Solid Phase Peptide Synthesis, Step By Step

In solution, making a chain of thirty residues means thirty coupling reactions and thirty separations, each of which loses material. The solid phase approach, introduced by Merrifield in 1963, changes the accounting. The chain is covalently tied to a cross-linked polystyrene or polyethylene glycol resin bead that is insoluble in the reaction solvents, so reagents can be used in large excess and then simply drained and rinsed off.

Synthesis runs from the C-terminus toward the N-terminus, which is the opposite direction to biological translation. The first residue is attached to a linker on the resin through its carboxyl group, and each new residue arrives with its own carboxyl activated and its alpha-amine masked by a temporary protecting group. The masking is what keeps the chain growing in one direction instead of polymerising on itself.

Two protecting-group strategies dominate, named for the group on the alpha-amine. Fmoc chemistry removes that group with a mild base, usually piperidine in dimethylformamide, and cleaves the finished chain from the resin with trifluoroacetic acid. Boc chemistry removes it with acid at every cycle and requires hydrogen fluoride for the final cleavage. Fmoc is the common commercial route because the repetitive step is the gentler one.

The cycle, four operations repeated

Each residue added costs the same four operations. Nothing about the cycle changes with chain length, which is why the method automates well and why its errors accumulate in a predictable shape.

• Deprotection: the temporary group on the chain's free alpha-amine is removed, exposing a nucleophile. In Fmoc chemistry this is typically 20% piperidine in dimethylformamide, in two applications of a few minutes each.
• Wash: the resin is rinsed repeatedly, commonly five to ten solvent volumes, to remove the cleaved protecting group and the base before the next reagent arrives.
• Coupling: the next amino acid, its carboxyl activated by a coupling reagent, is presented in excess and forms the amide bond to the exposed amine.
• Wash again, then optionally cap any amine that failed to react so that it cannot couple later in the sequence.

Side chains carry their own permanent protecting groups throughout, chosen so that they survive every deprotection cycle and come off together at the end. The final acid cleavage therefore does two jobs at once: it releases the chain from the linker and strips the side-chain protection, producing crude peptide in solution alongside a mixture of small organic fragments.

Why the arithmetic of repetition dominates

Crude purity is roughly the coupling efficiency raised to the power of the number of couplings. At 99.0% efficiency per coupling, a 20-residue chain leaves about 0.99 to the nineteenth power, or 82.6%, of the material as the intended sequence. At 30 residues the same efficiency gives about 74.7%. Drop the efficiency to 98.0% and a 30-residue chain falls to roughly 55.7%. Nothing unusual has gone wrong in either case; the exponent is simply doing its work.

This is the practical reason routine solid phase work sits below roughly 50 residues, with 30 to 40 residues a comfortable ceiling for a single linear assembly. Longer chains are usually built as fragments and joined, or made by expression in a biological system instead. The peptides sold as research material generally sit well inside the comfortable range, between about 5 and 45 residues.

The failure mode: a deletion sequence that hides in plain sight

The characteristic failure of the cycle is an incomplete coupling, where some fraction of chains on the resin do not receive the incoming residue. Those chains are not destroyed. At the next deprotection their amine is exposed again and they happily accept the following residue, producing a chain that is missing exactly one internal amino acid.

A one-residue deletion is a difficult impurity precisely because it is so similar to the target. Its mass differs by the residue mass, from 57 Da for glycine to 186 Da for tryptophan, so mass spectrometry resolves it easily. Its hydrophobicity often differs very little, so on a reverse-phase gradient it can elute close to or under the main peak and contribute area that a purity calculation assigns to the intended sequence.

Two habits limit it. Capping unreacted amines after each coupling converts a would-be deletion into a truncated chain that stops growing, which is easier to separate because it differs from the target by everything downstream rather than by one residue. Monitoring deprotection spectrophotometrically, by the absorbance of the released dibenzofulvene adduct at 301 nm, gives a per-cycle signal that a coupling has gone incompletely rather than a single verdict at the end.

What the published record covers, and what it does not

Our published purity figures are reverse-phase HPLC purity at a stated wavelength, reported as the percentage of total integrated peak area. That figure is a property of a chromatogram. It is not a sequence confirmation, and it is not net peptide content: counter-ions, residual water and residual solvent carry no chromophore at 214 nm and contribute mass without contributing peak area.

Identity is assayed by mass spectrometry, which confirms that the observed mass matches the expected mass for the sequence. A matching mass is strong evidence and it is not residue-by-residue sequence determination: an internal transposition of two residues leaves the mass unchanged. Sequence determination by amino acid analysis or by tandem fragmentation is not among the assays Aurum publishes. Sterility, endotoxin and pyrogen testing are not performed and not claimed, and the synthesis route speaks to none of them.

Common questions

Why build from the C-terminus?

Because the carboxyl end is what attaches to the resin linker, leaving the amine free to react. Building the other way round would require activating the amine end, which invites side reactions and racemisation.

What does the resin actually contribute?

Insolubility and filterability, nothing chemical to the final molecule. The resin and linker are discarded at cleavage, and in a clean process nothing of the bead remains in the product.

Is a microwave or heated coupling different chemistry?

No. Heating accelerates the same coupling reaction and can reduce cycle time from tens of minutes to a few. It also accelerates side reactions such as racemisation, so the conditions are chosen per sequence.

Can any sequence be made this way?

Not reliably. Aggregating sequences, long stretches without a helix-breaking residue, and certain adjacent residue pairs are known to couple poorly. Difficult sequences are real, documented, and not fixed by running the cycle again.

What happens to the crude material after cleavage?

It is precipitated out of the cleavage cocktail, usually into cold ether, then purified by preparative reverse-phase chromatography and lyophilized. The purification step, not the synthesis, is what sets the final purity figure.