Most research peptides are built by solid-phase peptide synthesis, or SPPS. Understanding how the method works explains where purity comes from, why impurities exist, and why every batch has to be tested.
The core idea
SPPS builds a peptide one amino acid at a time on a solid support. The growing chain stays anchored to a small insoluble resin bead while reagents are washed over it, so excess reagents and byproducts can be rinsed away at each step without losing the peptide. The chain is assembled from one end of the sequence to the other in a fixed direction.
The cycle, step by step
Each amino acid is added in a repeating cycle:
- Deprotection: a temporary protecting group on the open end of the chain is removed so the next amino
acid can attach.
- Coupling: the next amino acid, protected on its other reactive sites, is activated and joined to the
chain.
- Washing: excess reagents and byproducts are rinsed away before the next cycle.
The cycle repeats once per residue. A 30-residue peptide runs it 30 times.
Where impurities come from
No coupling step is perfectly complete. When a residue fails to couple on a given cycle, the result is a chain missing that amino acid, called a deletion sequence, or a chain that stopped early, called a truncation. These closely related molecules are the impurities that later appear on an HPLC trace. More residues means more cycles and more chances for them to accumulate, which is why longer peptides are harder to produce at high purity.
Cleavage and purification
Once the chain is complete it is cleaved from the resin and its remaining protecting groups are removed. The crude product is then purified, commonly by preparative HPLC, to separate the target sequence from the deletion and truncation products. Purification is what turns a crude mixture into research-grade material.
Why this connects to testing
Purity depends on synthesis and purification, and every batch is its own run, so purity has to be confirmed per batch. That is the job of the lot-specific Certificate of Analysis: HPLC to measure how much of the sample is the target sequence, and mass spectrometry to confirm the molecule is the one intended.
For laboratory research use only. Not for human consumption.