProt P02452).
"Collagen peptides" are something different again. They are fragments made by breaking collagen apart with hydrolysis, which cuts some of the peptide bonds. The result is a mixture of peptides of many different lengths, not a single compound with one sequence.
Is creatine a peptide?
No. Creatine, C₄H₉N₃O₂, is a single small molecule. It has a carboxylic acid group like an amino acid, joined to a nitrogen that carries a methyl group and a guanidino group (PubChem). There is only one unit and no peptide bond, so it is best described as an amino-acid derivative.
Is glutathione a peptide?
Yes. Glutathione is a tripeptide with an unusual link. Its formula is C₁₀H₁₇N₃O₆S, and it is made of three amino acids: glutamic acid, cysteine and glycine (PubChem). It is written γ-Glu-Cys-Gly.
The "γ" (gamma) marks the unusual part. Glutamic acid has two carboxyl groups: one on the backbone and one at the end of its side chain. In glutathione, the side-chain carboxyl forms the bond to cysteine, not the usual backbone one. A link through a side chain is called an isopeptide bond, and IUPAC's definition of a peptide includes it (IUPAC-IUB, 3AA-11). So glutathione is a tripeptide, just not a standard one.
Its cysteine carries a free thiol group (–SH), which can oxidise to form a disulfide. See oxidation and reduction.
Is carnosine a peptide?
Yes, carnosine is a dipeptide of β-alanine and histidine. Its full name is β-alanyl-L-histidine and its formula is C₉H₁₄N₄O₃ (PubChem). β-Alanine isn't one of the 20 standard amino acids. Its amino group sits one carbon further along the chain. It is still an amino acid, though, and the link to histidine is a normal amide bond, so carnosine counts as a peptide.
Is GHK a peptide?
Yes, GHK is a tripeptide. Its full name is glycyl-L-histidyl-L-lysine, C₁₄H₂₄N₆O₄: three standard amino acids joined by two ordinary peptide bonds (PubChem). It also binds copper ions. See GHK-Cu: the chemistry of a peptide–copper complex.
Is NAD+ a peptide?
No. NAD+ is a dinucleotide: two nucleotides joined through their phosphate groups. Its formula contains nitrogen and phosphorus, but no amino acids and no peptide bonds (PubChem). See NAD+: structure, stability and handling.
Is keratin a peptide?
Keratins are a family of proteins, so again they are long polypeptides. One human keratin, keratin 1, is listed at 644 residues (UniProt P04264). Like collagen, keratin is far longer than anything usually called a peptide.
Why the label matters in the lab
How a compound is classified decides how it is tested and described:
- Peptides are usually checked for purity by HPLC and for identity by mass spectrometry, and are described by their sequence.
- Proteins are often too large for the same methods to resolve fully. They are characterised by size and structure as well as sequence. See peptide and protein structure.
- Non-peptides, such as retinol, creatine or NAD+, have no sequence at all. They are described by their formula and structure. See empirical and molecular formulas.
Related guides
For how amino acids join and how peptides are named, see what is a peptide. For the building blocks themselves, see the 20 amino acids. For how chemists draw these molecules, see reading a structure diagram.
More on peptide chemistry
- The 20 amino acids: structures, side chains and propertiesAmino acid structure explained: the core all 20 amino acids share, how their side chains differ, L and D forms, and a chart of codes, masses and pKa.
- Peptide and protein structure: primary, secondary, tertiary and quaternaryThe four levels of peptide and protein structure, how helices and sheets form, what holds a chain in shape, and how structure is lost and measured.
- Amino acid codes and how to read a peptide sequenceAmino acid codes explained: the single-letter and three-letter abbreviations for the 20 amino acids, residue masses, and how to read ends and D-forms.