Science / Synthesis
How peptides are made
Synthesis
Most research and pharmaceutical peptides are built chemically, one amino acid at a time, on a solid support. Larger ones are often produced by living cells instead.
- 1963solid-phase synthesis described by R. B. Merrifield
- 1984Nobel Prize in Chemistry for the method
- C → Nthe direction the chain is built in
Building on a solid support
In solid-phase peptide synthesis the first amino acid is anchored by its carboxyl end to an insoluble resin bead. The chain then grows from the C-terminus towards the N-terminus, the opposite of the direction used by ribosomes. Because the growing chain stays attached to the bead, excess reagents are simply washed away after each step.
Protect, couple, repeat
Each incoming amino acid carries a temporary protecting group on its amino end, so that it can react only once. In the most widely used chemistry this group is Fmoc, which is removed with a mild base. Reactive side chains carry more durable protecting groups that stay on until the end.
One round has three parts: remove the temporary group from the end of the chain, wash, then add the next protected amino acid together with a coupling reagent that activates its carboxyl group. The round is repeated once per residue.
Release and clean-up
When the sequence is complete, a strong acid, usually trifluoroacetic acid, cuts the chain from the resin and strips the side-chain protecting groups. The result is a crude mixture: the intended peptide plus shorter and incomplete chains.
The mixture is separated by preparative high-performance liquid chromatography and the purified fraction is freeze-dried to a powder, typically as an acetate or trifluoroacetate salt.
Other routes
- Recombinant production: microorganisms or cultured cells are given the gene for the peptide and make it themselves. Human insulin has been produced this way since the early 1980s.
- Native chemical ligation: two unprotected peptide segments are joined in water, which brings chains too long for a single solid-phase run within reach.
- Semi-synthesis: a peptide made by cells is modified chemically afterwards, for example by attaching a fatty-acid chain.
Why length matters
No coupling step is perfect. If each one succeeds 99 times in 100, ten couplings in a row leave about 90 percent of the chains complete and forty leave about 67 percent, before purification. This is the practical reason long sequences are harder and costlier to make than short ones.
General chemistry, written for orientation. It does not describe how to use any substance.