Solid-Phase Peptide Synthesis, Explained
Merrifield's method explains most of what appears on a certificate of analysis: deletion sequences, why purity falls with length, and why price scales the way it does.
Solid-phase peptide synthesis is the reason a research peptide catalog can exist at all. Understanding roughly how it works explains most of what appears on a certificate of analysis — why deletion sequences exist, why purity falls with length, and why some compounds cost several times more than others of the same mass.
The core idea
Before 1963, peptides were built in solution, which meant purifying an intermediate after every single step. Bruce Merrifield’s insight was to anchor the growing chain to an insoluble resin bead. The chain stays put; reagents and by-products are washed away. Purification between steps becomes filtration, and what took months became feasible in days. The work won a Nobel Prize in 1984.
The cycle
Chains are built C-terminus to N-terminus — backwards relative to how sequences are written and how ribosomes work. Each residue takes four steps:
- Deprotection. Remove the temporary protecting group from the chain’s free end.
- Wash.
- Coupling. Add the next amino acid, itself protected, with a reagent that activates its carboxyl group.
- Wash.
Repeat per residue. At the end, cleave the finished chain from the resin and remove the side-chain protecting groups — usually with trifluoroacetic acid, which is where the TFA counter-ions on the final product come from.
Why length costs purity
Each coupling step is efficient but not perfect. Suppose 99.5 per cent completion per step — a good figure. Compound that across a sequence:
| Residues | Crude yield of full-length chain at 99.5%/step |
|---|---|
| 5 | ~98% |
| 10 | ~96% |
| 20 | ~91% |
| 30 | ~86% |
| 50 | ~78% |
The missing fraction is not waste — it is a population of closely related molecules, mostly chains missing one residue. These are deletion sequences, and they are the reason purification exists and the reason a chromatogram has secondary peaks close to the main one.
This also explains pricing. A 3-residue peptide and a 30-residue peptide are not ten times apart in cost; they are further, because the longer one needs more steps, produces a harder mixture to separate, and yields less.
Fmoc and Boc
Two protecting-group strategies dominate. Fmoc chemistry uses a base-labile group removed with piperidine, and final cleavage with TFA — it is the standard for most commercial synthesis. Boc chemistry uses acid-labile protection and requires hydrogen fluoride for final cleavage, which needs specialised equipment. Boc survives for difficult sequences where Fmoc struggles.
What this explains on a certificate
- Secondary peaks close to the main one — deletion sequences, structurally similar and hard to resolve.
- TFA counter-ions — from the cleavage step, and the reason peptide content is below vial mass.
- Why purity claims should be read with length in mind — 99% on a tripeptide and 99% on a 30-mer are not equally impressive results.
- Why mass confirmation matters most on long sequences — that is where a deletion is most likely and hardest to spot chromatographically.
Our guide to purity verification covers HPLC and mass spectrometry in detail, and the content versus purity note covers what the vial actually holds.
All products and information referenced are for in-vitro research and laboratory use only. Nothing here is medical advice, and no therapeutic claim is made or implied.