A custom synthesis delivers what the specification asked for, which is frequently not what the requester had in mind. Nine fields decide the outcome, and the expensive surprises almost always come from one that was left blank rather than one that was filled in wrongly. Writing the specification carefully takes twenty minutes and is the cheapest part of the whole transaction.
The nine fields
| Field | If you leave it blank |
|---|---|
| Sequence, N to C | Nothing can proceed; ambiguity here is fatal |
| N-terminal modification | You get a free amine, which may not be what you wanted |
| C-terminal modification | You get a free acid; amidation must be requested |
| Purity specification | Supplier's default, often 95% |
| Salt form | Trifluoroacetate, by default of the purification |
| Quantity and basis | Ambiguous whether it means powder or peptide |
| Analytical package | Identity and purity only |
| Bridges and cyclisation | A linear peptide with free cysteines |
| Solubility or format requests | Lyophilised powder, no excipient, as-is |
Which omission causes the most trouble?
The quantity basis, because it is the one people do not realise is ambiguous. "5 mg" can mean 5 mg of powder in the vial or 5 mg of peptide after correcting for counter-ion and water, and those differ by 15–30%.
Suppliers differ on which they mean, and both readings are defensible. Net peptide content is the figure that resolves it, so a specification should say "5 mg net peptide" or "5 mg gross" explicitly. This single phrase prevents the most common disappointment with a custom order.
Why specify the termini when the sequence is given?
Because a sequence in letters describes the residues, not the ends. The same written sequence can be delivered with a free N-terminal amine or an acetyl cap, and with a free C-terminal acid or an amide, and those are four different molecules with four different masses.
Where the peptide is modelled on a natural fragment, the native form is often amidated and the default synthesis is not. Terminal capping also changes protease resistance, so this is not a cosmetic difference — it changes both identity and behaviour.
Where the cost sits, and where it does not
Some fields move the price substantially and others barely at all, which is worth knowing before negotiating.
Length moves it the most. Yield compounds across couplings, so each additional residue costs more than the last. The gap between a five-residue peptide and a forty-four residue one is not nine times; it is far more, and at eighty-three residues stepwise synthesis stops being the sensible route at all.
Non-natural residues move it. D-amino acids, Aib and the like cost many times their natural equivalents as raw materials.
Purity moves it steeply at the top. Going from 95% to 98% costs more than going from 85% to 95%, because the last few points mean cutting narrowly around the main peak and discarding the shoulders.
The analytical package moves it modestly. Each additional test is a real fee but a small one against the synthesis.
Salt exchange moves it a little. It is an extra processing step with its own yield loss.
What purity should I actually ask for?
The lowest that will not compromise the work, which is lower than most people specify. For a binding assay, 95% is usually ample. For anything where an impurity might be active at the same target, or where the material is a reference standard, the higher specification earns its cost.
Specifying 98% out of habit on an exploratory compound is a common way to pay for precision nobody will use. Purity is a proportion of peptide-related material, and a higher number does not improve anything the experiment is measuring unless the impurities would have interfered.
What analytical package is worth requesting?
Identity and purity come as standard. The additions worth considering are content, which is the figure that changes your arithmetic; endotoxin, if the material is going into cells; and the raw chromatogram rather than only a number.
Asking for the trace costs nothing and is the single most useful addition, because it shows what a figure hides — a shoulder on the main peak, an unresolved neighbour, a run too short to show late material.
Things worth saying that are not fields
A few sentences outside the specification table prevent most of the remaining surprises.
State what it is for, at least in general terms. A supplier who knows the material is going into cell culture will flag the endotoxin question themselves. One who knows it is a reference standard will treat the content figure differently.
Ask what they expect to be difficult. An experienced synthesis chemist can usually look at a sequence and name the aggregation-prone stretch. That is worth knowing before committing, because it predicts both the price and the realistic purity.
Agree what happens if the specification is not met. A synthesis that comes in at 93% against a 95% specification is a normal occurrence, and whether that is delivered at a discount, re-purified, or re-run should be settled in advance rather than discovered.
Should I specify the synthesis route?
Almost never. The route is the supplier's expertise and constraining it removes their ability to solve problems you cannot see. What is reasonable to specify is the outcome — sequence, purity, salt form, what must be absent — and to leave how they get there to them.
The exception is where a particular reagent or process must be avoided for a downstream reason, in which case say so explicitly and say why. A constraint with a reason attached gets engineered around; one without gets followed literally in whatever way is cheapest.
How should the specification travel with the order?
Attached to the purchase order and referenced on it, so the specification and the transaction are one document rather than two. A quotation that reflects a specification is also the thing to check on delivery, field by field, against the certificate that arrives.
That final comparison takes two minutes and is where any mismatch surfaces while it is still correctable. Discovering six months later that the material was delivered as the acid rather than the amide is a problem with no good remedy.
Reading the quotation that comes back
A quotation is the supplier's interpretation of the specification, and comparing the two is the last cheap opportunity to catch a misunderstanding.
Check that the sequence is reproduced exactly, including modifications and any bridges. Check the quantity basis has been echoed rather than silently reinterpreted. Check the purity specification and the salt form. Those four account for nearly every discrepancy that surfaces on delivery.
A quotation that restates the specification in the supplier's own words is a good sign, because it demonstrates the request was read rather than pasted. One that returns only a price and a lead time has not confirmed anything.
How long should a custom synthesis take?
Long enough that it is worth asking before committing, and the answer scales with the same variables as the price. A short unmodified peptide is a different proposition from a long one with non-natural residues and a bridge, and a supplier's estimate is also a signal of how routine they consider the job.
What is worth avoiding is treating a quoted timeline as a commitment that absorbs all risk. A difficult sequence can need a second attempt, and a specification that allows for that outcome is more useful than one that pretends it cannot happen.
Should a small trial quantity be ordered first?
For a novel or difficult sequence, often yes. A small-scale synthesis establishes whether the sequence behaves, what purity is realistically achievable, and whether the material does what the project needs — before committing to a quantity that assumes all three.
The cost is that per-milligram pricing is worse at small scale, since synthesis and testing are largely fixed costs for a batch. For a sequence with any uncertainty attached, that premium is usually cheaper than a large batch that turns out to be wrong.
What belongs in the record once it arrives?
The specification as sent, the quotation as accepted, and the certificate as delivered, filed together. Those three documents describe what was asked for, what was agreed and what came, and a discrepancy is only visible when all three sit side by side.
This matters more for custom material than for catalogue items, because there is no product page to refer back to. The specification is the only description of what the compound was supposed to be, and a novel sequence will not have a registry number to fall back on either.
Written carefully, a specification is also a reusable asset. The next order of the same compound, or a close analogue, starts from a document that already names every field and records what the supplier delivered against it — which turns a bespoke transaction into something closer to reordering a catalogue item.
All products referenced here are supplied for laboratory and research use only. They are not drugs, foods, supplements or cosmetics, and are not for human or veterinary use.
