Solid-phase peptide synthesis, step by step

How to produce a peptide of a defined sequence is not a question about a clever reaction. It is one short cycle repeated with discipline: deprotect, wash, couple, wash. Every residue costs those same four operations, and the quality of the powder that reaches a vial is decided by how completely each was driven.

The overview of how research peptides are made sets out the whole route from resin bead to sealed vial. This page stays inside one stage of it: the solid-phase peptide synthesis protocol as it is actually run, and the ways a sequence resists it. It is process chemistry for laboratories evaluating research material, not dosing, preparation or administration guidance, and nothing we sell is for human or veterinary consumption.

Before the first residue: resin, loading and linker

Two decisions precede the first amino acid, and neither can be revisited once the chain is growing. The first is the support, where the governing property is swelling: polystyrene cross-linked with about one per cent divinylbenzene is glassy when dry and expands several-fold in DMF, NMP or dichloromethane. Almost every anchor point sits inside that swollen gel rather than on the bead's surface, so a poorly solvated resin is one whose interior chains are unreachable — the reaction reads as slow when most of it never started. PEG-grafted supports swell more evenly, which is why aggregation-prone sequences go on them.

Loading — millimoles of anchor point per gram of dry resin — is the same trade in another currency: high loading yields more crude material per vessel but crowds the chains together, encouraging the interchain hydrogen bonding that stalls couplings, so a long sequence goes onto resin nearer 0.2 mmol/g than 0.7.

The second decision is the linker joining peptide to bead, which fixes what the C-terminus becomes and how aggressive its release must be. The one worth adding to the familiar pair is 2-chlorotrityl chloride resin: it gives an acid, but one liberated by one to two per cent TFA in dichloromethane, mild enough to recover a fragment with its side-chain protection intact. The choice stays legible in the product. Ipamorelin is catalogued as a pentapeptide of formula C38H49N9O5: five backbone carbonyls account for all five oxygens, since no side chain in that sequence carries one, and a free C-terminal acid would need a sixth. What the formula describes is a C-terminal amide, the output of an amide linker.

The solid-phase peptide synthesis steps, one residue at a time

Assembly runs opposite to the direction sequences are written: the C-terminal residue is anchored to the bead, and each new residue joins onto the amine at the far end. Peptide synthesis for research is mostly run on instruments that repeat the operations below without variation.

Deprotection. The N-terminal Fmoc group comes off with roughly 20 per cent piperidine in DMF, conventionally a short exposure then a longer one. Piperidine abstracts the acidic fluorenyl 9-proton, the group collapses to dibenzofulvene, and a second piperidine traps that fulvene as a stable adduct so it cannot re-alkylate the peptide. The adduct absorbs strongly near 300 nm, so an instrument records a deprotection trace for every residue — and a peak that broadens and tails across successive cycles is the earliest warning that a chain has begun to resist.

Piperidine also drives the best-known base-mediated side reaction: at an aspartate, the backbone nitrogen of the following residue can close onto the side-chain ester to form an aspartimide, which reopens to the beta-linked isomer and to epimers. Asp-Gly is the notorious motif, but the junction at risk is always Asp-X: what matters is the residue after the aspartate, not the one before. In BPC-157's sequence GEPPPGKPADDAGLV the adjacent aspartates give an Asp-Asp and an Asp-Ala junction, either of which can cyclise. The countermeasures are a bulkier aspartate ester, backbone amide protection on the residue that follows, or an acidic additive such as Oxyma in the piperidine.

Washing. Between every step the vessel is drained through a frit and the resin washed repeatedly with fresh DMF. Excess reagent, spent base and by-products leave in the filtrate; no intermediate is ever isolated. Skimping is not a neutral saving of time: piperidine carried forward quenches the activated ester.

Activation and coupling chemistry

A carboxylic acid will not acylate an amine at any useful rate, so the incoming residue's carboxyl is activated in situ. Three reagent families cover nearly all of it.

• Carbodiimides with an additive. DIC plus HOBt, HOAt or Oxyma Pure. The carbodiimide forms an O-acylisourea; the additive intercepts it as a less reactive but far better-behaved active ester, suppressing racemisation and rearrangement to the dead N-acylurea. DIC rather than its older cousin DCC: DCC's urea by-product is insoluble in DMF and precipitates inside the resin bed where washing cannot shift it, while DIC's diisopropylurea leaves with the filtrate.
• Uronium and aminium salts. HBTU, HCTU, HATU and COMU, fired with a tertiary base such as DIPEA or collidine. Faster and more forcing, the usual choice at a hindered junction. One discipline belongs to this family alone: the salt is charged slightly under the amino acid and never over it, because reagent the acid does not consume attacks the resin-bound amine and caps it as an unreactive guanidinium — a chain ended, not merely slowed.
• Phosphonium salts. PyBOP and PyAOP, also base-activated, forming the same benzotriazolyl esters. Their electrophilic centre is phosphorus rather than the carbon that becomes a guanidinium, so a surplus cannot cap the growing chain — which is why they are the reagents to reach for when a coupling has to be forced or repeated.

