The idea that won a Nobel Prize
Before 1963, peptides were assembled in solution, and it was hell: after every step the product had to be isolated and purified. Merrifield's move was to anchor the first amino acid to an insoluble resin — then everything unwanted can simply be washed away while the product stays on the support. That idea turned weeks of work into hours and eventually earned a Nobel Prize.
Modern synthesisers run these cycles automatically: activate the amino acid, couple it to the growing chain, remove the protecting group, wash. Repeat as many times as the sequence has links.
Where impurities are born
No cycle runs at a hundred percent. If every coupling works at 99%, by link thirty only about 74% of chains are fully correct. The rest are chains with skipped or interrupted links — deletion and truncation sequences. They're chemically very similar to the target peptide, which is exactly why purification is serious standalone work.
After assembly, the chain is cleaved from the support with acid, which also strips the side-chain protecting groups. That cocktail leaves traces of its own — which is why chromatography is unavoidable next.
Purification and drying
The main purification tool is reversed-phase HPLC: the mixture runs through a column where chains separate by hydrophobicity, and the fraction containing the target peptide is collected. The same technique is later used to check the purity of the finished material.
The finale is lyophilisation: the solution is frozen and the water removed by sublimation under vacuum. The result is a loose white powder in which the peptide stays stable for months under proper storage. If you want the details, we have separate articles on lyophilisation and on why purity is never 100%.
References
- Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide — Journal of the American Chemical Society, 85(14), 2149–2154 (DOI 10.1021/ja00897a025), 1963.
- Reversed-Phase High-Performance Liquid Chromatography in Peptide and Protein Analysis — Encyclopedia of Analytical Chemistry (DOI 10.1002/9780470027318.a1631), 2000.