Peptides are short chains of amino acids joined end to end. They sit in the space between small-molecule chemistry and the large, folded proteins of biology. That middle ground is exactly why they draw so much research attention.
Amino acids: the building blocks
Every amino acid has the same backbone: an amino group (–NH₂), a carboxyl group (–COOH) and a central carbon. What makes each one different is its side chain. Twenty standard amino acids are encoded by the genetic code. Their side chains range from small and greasy (alanine, leucine) to charged (lysine, aspartate) to ring-shaped (phenylalanine, tryptophan).
Chemists usually write sequences with one-letter codes. For example, G is glycine, K is lysine and H is histidine.
The peptide bond
When the carboxyl group of one amino acid reacts with the amino group of the next, they form an amide bond, called a peptide bond, and release a molecule of water. Repeat that step and you get a chain with a defined direction. One end has a free amino group (the N-terminus) and the other a free carboxyl group (the C-terminus). Sequences are always written from N to C.
Peptide or protein?
There's no strict cut-off, but a common convention is:
| Term | Typical length | Notes |
|---|---|---|
| Dipeptide / tripeptide | 2–3 residues | e.g. glutathione (a tripeptide) |
| Oligopeptide | up to ~20 residues | Often flexible in solution |
| Polypeptide | ~20–50 residues | May begin to fold into stable shapes |
| Protein | 50+ residues | Usually folds into a defined 3D structure |
Why peptides are interesting to researchers
- Selectivity. Peptides can bind their targets very precisely, because many natural signalling molecules (hormones, neurotransmitters) are peptides themselves.
- Synthetic accessibility. Unlike most proteins, peptides can be built entirely by chemical synthesis, residue by residue. That lets researchers swap in unnatural amino acids or add modifications.
- Tunable properties. Small changes, such as cyclising the chain, adding a fatty-acid tail or replacing one residue, can greatly change stability and behaviour.
The challenges
Peptides are broken down quickly by enzymes called proteases, often have short half-lives in biological systems, and generally cross cell membranes poorly. Much of modern peptide chemistry is about solving those three problems. We cover the main strategies in Where peptide research is heading.
Key takeaway: a peptide is defined by its sequence. Because synthesis can introduce errors, confirming that sequence and its purity is the core job of peptide analysis.
Related articles
- How laboratories validate analytical methods
- Antimicrobial peptides: an ancient defence against modern superbugs
- How clinical trials are designed
Sources and further reading
- Pauling L, Corey RB, Branson HR. The structure of proteins: two hydrogen-bonded helical configurations of the polypeptide chain. PNAS 1951;37:205–211. doi:10.1073/pnas.37.4.205 · PMID: 14816373
- Lau JL, Dunn MK. Therapeutic peptides: historical perspectives, current development trends, and future directions. Bioorg Med Chem 2018;26:2700–2707. doi:10.1016/j.bmc.2017.06.052 · PMID: 28720325
- Muttenthaler M, King GF, Adams DJ, Alewood PF. Trends in peptide drug discovery. Nat Rev Drug Discov 2021;20:309–325. doi:10.1038/s41573-020-00135-8 · PMID: 33536635