Solid-phase peptide synthesis (SPPS) is how nearly every research peptide is made today. The method builds a chain one amino acid at a time while the growing peptide stays anchored to tiny resin beads, so excess reagents can simply be washed away after each step. Understanding the cycle explains why synthetic peptides need purification, why certain impurities appear, and what a certificate of analysis is really measuring.
Quick answer
What is solid-phase peptide synthesis? SPPS assembles a peptide on insoluble resin beads. Each cycle removes a temporary protecting group, couples the next protected amino acid, and washes away excess reagents. When the chain is complete, it is cleaved from the resin, side-chain protecting groups are removed, and the crude product is purified, usually by reversed-phase HPLC.
The idea that changed peptide chemistry
Before 1963, peptides were assembled in solution, and every intermediate had to be isolated and purified before the next residue could be added. Bruce Merrifield’s insight, published that year in the Journal of the American Chemical Society, was to attach the first amino acid to an insoluble polymer support. The chain stays on the solid phase, reagents can be used in excess to drive reactions forward, and purification between steps becomes a matter of filtering and washing. Merrifield received the 1984 Nobel Prize in Chemistry for the method.
The synthesis cycle, step by step
SPPS builds the chain from the C-terminus toward the N-terminus, which is the reverse of how sequences are written; see how to read a peptide sequence for the naming convention.
- Anchor. The C-terminal amino acid is attached to the resin through a linker that will later determine whether the peptide ends as an acid or an amide.
- Deprotect. The temporary protecting group on the anchored residue’s amino group is removed. In Fmoc chemistry, a mild base such as piperidine does this.
- Wash. Solvent washes remove the reagents and by-products.
- Couple. The next amino acid, with its own amino group and reactive side chain protected, is activated and bonded to the free amino group on the resin.
- Wash again, then repeat from step 2 for every residue in the sequence.
- Cleave. A strong acid cocktail, typically based on trifluoroacetic acid (TFA) with scavengers, releases the finished chain and strips the side-chain protecting groups.
Fmoc and Boc: the two main chemistries
| Fmoc/tBu | Boc/Bzl | |
|---|---|---|
| Temporary amino protection | Fmoc, removed by base | Boc, removed by acid |
| Final cleavage | TFA-based cocktail | Very strong acid such as hydrogen fluoride |
| Current use | The method of choice for most synthesis today | Older approach, still used for some difficult sequences |
The review by Behrendt, White and Offer listed below describes Fmoc SPPS as the method of choice and covers the improvements in building blocks and side-reaction control that made it dominant.
Why chain length makes synthesis harder
No coupling step is perfectly complete. Small per-step losses compound across the whole chain, so the theoretical share of full-length product falls as sequences get longer. The table below is a simple calculation, not a measurement of any specific synthesis:
| Per-step efficiency | 10 couplings | 20 couplings | 30 couplings | 50 couplings |
|---|---|---|---|---|
| 98% | 81.7% | 66.8% | 54.5% | 36.4% |
| 99% | 90.4% | 81.8% | 74.0% | 60.5% |
| 99.5% | 95.1% | 90.5% | 86.0% | 77.8% |
The rest of the crude material is mostly chains that are almost right: missing a residue, stopped early, or carrying a side reaction. That is why purification is not optional, and why our guide to peptide impurities focuses on these near-miss molecules.
From crude peptide to documented lot
After cleavage, the crude peptide is typically purified by preparative reversed-phase HPLC, which separates the target chain from related impurities. The pooled fractions are freeze-dried into the powder described in Lyophilized peptides 101. Because TFA is used in cleavage and often in purification, the peptide usually leaves this stage as a TFA salt unless it is exchanged to another counter-ion, a topic covered in TFA vs acetate peptide salts.
The finished lot is then analyzed: analytical HPLC for chromatographic purity and mass spectrometry for identity. Our guide on reading an HPLC chromatogram shows what that analysis looks like on a certificate, and the public COA ledger is where Revoryn Labs publishes available batch documentation.
Key takeaways
- SPPS builds a peptide one residue at a time on resin beads, from the C-terminus to the N-terminus.
- Anchoring the chain lets excess reagents be washed away, which made synthesis fast and scalable.
- Fmoc chemistry is the dominant approach today; final cleavage typically uses TFA.
- Per-step losses compound, so longer sequences produce more near-miss impurities.
- Purification, freeze-drying and analysis turn crude synthesis output into a documented lot.
Frequently asked questions
Who invented solid-phase peptide synthesis?
R. Bruce Merrifield, who described the method in 1963 and received the 1984 Nobel Prize in Chemistry for it.
What does Fmoc stand for?
Fluorenylmethyloxycarbonyl, a protecting group that blocks the amino group of each incoming amino acid and is removed with mild base before the next coupling.
Why is the peptide built from the C-terminus?
The first residue is anchored to the resin through its carboxyl end, so each new amino acid is added to the free amino end. The chain therefore grows toward the N-terminus.
Does SPPS produce a pure peptide directly?
No. The crude product contains related impurities from incomplete reactions and side reactions, so it is purified, usually by reversed-phase HPLC, and then analyzed before release.
Sources
- Merrifield RB. Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide. J Am Chem Soc. 1963;85(14):2149-2154. doi:10.1021/ja00897a025
- Behrendt R, White P, Offer J. Advances in Fmoc solid-phase peptide synthesis. J Pept Sci. 2016;22(1):4-27. doi:10.1002/psc.2836
- D’Hondt M, Bracke N, Taevernier L, et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30. doi:10.1016/j.jpba.2014.06.012
- The Nobel Prize in Chemistry 1984: Bruce Merrifield. nobelprize.org
For laboratory and in-vitro research use only. Not for human or veterinary use.
For laboratory and in-vitro research use only. Not for human or veterinary use. See our research-use disclaimer.
