What actually happens to your order once a peptide factory accepts it?

For most buyers, the honest answer is: not much that they get to see. You send a sequence, you get a quote, and some weeks later a box arrives with a certificate of analysis taped to the lid. Everything in between is a black box with a tracking number on it.

That black box deserves more attention than it gets. The parts you can't see are exactly the parts that decide whether batch ten matches batch one. So here's the whole route, stop by stop, with one thing worth watching at each stage.

Stage 1: Sequence review, before anything gets weighed

The first useful thing a competent site does with your order is look for reasons to call the sequence difficult. Chain length. Hydrophobic stretches. Regions that aggregate on the resin. Disulfide bridges that have to be formed in the right order. Non-natural residues. A C-terminal amide instead of an acid. Each of those changes the chemistry, and each changes the plan.

Then comes the plan itself: resin choice, since a low-loading resin can take pressure off aggregation; coupling reagents; whether pseudoprolines are needed to break up beta-sheet formation; whether to run couplings under microwave. And what crude purity the site expects to end up with.

That last one is the question worth asking. Before anyone synthesises anything, ask what crude purity they expect from your sequence. A site that has run similar sequences will give you a figure without checking with anyone. A site that hasn't will answer in generalities.

Stage 2: Synthesis, or thirty chances to lose yield

Solid-phase peptide synthesis is a loop. Couple an amino acid, wash, strip the protecting group, wash, couple the next one. Thirty residues means thirty couplings, each with its own yield — and yields multiply rather than add. Couple thirty times at 99% efficiency and you're at 74% before purification has even started.

That multiplication is why crude purity falls off as chains get longer, and why a sequence with one awkward stretch tends to come apart at precisely that spot. A 10-mer might come off the resin at 85–90% crude. A 30-mer with a sticky middle might come off at 45%.

Two details separate careful sites from the rest. In-process checks: a Kaiser or chloranil test on the resin, or automated monitoring, catches a failed coupling before twenty more residues pile on top of it. And capping after each cycle, which acetylates unreacted sites so they can't grow truncated sequences that sit one residue away from your target and sail through purification with it.

Stage 3: Cleavage, where yield quietly disappears

To get the peptide off the resin, the resin goes into a cleavage cocktail, usually trifluoroacetic acid with scavengers and water. The same step strips the side-chain protecting groups. The peptide is then precipitated in cold ether, filtered, dried, and what you have is crude material.

Cleavage is less glamorous than synthesis and it loses more batches than people expect. Incomplete cleavage leaves peptide still stuck to resin. The wrong scavenger mix lets sensitive residues oxidise or alkylate. The precipitation conditions decide whether the crude comes down as a fine powder or as an oil you can't filter.

Ask for the recovery figure. How much crude did they actually get, against what theory? A site that knows its process tracks this number. A site that doesn't will hand you a purity percentage and change the subject.

Stage 4: Purification, and why "98%" needs a method beside it

Most crude peptide goes onto a preparative HPLC column, gets eluted with a gradient of acetonitrile in water, and is collected in fractions. Fractions get checked on an analytical HPLC, pooled, and re-analysed.

Now, the thing about purity: it isn't a property of your molecule. It's a property of the method used to measure it. The same material can read 97% on one method and 99% on another, depending on column, gradient, wavelength, and whether the integration counts the shoulders. Two certificates that both say "98%" are not the same document unless the methods behind them match.

One pass also doesn't always finish the job. A single preparative pass typically takes a normal sequence to 90–95%. Pushing past 98% on a difficult one often means a second pass, sometimes on different column chemistry. Every extra pass costs material, with recoveries of 50–70% per pass being normal — which is how two sites can quote for the same sequence and land a factor of three apart.

Stage 5: Lyophilisation, where shelf life gets decided

Freeze-drying pulls the water and organic solvent off the purified solution under vacuum and leaves a stable powder. Simple in outline, and where a surprising amount of shelf life is settled.

Water content is the number to care about here. Residual moisture drives hydrolysis and aggregation over time, and it makes the material harder to weigh accurately. Karl Fischer titration is the standard test, and it belongs on the certificate for every batch, not once for the product in general.

Two other things happen at this stage. The fill: if the application needs a precise mass per vial, fill accuracy and its validation matter as much as purity does. And the drying cycle, which has to suit the fill volume and the buffer. A cycle validated for 10 mg vials will not dry a 500 mg fill properly. Properly here means the middle of the cake, not the surface.

Stage 6: QC and release, where the signature matters

The test list is fairly standard: purity by HPLC with the method spelled out, identity by mass spectrometry against the theoretical mass of the sequence, water content, appearance, and depending on where the material is going, counter-ion content, residual solvents, and the additional microbiology work some applications require.

Then release. A named person signs the certificate, and a sample from that batch goes into the freezer as a retained sample. Both matter. The signature means somebody put their name on it. The retained sample means that if you re-test a vial two years later, there's a baseline to compare against. If a supplier can't tell you who signed the last certificate or where the retained sample is kept, they aren't releasing batches. They're printing certificates.

One question worth asking at each stage

• Sequence review:What crude purity do you expect from this sequence, and why?
• Synthesis:Do you cap after each coupling?
• Cleavage:What was the crude recovery, against theory?
• Purification:How many preparative passes, and on what method?
• Lyophilisation:What's the water content per batch, and what's the fill tolerance?
• QC and release:Who signs the release, and how long are retained samples kept?

Where the delays actually come from

Most of the waiting in a peptide order isn't synthesis. It's the queue for a reactor slot, which can run days for a short batch and weeks for a large one. It's a second purification pass after the first one missed its target. It's the column changeover between products. It's a protected amino acid sitting on a three-week lead time. The lyophilisation cycle itself runs for a day or more per batch and can't be hurried by paying more. And then there's QA review before release, which is real work and therefore real time.

So ask for the timeline broken into stages instead of one number. A site that quotes "4–6 weeks" with no breakdown either doesn't know its own process or doesn't want you to see it.

What this means for picking a peptide factory

Those six stages double as the fastest test you can run on a supplier.

Trading houses can't answer the Stage 1 or Stage 3 questions, because they don't run the reaction. They can forward a certificate, but they can't tell you what crude purity your sequence gave or why the second purification pass was needed.

So ask two or three of the questions above and listen carefully. The sites that manufacture answer from memory, because they were in the room when it happened. The ones that don't will offer to check with the factory and get back to you, and the reply will arrive suspiciously close to what you were hoping to hear.

Peptide factory FAQ