How Research Peptides Are Made: Solid-Phase Peptide Synthesis Explained
The short answer
Most research peptides are produced by solid-phase peptide synthesis (SPPS), a method introduced by Bruce Merrifield in 1963 in which the growing chain stays anchored to an insoluble resin while amino acids are added one at a time. Each cycle involves removing a protecting group from the chain’s exposed amino terminus and coupling the next protected amino acid, after which excess reagents are simply washed away. The approach is efficient but not perfect: small losses at every cycle compound, so longer sequences give lower yields and more complex impurity profiles.
Merrifield’s central idea
Before SPPS, peptides were assembled in solution, with the intermediate purified after each coupling. That approach is workable for very short sequences and becomes impractical quickly. Merrifield’s contribution, for which he received the Nobel Prize in Chemistry in 1984, was to attach the C-terminal residue to a cross-linked polystyrene bead so that the peptide became a solid that could be filtered and washed. Reagents could then be used in large excess to drive couplings towards completion, and purification between steps collapsed into a simple filtration.
The resin is typically a lightly cross-linked polymer that swells in the reaction solvent, exposing the internal sites where chains are anchored. Choice of resin and of the linker joining the peptide to it determines both the conditions needed for final release and the chemical form of the C-terminus — a free acid or an amide, for example.
The coupling cycle
Synthesis proceeds from the C-terminus towards the N-terminus, the reverse of biological translation. Each cycle repeats the same sequence of operations:
- Deprotection. The temporary protecting group on the chain’s N-terminal amine is removed, exposing a free amine.
- Washing. Deprotection reagents and cleaved by-products are washed from the resin.
- Coupling. The next amino acid, itself protected at its own amine and on any reactive side chain, is activated and reacted with the free amine to form the new peptide bond.
- Washing. Excess reagents are removed and the cycle repeats.
Coupling requires activation of the incoming carboxyl group, since a carboxylic acid and an amine do not react usefully on their own. Carbodiimides such as DIC were used early and remain in use; uronium and phosphonium salts such as HBTU, HATU and PyBOP, generally with an additive and a tertiary amine base, are now common because they couple quickly and suppress racemisation. Difficult couplings may be repeated, run at elevated temperature, or performed with a different reagent; unreacted chains are sometimes deliberately capped, typically by acetylation, to prevent them from re-entering later cycles and generating deletion sequences that are hard to separate.
Protecting groups: Fmoc versus Boc
SPPS depends on an orthogonal protection strategy — a temporary group removed at every cycle, and semi-permanent side-chain groups that survive until the end. The two dominant schemes are named for the temporary group.
Boc chemistry uses the acid-labile tert-butyloxycarbonyl group, removed each cycle with trifluoroacetic acid. Side chains carry benzyl-type protection, and final cleavage requires a strong acid, historically anhydrous hydrogen fluoride. Boc chemistry handles certain aggregation-prone and hydrophobic sequences well, but the requirement for HF and specialised apparatus limits its use.
Fmoc chemistry uses the base-labile 9-fluorenylmethyloxycarbonyl group, removed with a secondary amine, usually piperidine in DMF. Side chains carry acid-labile protection, so the final cleavage uses TFA rather than HF. Because the two removal conditions are chemically orthogonal — base for the temporary group, acid for the permanent ones — and because no strong acid is needed at each cycle, Fmoc chemistry has become the default for most research-scale synthesis. It is also convenient to monitor, since the dibenzofulvene released on deprotection absorbs ultraviolet light and can be measured to follow reaction progress.
Cleavage and workup
Once the sequence is complete, the peptide is released from the resin and side-chain protecting groups are removed, usually in a single acidolytic step. In Fmoc chemistry this is typically a TFA-based cocktail containing scavengers such as water, triisopropylsilane, thioanisole or ethanedithiol. Scavengers are not optional detail: deprotection generates reactive carbocations that will otherwise alkylate sensitive side chains, particularly tryptophan, methionine, cysteine and tyrosine, producing modified by-products.
The crude peptide is generally precipitated in cold diethyl ether, collected, dissolved and then purified by preparative reversed-phase HPLC. Fractions are analysed, pooled and lyophilised. Where a sequence contains cysteine residues intended to form disulfide bonds, an additional oxidative folding step is required, and getting the correct connectivity in peptides with multiple disulfides is a well-recognised difficulty.
Why longer sequences are harder
The arithmetic of stepwise synthesis is unforgiving. If each cycle proceeds to 99 per cent completion, a 10-residue peptide finishes at roughly 90 per cent of theoretical full-length material, while a 50-residue peptide finishes at around 61 per cent — before accounting for losses at cleavage and purification. At 98 per cent per cycle, the 50-residue case falls to about 36 per cent.
Real syntheses are also not uniform. Certain sequences aggregate on the resin as the chain lengthens, with the growing chains forming hydrogen-bonded structures that bury the reactive terminus and slow both deprotection and coupling. These “difficult sequences” are addressed with pseudoproline dipeptides, backbone protection, solvent changes, microwave heating, or by splitting the target into fragments that are made separately and joined, for example by native chemical ligation.
Where impurities come from
Understanding the origin of impurities explains why analytical characterisation matters. Common categories include deletion sequences from incomplete coupling; truncated sequences from capped chains; incompletely deprotected material where a side-chain group survived cleavage; oxidation products, particularly at methionine; alkylation adducts from inadequate scavenging; racemised residues from over-activation, cysteine and histidine being most susceptible; aspartimide-related by-products arising in aspartate-containing sequences under repeated base treatment; and, in disulfide-containing peptides, mis-paired or intermolecular disulfide isomers.
Several of these species differ only slightly from the target in mass and hydrophobicity, which is precisely why they can be difficult to resolve chromatographically and why a single analytical method is rarely sufficient to characterise a synthetic peptide.
Research use only
The products discussed on this site are supplied strictly for laboratory research use only. They are not medicines, are not licensed or approved for human or veterinary consumption, and must not be administered to humans or animals. Nothing in this article constitutes medical advice, a therapeutic claim, or a recommendation for use of any kind. Descriptions of chemistry and published research are provided for informational purposes and should not be read as evidence of safety or efficacy for any application.
Image: “TBTU SPPS” by Daniel Dróżdż, licensed under CC BY 4.0.
