Custom peptide synthesis: MOQ, lead time, purity grade and price logic

Two quotes for the same ten-residue peptide can differ by a factor of five and both be defensible. This guide explains what the price is actually paying for, why lead time is measured in weeks, and how to write an RFQ that produces comparable quotes instead of noise.

1. What “custom synthesis” actually covers

Catalog peptides are sequences someone already made and keeps in stock. Custom synthesis means the sequence is built for your order: synthesised, cleaved, purified, characterised, and lyophilised to your specification. That changes three things — there is a setup cost, a fixed schedule, and a quality specification you define rather than inherit.

In practice, buyers arrive with four kinds of request:

• A known sequence at a different scale — same peptide, more material, same or better documentation. The most predictable case.
• A modified version of a catalog item — added acetylation, amidation, a fluorescent tag, a biotin, a non-natural residue. Price usually moves more than expected.
• A novel sequence — designed in-house, never synthesised. Requires a feasibility check before quoting.
• A panel or library — dozens of short sequences, often overlapping. Pricing logic is completely different from single-peptide work.

2. Three scales, three cost structures

Scale changes what dominates the cost. This is the single most useful thing to understand when comparing quotes, because a supplier quoting a gram price and a supplier quoting a milligram price are describing different businesses.

The consequence: per-milligram price falls steeply with scale, but never to zero. A sequence quoted at roughly $100–150 per milligram at 1 mg might come to $1,500–4,000 for a gram — a 30–100× drop in unit price, not a proportional one. Buyers who expect linear pricing are usually the ones who conclude the small-scale quote is a rip-off.

3. Purity grades: where the price steps come from

Purity is the second price lever after scale, and it is non-linear. Each step up removes progressively smaller amounts of impurity at disproportionately higher cost, because purification yield falls as the target purity rises.

Two practical notes. First, purity is defined by the analytical method: a 98% figure from a fast 10-minute gradient and a 98% from a 40-minute gradient are not the same material. Ask for the method conditions. Second, above about 98%, the number becomes less meaningful than the impurity profile — a 99.2% product with one 0.5% deletion sequence may be worse for your assay than a 98.5% product with a spread of small impurities. This is covered in detail in how to read a peptide COA.

4. Why lead time is three weeks, not three days

Buyers new to this category often read a three-week lead time as inefficiency. It is not. A short custom peptide passes through a fixed sequence of steps, several of which cannot be compressed without risking the batch:

1. Feasibility and sequence review (1–2 days) — checking for aggregation-prone stretches, difficult couplings, and incompatible modifications.
2. Resin loading and chain assembly (3–7 days) — one coupling per residue, each with coupling and deprotection steps; longer sequences scale almost linearly, and difficult positions need double coupling.
3. Cleavage and side-chain deprotection (1 day) — with a cocktail chosen for the specific residues present.
4. Purification (2–5 days) — preparative HPLC, often with a re-run for high-purity grades, plus fraction analysis.
5. Analytical QC (2–3 days) — HPLC purity, LC-MS identity, water content, net peptide content, and the certificate itself.
6. Lyophilisation and packing (1–2 days) — plus documented storage conditions.

Add shipping and customs, and three weeks is a realistic floor for a typical 10–20 residue peptide at ≥98%. A supplier promising five days for a novel sequence at 99% is either quoting a catalog item, or planning to skip the QC steps you are paying for. Ask which steps are included and what happens if the first purification run misses the purity specification — a real answer involves a re-purification, at the supplier's cost, and a revised date.

5. What actually drives the price of a sequence

Beyond scale and purity, cost is driven by the physical difficulty of the synthesis. The variables that matter, roughly in order of impact:

• Length. Cost rises faster than linearly past ~20 residues, because each coupling has a yield and the cumulative yield multiplies.
• Hydrophobicity and aggregation. Sequences that aggregate on resin couple poorly and purify badly. A hydrophobic 15-mer can cost more than a hydrophilic 30-mer.
• Cysteine content and disulfide bonds. Multiple cysteines require selective protecting groups; forming a defined disulfide bridge adds an oxidation and verification step, and the wrong isomer is a real failure mode.
• Oxidation-sensitive residues. Methionine and tryptophan need handling that avoids oxidation throughout — a quality cost, not just a price cost.
• Modifications. Acetylation and amidation are routine. Biotin, fluorescent labels (FITC, FAM), phosphorylation, PEGylation, and non-natural amino acids add synthesis steps and often a second purification.
• Counter-ion / salt form. Trifluoroacetate is default; acetate exchange is an extra step and matters for cell-culture work where TFA is undesirable.

6. Sequences that are difficult — or unrealistic

A credible supplier will tell you when a request is a bad idea before taking the order. Common cases:

• Beyond ~50–70 residues by solid-phase synthesis. Yield and purity collapse; the realistic routes are native chemical ligation or recombinant expression, both different products commercially.
• Highly repetitive or hydrophobic stretches. Coupling efficiency drops and the peptide becomes hard to dissolve for purification — sometimes the limit is solubility, not chemistry.
• Sequences requiring a specific disulfide pattern with four or more cysteines. Possible, but expect a substantial price premium and a longer schedule.
• Very short peptides (2–4 residues). Cheap to make, surprisingly annoying to purify and to analyse; documentation quality varies more than price.

If a supplier accepts every request without comment, that is information about the supplier, not about the chemistry.

7. MOQ logic: why 1 mg costs more per milligram than 1 g

A minimum order quantity in this category is not a sales tactic; it reflects a fixed cost floor. A batch has to be scheduled, a resin prepared, a purification run set up, and a full analytical package produced regardless of whether the yield is 2 mg or 2 g. Below a certain quantity, you are buying the batch record, not the powder.

This produces a pattern worth recognising: quotes cluster around a floor price for small quantities because the underlying work is identical. A supplier offering 1 mg at a third of the market floor is usually planning to skip purification or QC — which is precisely what you cannot verify until the material arrives.

8. How to write an RFQ that gets usable quotes

Most quote discrepancies come from vague requests. Include all eight items and the quotes become comparable:

1. 1
   Sequence — one-letter code, plus any non-standard residues spelled out.
2. 2
   Modifications — N-terminal acetylation, C-terminal amidation, labels, disulfide pattern, phosphorylation sites.
3. 3
   Quantity — and whether it is the delivered amount or the synthesised amount.
4. 4
   Purity grade — with the analytical method you expect (HPLC-UV at a stated wavelength).
5. 5
   Salt form — TFA default, or acetate if your assay requires it.
6. 6
   Documentation — COA with chromatogram, MS, water content, net peptide content; SDS and TDS if required by your EHS team.
7. 7
   Destination and Incoterm — country, DAP or DDP, and whether you need the shipment to arrive with batch labels intact.
8. 8
   Timeline — your real deadline, and whether a partial delivery would help.

Send that and you will get answers you can compare line by line — including the useful ones that say “this sequence will not reach 99% reliably, here is what we recommend instead”.

9. Documentation that should arrive with the batch

A custom batch without a matching document set is not usable in a regulated environment, whatever the purity. Every delivery should carry:

• Batch-specific COA with HPLC trace and method conditions, LC-MS identity against theoretical mass, water content and net peptide content
• SDS in GHS format for the actual product form
• TDS with solubility, recommended handling and storage conditions
• Packing list with batch labels that match the COA numbers
• For contract work: the batch record summary or a statement of the process route, under NDA if required

If you are qualifying a supplier for the first time, the document checklist in how to verify a peptide supplier covers the commercial side of the same conversation.