A laboratory compares two chromatograms for synthetic peptides of the same nominal purity. The first, a ten-residue chain, shows a clean main peak with a few small, well-separated neighbors. The second, a thirty-residue chain, shows a main peak with a cluster of shoulders crowding its edges. The difference is not a matter of one supplier being careful and the other careless. It follows from how solid-phase peptide synthesis builds a chain, one residue at a time, and from the arithmetic of repeating an imperfect step many times.
Building a chain on a bead
Almost all research peptide is made by solid-phase peptide synthesis. The growing chain is anchored to an insoluble resin bead throughout assembly. Because the chain cannot dissolve, excess reagents and byproducts are removed simply by draining and washing the resin. What would otherwise be a purification step after every reaction becomes a rinse. That simplification is what made routine synthesis of long peptides practical.
Assembly runs from the C-terminus toward the N-terminus, the opposite direction to biological protein synthesis. The first residue is attached to the resin through a linker, and each subsequent residue is joined to the free amino group of the one before it. How the choice of resin and linker shapes the final product is covered in resin choice and loading in solid-phase peptide synthesis.
The four-step cycle, repeated for every residue
In Fmoc chemistry, the standard approach for research-grade material, each residue is added by the same cycle:
- Deprotect. A base, most often piperidine, removes the Fmoc group protecting the N-terminal amine of the chain, exposing it for the next coupling.
- Wash. The base and the released protecting-group fragments are rinsed away.
- Couple. The next amino acid, carrying its own Fmoc group and side-chain protection, is chemically activated and forms an amide bond to the exposed amine.
- Wash. Unused activated amino acid and coupling byproducts are rinsed away, leaving the resin ready for the next deprotection.
One cycle adds one residue. A twenty-five residue peptide therefore needs twenty-five cycles, and every one of them is an opportunity for an incomplete reaction.
Why length is so costly: compounding yield
No coupling reaches every chain on the resin. Suppose each cycle succeeds on 99 percent of the chains, a respectable figure. The fraction of chains carrying the complete, correct sequence after n cycles is 0.99 raised to the power n.
| Residues added | Chains with the full sequence at 99% per step |
|---|---|
| 10 | about 90% |
| 20 | about 82% |
| 30 | about 74% |
| 40 | about 67% |
The chains that miss a coupling are not removed. They continue through every later cycle and finish as deletion sequences, one residue short of the target. Because a deletion sequence differs from the product by a single residue, it usually behaves almost identically on a reverse-phase column and elutes very close to the main peak. That is the cluster of shoulders in the opening example, and it is why long peptides are both harder and more expensive to make well. The broader catalog of synthesis byproducts is described in peptide synthesis impurities.
Capping: making failures easier to remove
Many protocols add a capping step after coupling. Any chain that failed to react is acetylated at its N-terminus, which blocks it permanently. A capped chain stops growing, so instead of carrying on as a near-copy of the product missing one internal residue, it ends as a clearly shorter truncated chain.
Capping does not increase the amount of correct product; a chain that failed to couple was already lost. What it improves is purification. A short truncated chain differs noticeably from the target in hydrophobicity and separates cleanly, whereas an uncapped deletion sequence may not separate at all.
Cleavage, purification and drying
When the final residue is in place, the chain is released from the resin with strong acid, usually trifluoroacetic acid (TFA) mixed with scavengers. The same step removes the side-chain protecting groups. Scavengers are added to capture the reactive species that form during deprotection, which would otherwise modify vulnerable residues. If a protecting group survives this step, the product carries an extra mass that mass spectrometry can detect.
The crude peptide is then purified by preparative reverse-phase HPLC, and the collected fraction is freeze-dried to a solid. Because TFA is present both at cleavage and, commonly, in the purification mobile phase, many synthetic peptides are isolated as trifluoroacetate salts. What that means for the composition of the solid is explained in TFA vs acetate peptide counter-ion.
What this means when reading a purity result
Knowing the cycle helps a buyer read analytical data sensibly. Closely eluting impurities near the main peak are most likely deletion sequences. Clearly earlier peaks are often truncations. Species heavier than the target may reflect incomplete removal of protecting groups. Longer sequences can be expected to carry more of all of these before purification, which places more demand on the purification step.
A reverse-phase HPLC purity figure summarizes how well purification separated the product from these relatives. Battle Born publishes an independent reverse-phase HPLC result for each product, and how such a result is produced is described in reverse-phase HPLC and peptide purity.
Questions
Why does synthesis run from the C-terminus?
The first residue is anchored to the resin by its carboxyl end, leaving its amine free for the next coupling. Each new residue is added to the growing N-terminus, so the chain extends in the C-to-N direction.
Is a deletion sequence the same as a truncation?
No. A deletion sequence is missing one internal residue but otherwise complete. A truncation stops short, often because a failed chain was capped and could grow no further.
Why are long peptides harder to purify?
Yield losses compound over more cycles, so the crude product contains more near-identical deletion sequences, and those are the hardest species to separate from the target.
Does a higher per-step efficiency make a large difference?
Yes, because the effect compounds. Raising each coupling from 99 to 99.5 percent roughly halves the fraction of incomplete chains for a given length, which is why synthesis chemists work hard to push individual steps as close to completion as they can.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.