Short answer: Peptide synthesis is the process of linking amino acids together in a specific sequence [1]. Most peptides today are manufactured by solid-phase peptide synthesis (SPPS) [6]: the first amino acid is anchored to a solid polymer, the chain is built one residue at a time, and by-products and leftover starting materials are washed away after every step [2]. The finished chain is cut off the support only at the end [2].
This article is educational chemistry for laboratory readers. It is not guidance for use in people or animals.
The basic problem: making one bond at a time, in the right order
A peptide bond forms when the carboxyl group of one amino acid couples with the amino group of another, releasing water. This does not happen on its own, so chemists use coupling reagents to activate the amino acid [1].
If you simply mixed two unprotected amino acids with a coupling reagent, you would get a mixture of the four possible dipeptides, and tri- and longer peptides as well [1]. To make exactly one product, chemists protect the groups that should not react:
- a temporary protecting group on the α-amino group, removed after each coupling so the next residue can be added;
- a permanent protecting group on the C-terminus and on reactive side chains (for example, the side-chain amine of lysine), which must survive every cycle and come off only at the end [1].
Why “solid phase” changed everything
The classic step-by-step approach in solution is simple in theory but difficult in practice: after each step the product has to be separated from by-products and unreacted starting material, and some product is lost every time [2].
The Nobel Committee illustrated the cost with simple arithmetic. If each of 100 steps gives a 90% yield, the overall yield is about 0.003%. Raising each step to 99.5% or better — which solid-phase methods made achievable — brings the overall yield for the same 100 steps to about 61% [2].
R. Bruce Merrifield’s solution, published in 1963 as “Solid Phase Peptide Synthesis. I. The Synthesis of a Tetrapeptide” [3], was to attach the first amino acid through its carboxyl group to a solid polymer. Between steps, by-products and leftover reagents are removed simply by filtering and washing the polymer. Only after the whole chain is assembled is the peptide released [2]. The method was also well suited to automation, and automated peptide synthesizers followed [2]. Merrifield received the 1984 Nobel Prize in Chemistry “for his development of methodology for chemical synthesis on a solid matrix” [4].
The SPPS cycle, step by step
Each added amino acid goes through the same repeating loop [1][2]:
1. Deprotect the temporary group on the resin-bound chain’s N-terminus.
2. Wash the resin.
3. Couple the next activated, protected amino acid.
4. Wash again to remove excess reagent and by-products.
After the last residue, a final cleavage step releases the peptide from the resin and removes the permanent side-chain protecting groups. The crude peptide is then purified and checked analytically; synthetic peptides purified by HPLC are usually obtained as TFA salts [7] (see HPLC peptide testing and mass spectrometry testing).
Bachem notes that there is a standard SPPS protocol that works for most peptides — although the quality of the raw peptide can be poor — whereas solution-phase synthesis requires careful case-by-case planning [1].
Fmoc vs Boc: the two main SPPS chemistries
The temporary α-amino protecting group defines the overall strategy [1]:
Fmoc (9-fluorenylmethoxycarbonyl)
- Removed by: base (piperidine) [1]
- Final cleavage: acid (standard global TFA cleavage) [5]
- Key point: orthogonal; temporary and permanent groups come off under different conditions [5]
- Removed by: acid (trifluoroacetic acid, TFA) [1]
- Final cleavage: anhydrous hydrogen fluoride (HF) [5]
- Key point: repeated acid steps can partially remove side-chain protecting groups each cycle and cause progressive loss of peptide from the support [5]
- it avoids handling anhydrous HF;
- it is easy to automate, because no corrosive TFA is needed in the synthesis cycles and Fmoc removal releases a fluorene group with strong UV absorbance that gives a useful indicator of synthesis progress;
- its milder chemistry tolerates modifications such as phosphorylation and glycosylation, which are not stable to HF cleavage;
- high-quality Fmoc building blocks are available at low cost because of large-scale production [5].
Solid-phase vs solution-phase synthesis
Solution-phase (liquid-phase) synthesis has not disappeared. Bachem, which says most of the peptides it sells or makes under contract are produced by SPPS, still uses solution synthesis for some generic peptides, very short peptides such as dipeptides, and peptides modified at the C-terminus [1]. Bachem names rapidity and ease of automation as the essential advantages of SPPS [1].
Bachem also describes chemical synthesis as simpler than biological (recombinant) production, especially for short peptides of up to about 60 amino acids, and more flexible in scale, sequence, and modifications [1].
How long can a synthetic peptide be?
There is no clean cutoff. The Journal of Peptide Science review notes that “around 50 amino acids” is often quoted as the length that can be routinely synthesized, but calls that figure meaningless in practice, because many much shorter sequences are extremely difficult and success is not guaranteed [5]. Tools such as pseudoprolines and backbone protection have extended the reach of SPPS to longer, aggregation-prone sequences, and larger proteins are often assembled from synthetic fragments by native chemical ligation [5].
Why synthesis chemistry shows up on a COA
The way a peptide is made explains most of what an analytical certificate is looking for:
- Related impurities. SPPS can generate deletion and insertion sequences, diastereomers from racemization, incompletely deprotected side-chain adducts, oxidation products, and dimers or oligomers [6]. HPLC is used to separate and quantify these against the main peak.
- Sequence-specific side reactions. The review identifies aspartimide formation — triggered when aspartic-acid-containing sequences are exposed to strong base — as the most serious side reaction in Fmoc chemistry, producing by-products some of which co-elute with the target peptide [5]. This is one reason identity confirmation by mass spectrometry matters alongside HPLC.
- Counter-ions and net peptide content. Peptides purified with TFA-containing mobile phases typically carry trifluoroacetate counter-ions, which add mass without being peptide. That is why net peptide content is a separate number from HPLC purity [6][7]. See net peptide content vs. purity.
- Lyophilized form. Purified peptides are usually freeze-dried into a powder for shipping and storage; see lyophilized peptides explained.
FAQ
What is solid-phase peptide synthesis?
A method in which the growing peptide chain is anchored to an insoluble polymer so that reagents and by-products can be washed away after every step, with the finished peptide released at the end [2].
Who invented SPPS?
R. Bruce Merrifield, who published the method in 1963 [3] and received the 1984 Nobel Prize in Chemistry for it [4].
Is Fmoc or Boc SPPS more common?
Fmoc. A 2016 review calls Fmoc SPPS the method of choice today [5].
Which direction is a peptide synthesized?
From the C-terminus to the N-terminus, although sequences are written starting at the N-terminus [1].
Does synthesis method affect purity?
Yes. Each coupling and deprotection step can leave small amounts of related impurities, which is why synthetic peptides are purified and then checked by HPLC and mass spectrometry [2][5][6].
Bottom line for laboratory buyers
Most synthetic peptides start life on a solid support, and Fmoc SPPS is today's method of choice [5][6]. Knowing how that cycle works makes the numbers on a certificate easier to read: HPLC purity tracks how well related impurities were removed, mass spectrometry confirms the right sequence was built, and net peptide content accounts for counter-ions and water left over from purification. Dynamite Research Peptides publishes batch documentation for each of those questions — see our quality page and the research peptide glossary.
All products are for research use only — not for human or animal consumption.