Peptide Synthesis: SPPS, Purification and Lot Variance

Understanding how a peptide is made explains almost everything about how it can fail, and what a supplier’s specifications are actually promising.

Solid-phase peptide synthesis

Since Merrifield’s work in the 1960s, peptides have been assembled on an insoluble resin support, one residue at a time, from the C-terminus toward the N-terminus. Each cycle repeats four operations: deprotect the N-terminal group of the growing chain; wash; couple the next protected amino acid using an activating reagent; wash again. Because the growing chain is anchored to resin, excess reagents are simply washed away — which is what makes the process automatable.

Where impurities come from

No coupling step is perfectly efficient. At 99.5% efficiency per cycle, a 30-residue peptide finishes at roughly 86% of theoretical yield, and the shortfall is not lost — it is present as truncated and deletion sequences that differ from the target by one or more residues. These are the closest-eluting impurities and the hardest to resolve chromatographically, which is exactly why gradient design determines how honest a purity figure is.

• Deletion sequences — a residue that failed to couple in one cycle.
• Truncated sequences — chains that stopped elongating entirely.
• Incomplete deprotection — side-chain protecting groups surviving final cleavage.
• Oxidation — methionine, cysteine and tryptophan are all susceptible during cleavage and workup.
• Racemisation — epimerisation at a stereocentre, producing a diastereomer of identical mass. Mass spectrometry cannot detect this; only chiral or high-resolution chromatography can.

Cleavage and purification

The completed chain is cleaved from the resin and globally deprotected, typically with trifluoroacetic acid and scavengers. The crude material is then purified by preparative reverse-phase HPLC, collected across the main peak, and lyophilised. The width of the collection window is a direct trade-off: narrow collection raises purity and lowers yield.

Lot-to-lot variance is the real signal

Any competent facility can produce one excellent lot. The measure of a supplier is the spread across lots — whether purity, water content and endotoxin cluster tightly around specification or wander. That is only visible if the supplier publishes every certificate rather than a selected one, which is a good reason to prefer suppliers that do.