What are peptides, and how do they differ from proteins? If you are new to research peptides, the terminology can be confusing. This guide explains peptide structure, how synthetic peptides are made, why they are supplied as freeze-dried powder, and how peptide purity is measured.
This article is for educational and laboratory research purposes only.
What are peptides?
According to the National Human Genome Research Institute, a peptide is a short chain of amino acids, typically 2 to 50, linked by chemical bonds called peptide bonds. Longer chains are usually described as polypeptides or proteins. The boundary is a convention rather than a strict rule.
Peptide naming by length
- Dipeptide: two amino acids.
- Tripeptide: three amino acids, for example GHK (glycine-histidine-lysine) or KPV (lysine-proline-valine).
- Oligopeptide: a short chain, often up to around ten amino acids, such as Epithalon, a tetrapeptide.
- Polypeptide: a longer chain, such as the 43-amino-acid protein thymosin beta-4, from which TB-500 is derived.
Peptide structure
Every amino acid has an amino group, a carboxyl group and a side chain. A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of the next, releasing a molecule of water. Peptide structure is then described at several levels:
- Primary structure: the sequence of amino acids, written from the N-terminus to the C-terminus.
- Secondary structure: local folding patterns such as helices and sheets, which become more relevant in longer peptides.
- Modifications: changes such as N-terminal acetylation, C-terminal amidation, cyclisation, D-amino acids or pegylation, which researchers use to alter stability. PEG-MGF is an example of a pegylated peptide.
How are research peptides made?
Most synthetic research peptides are produced by solid-phase peptide synthesis (SPPS), a method developed by R. Bruce Merrifield, who received the 1984 Nobel Prize in Chemistry for the work. In SPPS, the first amino acid is anchored to an insoluble resin bead and the chain is built one residue at a time. Each cycle removes a protecting group, couples the next protected amino acid and washes away excess reagents. When the sequence is complete, the peptide is cleaved from the resin and purified.
Why peptides are supplied lyophilised
After purification, peptides are usually lyophilised (freeze-dried). Removing water makes the peptide far more stable during shipping and storage. That is why most vials in our research peptides range are supplied as lyophilised powder and are reconstituted in the lab with a suitable diluent such as bacteriostatic water. See our guide to storing peptides for handling advice.
How is peptide purity measured?
Peptide purity describes how much of the material is the intended sequence rather than related impurities such as truncated or deleted sequences left over from synthesis. Two analytical techniques are standard:
- High-performance liquid chromatography (HPLC): separates the components of a sample. Purity is commonly reported as the percentage of the total peak area that belongs to the main peak.
- Mass spectrometry (MS): confirms identity by measuring the molecular mass and comparing it with the expected mass of the sequence.
Reading a purity figure
An HPLC purity figure tells you about related peptide impurities, but not everything in the vial. Lyophilised peptides also contain counter-ions (such as acetate or trifluoroacetate) and residual water, so the net peptide content by weight is lower than the gross weight. When comparing high purity research peptides, check what method a figure refers to and how it was measured.
Research peptides vs medicines
Research peptides are supplied as reference materials for in-vitro laboratory work. They are not manufactured or approved as medicines and must not be administered to people or animals. Every research product at SJ Peptides is labelled accordingly; see our research use disclaimer.
Conclusion
Peptides are short chains of amino acids joined by peptide bonds. Synthetic research peptides are typically made by solid-phase synthesis, purified, freeze-dried for stability and characterised by HPLC and mass spectrometry. Understanding peptide structure, synthesis and purity makes it easier to choose and handle research materials. Explore the SJ Peptides shop or read more on our research blog.
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