Solid-Phase Peptide Synthesis (SPPS) Explained

Almost every synthetic research peptide, from a five-residue fragment to a lipidated 31-mer, is built by solid-phase peptide synthesis (SPPS). Understanding how SPPS works explains why peptides are sold as purified powders, why HPLC purity is never exactly 100%, and what the small peaks on a chromatogram actually are.

The problem SPPS solved

A peptide is a chain of amino acids joined by amide (peptide) bonds. Making one bond in solution is easy. Making thirty of them in the right order is not: every intermediate has to be isolated and purified, yields fall with each step, and longer intermediates become hard to dissolve.

In 1963 R. Bruce Merrifield published a different approach. He anchored the first amino acid to an insoluble polymer bead (a cross-linked polystyrene resin) and grew the chain on that bead. Because the growing peptide stays attached to a solid, excess reagents and by-products can simply be washed away after each step. The method was soon automated. Merrifield received the 1984 Nobel Prize in Chemistry for it.

The basic sequence of operations

SPPS builds the chain from the C-terminus toward the N-terminus, one residue at a time. Each addition repeats the same steps:

1. Deprotection. Remove the temporary protecting group from the α-amino group of the last residue on the resin, exposing a free amine.
2. Wash. Flush away the deprotection reagent and by-products.
3. Activation and coupling. Add the next amino acid, with its own α-amine protected and its carboxyl group activated, so that it forms an amide bond with the free amine on the resin.
4. Wash. Remove excess amino acid and coupling reagents.
5. Optional capping. Acetylate any amines that failed to couple so they cannot grow further.

After the final residue, the completed peptide is cleaved from the resin and its side-chain protecting groups are removed in one step. The crude product is then purified, usually by preparative reverse-phase HPLC, and lyophilised.

Protecting-group strategies: Boc/Bzl and Fmoc/tBu

Amino acids are reactive in more than one place. Lysine has a second amine, aspartic acid a second carboxyl, cysteine a thiol. To control which groups react, chemists use two classes of protecting group:

• A temporary group on the α-amine, removed at every addition.
• Permanent groups on reactive side chains, removed only at the end.

The two groups must be removable under different conditions, a property called orthogonality.

Feature	Boc/Bzl (Merrifield's original)	Fmoc/tBu (current standard)
α-Amine protection	tert-Butyloxycarbonyl (Boc)	9-Fluorenylmethoxycarbonyl (Fmoc)
Removed by	Trifluoroacetic acid (TFA)	Piperidine (a mild base)
Side-chain protection	Benzyl-based groups	tert-Butyl and trityl groups
Final cleavage	Anhydrous hydrogen fluoride (HF)	TFA with scavengers
Main advantage	Good for difficult, aggregation-prone sequences	Milder; no HF; easy to automate and monitor

The Fmoc group, introduced by Carpino and Han in 1972, is now used for the large majority of research and commercial peptides. Its removal releases a dibenzofulvene adduct that absorbs UV light, so synthesisers can monitor each deprotection in real time.

Coupling chemistry

A carboxylic acid does not react with an amine at room temperature on its own. Coupling reagents convert the carboxyl group into an activated ester or similar intermediate that reacts quickly and cleanly. Common families include:

• Carbodiimides such as DIC, usually combined with an additive such as Oxyma Pure or HOBt
• Aminium/uronium salts such as HBTU, HATU and HCTU
• Phosphonium salts such as PyBOP

Each coupling is typically run with a several-fold excess of activated amino acid to push the reaction close to completion. Completion can be checked with colourimetric tests such as the Kaiser (ninhydrin) test, which turns blue in the presence of free primary amines. If a coupling is incomplete it is repeated ("double coupling") before moving on.

Why near-complete coupling matters

Small inefficiencies compound. If each step proceeds at 99%, a 30-residue peptide built in 30 couplings would have only about 0.99³⁰ ≈ 74% of chains with the full, correct sequence before purification. At 99.5% per step, that rises to about 86%. This arithmetic is why long sequences need careful optimisation and why purification is essential.

Cleavage and global deprotection

In Fmoc/tBu chemistry the finished peptide-resin is treated with concentrated TFA. This cleaves the anchor bond to the resin and removes the acid-labile side-chain groups at the same time. The process releases highly reactive carbocations (for example tert-butyl cations), which can re-attach to electron-rich side chains such as tryptophan, methionine, tyrosine and cysteine. To prevent this, the cleavage cocktail contains scavengers: water, triisopropylsilane and, for some sequences, thiols such as ethanedithiol.

After cleavage, the peptide is precipitated in cold ether, collected and dried. This crude material is the starting point for purification. Because TFA is present throughout, the purified peptide is usually isolated as a TFA salt unless it is later exchanged to acetate or hydrochloride.

Where impurities come from

Every chromatogram of a synthetic peptide shows the main product plus minor related species. Most of them trace directly back to steps in the synthesis.

Impurity	Origin	How it appears
Deletion sequence	A coupling fails, but the free amine couples in a later step; one residue is missing	Mass lower by one residue; often elutes close to the main peak
Truncated sequence	Chain growth stops (for example after capping)	Shorter peptide, frequently N-acetylated
Incomplete deprotection	A side-chain group survives cleavage	Mass higher by the protecting group (e.g. +56 Da for tBu)
Racemisation	An L-amino acid converts partly to its D-form during activation	Same mass; diastereomer separated only by careful HPLC
Aspartimide formation	Asp side chain cyclises under basic Fmoc removal	Mass −18 Da; can reopen to β-aspartyl peptides
Oxidation	Methionine or tryptophan oxidised during handling	Mass +16 Da
Insertion	Double addition of one residue	Mass higher by one residue

Racemisation deserves particular mention because mass spectrometry cannot detect it: a peptide with one D-residue has exactly the same mass as the intended sequence. Cysteine and histidine are especially prone. Modern coupling additives and careful choice of base keep it low, and a well-developed HPLC method can usually resolve the diastereomers.

From synthesis to specification

Purification removes most of these species, but structurally similar ones, especially single deletions and diastereomers, can co-elute with the main peak. That is why a credible certificate of analysis shows the chromatogram rather than a bare number, and pairs HPLC purity with mass spectrometry for identity. For a walk through those documents, see how to read a certificate of analysis.

Modified peptides add steps. Semaglutide, for example, carries a fatty-acid side chain attached through a linker to a lysine, which is introduced using an orthogonally protected lysine building block. Short sequences such as ipamorelin or BPC-157 are comparatively straightforward to make at high purity, while longer or more hydrophobic sequences may need special resins, pseudoproline building blocks or elevated temperature to suppress aggregation on the resin.

References

• Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. J Am Chem Soc. 1963;85(14):2149–2154.
• Carpino LA, Han GY. The 9-fluorenylmethoxycarbonyl amino-protecting group. J Org Chem. 1972;37(22):3404–3409.
• Kaiser E, Colescott RL, Bossinger CD, Cook PI. Color test for detection of free terminal amino groups in the solid-phase synthesis of peptides. Anal Biochem. 1970;34(2):595–598.
• Chan WC, White PD, eds. Fmoc Solid Phase Peptide Synthesis: A Practical Approach. Oxford University Press; 2000.
• Isidro-Llobet A, Álvarez M, Albericio F. Amino acid-protecting groups. Chem Rev. 2009;109(6):2455–2504.
• El-Faham A, Albericio F. Peptide coupling reagents, more than a letter soup. Chem Rev. 2011;111(11):6557–6602.