Two excesses are in play and worth keeping apart. Reagent to acid is a stoichiometry, near 1:1 and deliberately under it for the uronium and aminium salts. The active ester, once formed, is delivered to the resin at three- to five-fold excess over the resin-bound amine, because that excess costs only reagent and washes out again. Base exposure is kept short: cysteine and histidine epimerise most readily while activated.

Capping. Amines left over from an incomplete coupling are acetylated with acetic anhydride and a base, immediately, before the next deprotection puts fresh free amines beside them. Capping rescues nothing: it ends those chains deliberately, for the separation reasons set out in how research peptides are made.

Difficult sequences, and when to couple twice

Some stretches resist for structural rather than chemical reasons. Between roughly the fifth and fifteenth residue a growing chain can hydrogen-bond to its neighbours and organise into beta-sheet: the gel collapses locally, the terminal amine becomes inaccessible, and deprotection and coupling slow together. The symptoms are recognisable — resin that shrinks or clumps, a broadening deprotection trace, a colour test still positive after a routine coupling, over several residues rather than one.

The responses are of two kinds. Solvent and temperature: NMP or a DMF/DMSO mixture to break up hydrogen bonding, or controlled heating, which helps the physics but accelerates aspartimide formation and epimerisation with it. And backbone disruption: a pseudoproline dipeptide at serine or threonine, or Dmb or Hmb protection, removed at cleavage.

Whether to couple a residue once or twice is decided by rule rather than by hope: at beta-branched residues meeting one another; at arginine, bulky under Pbf protection and prone to lactamisation once activated; at the residue coupled onto a proline, where the acylation is of a hindered secondary amine — in the written sequence, the neighbour immediately before that proline; and anywhere inside a stretch known to aggregate. A twenty-nine-residue sequence such as CJC-1295 No DAC takes twenty-eight consecutive couplings, and no later step repairs a shortfall in any one of them.

The Kaiser test and other colourimetric monitoring

Completion is measured, not assumed. The Kaiser test, published by Kaiser and co-workers in 1970, is the standard bench check: a few washed beads are heated with ninhydrin for about five minutes; free primary amines give an intense blue, and a completed coupling leaves beads and solution yellow.

Its limits are well known: it reads primary amines, so it is blind to proline — a sequence carrying four of them, as BPC-157's does, needs another test at those junctions — and it is qualitative, sampling a few beads.

Final deprotection, then cleavage

The cycle ends at capping, which leaves one operation easy to forget and impossible to skip. Fmoc is base-labile, not acid-labile: it survives a TFA cleavage cocktail intact. A synthesis taken straight from the last coupling into cleavage therefore yields Fmoc-peptide — correct in sequence, heavier than the target by the 222 Da of a group that should not be there. The last coupling must be followed by one more piperidine deprotection, on resin, before any acid goes in.

With the N-terminus free, a single acidolysis severs the linker and strips the acid-labile side-chain protection together. The workhorse cocktail is TFA with triisopropylsilane and water in roughly 95:2.5:2.5 proportions, two to three hours at room temperature, its minor components acting as scavengers for the reactive species released as protecting groups leave.

The cocktail is read off the sequence, not taken from habit. Tryptophan, methionine, cysteine and tyrosine call for a richer scavenger mixture, such as the classic Reagent K. Arginine sets the clock: Pbf comes off slowly, and several arginines can need three hours or more — exactly the period over which acid damage accumulates elsewhere.

Where the protocol hands over

Past cleavage the resin is filtered off, the peptide precipitated into cold ether near −20 °C and freeze-dried to a crude powder still holding truncations, deletions, epimers and by-products. Preparative chromatography and batch analysis are covered under how research peptides are made; why the mass in a vial is not all peptide is in peptide purity vs net peptide content; keeping that solid intact afterwards is lyophilised peptide storage.

None of this is visible from outside a manufacturer, which is why a protocol is context rather than evidence — the questions worth putting to any supplier, ourselves included, are in RP-HPLC vs mass spectrometry. NorthScientific supplies research-use-only peptides as lyophilised powder in crimp-sealed glass vials, held as UK stock and dispatched tracked Monday to Friday — details under shipping. We do not publish independent third-party analysis for the material we hold, and we make no claim about the synthesis route behind it. The range is in the catalogue.

Solid-phase is one of two families of peptide synthesis methods; the classical alternative, run entirely in solution, is compared against it in liquid-phase peptide synthesis vs SPPS.

Materials referenced: BPC-157 · Ipamorelin · CJC-1295 No DAC · full catalogue