Chemistry · 7 min read
How Solid-Phase Peptide Synthesis Creates Impurities
A plain-language guide to Fmoc solid-phase peptide synthesis and the deletion, truncation, racemization, and oxidation impurities that HPLC and LC-MS detect.
Almost every research peptide is made by solid-phase peptide synthesis (SPPS). The chain is built one amino acid at a time on an insoluble resin bead, then cleaved off and purified. Each cycle is highly efficient but not perfect, and the small failures accumulate into the impurities that a certificate should reveal.
The Fmoc cycle
- Remove the temporary Fmoc protecting group from the growing chain
- Couple the next Fmoc-protected amino acid with an activating reagent
- Wash away excess reagents
- Repeat until the sequence is complete, then cleave from the resin with TFA and remove side-chain protecting groups
Common impurity types
- Deletion sequences: a coupling step fails, so one residue is missing
- Truncated sequences: the chain stops growing early, often after a capping step
- Incomplete deprotection: a side-chain protecting group survives cleavage
- Racemization: an amino acid converts to its mirror-image form
- Aspartimide formation at aspartic acid, especially next to glycine
- Oxidation of methionine or tryptophan, and deamidation of asparagine or glutamine
Why longer peptides are harder
If each coupling is 99% efficient, a 10-residue peptide yields roughly 90% full-length chains before purification, while a 40-residue peptide yields roughly 67%. Longer sequences depend more heavily on preparative HPLC, and their certificates deserve closer reading.
How the impurities are detected
Most of these by-products differ from the target by one residue or a small mass shift. HPLC separates them as small peaks near the main peak, and LC-MS identifies them by mass. See reading an HPLC chromatogram.
Compare sequence lengths in the Peptide Library, from short sequences such as KPV to longer ones such as LL-37.
Educational content for laboratory professionals. North Wall products are for in-vitro research only and are not for human or veterinary use.