How to Write a Custom Peptide Synthesis Brief

The order that starts an argument

Most disputes over a custom peptide are not scientific. They begin with an order that reads "synthesize this sequence, 100 mg, high purity" and stops there. The vendor makes a reasonable interpretation of every unstated detail — free acid at the C-terminus, TFA counter-ion, purity judged by their default gradient — and delivers exactly what they were told rather than what you assumed. The material may be usable; it may also be unusable in the assay you built around a different salt form. Nobody was careless. The brief simply left the decisions to whoever answered them first.

A synthesis brief is the document that closes those gaps before they cost you a delivery cycle. It is not paperwork for its own sake. Each field it forces you to fill is a decision that would otherwise be made silently, by someone who cannot see your downstream use. Writing the brief well is mostly a discipline of naming what you already implicitly need.

Sequence notation that cannot be misread

The sequence is where ambiguity is least forgivable and most common. Single-letter code is compact but fragile: it has no unambiguous way to write D-amino acids, non-natural residues, or modifications, and a transposed letter is invisible. Three-letter code is longer but self-checking, and it accommodates the notation vendors actually use for anything beyond the twenty proteinogenic residues. When a sequence contains non-standard building blocks, spell them out with the exact designation you expect on the certificate of analysis.

Chirality deserves explicit treatment. A lower-case letter or a D- prefix is easy to drop in transcription, and a peptide built from the wrong enantiomer at one position is a different molecule with different behavior. State the stereochemistry residue by residue where it is non-standard rather than trusting a convention to carry it. If you have a reference identifier for an unusual residue — a supplier catalog name or a structure — include it so the manufacturing chemist resolves your intent, not their best guess.

• Prefer three-letter code for anything with modified or D-residues; reserve single-letter code for simple all-L sequences.
• Mark every non-standard stereocenter explicitly rather than relying on case.
• Attach a structure or catalog reference for unusual building blocks.

Termini and the modifications that hang off them

The N- and C-termini are a frequent source of silent mismatch. A C-terminal amide and a C-terminal free acid differ by one charge and by their resistance to certain enzymes; either can be the intended product, so the brief has to say which. The same applies to the N-terminus, where acetylation is a common and deliberate choice. Leaving termini unstated invites the vendor's default, which is often the free acid and free amine — a sensible baseline that may be wrong for you.

Beyond termini, list every intended modification with its position: phosphorylation, a fluorophore or biotin tag, a spacer, a fatty-acid chain, or an internal cyclization. Each modification changes the synthetic route, the purification, and sometimes the achievable purity. Say where a label sits and how it attaches, because "add a fluorescent tag" can be realized several ways with different spectral and solubility consequences. If a tag must be at a specific residue for your assay geometry, that constraint belongs in the brief, not in a follow-up email.

Form: salt, counter-ion, and net peptide content

A peptide is delivered as a salt, and the salt is part of what you are buying. Sequences purified by reversed-phase methods using trifluoroacetic acid commonly arrive as the TFA salt, which is fine for many purposes and problematic for others — TFA can interfere with certain cell-based and spectroscopic work. If your application needs an acetate salt or a specified counter-ion, request a salt exchange explicitly and expect it to add a step. Silence here defaults to whatever the purification produced.

Related to salt form is the distinction between gross mass and net peptide content. A vial labeled 100 mg contains peptide plus counter-ions, bound water, and residual solvent; the mass of actual peptide is lower and is reported as peptide content or net peptide. If your experiments depend on an accurate amount of peptide — most quantitative work does — specify whether the quantity target refers to gross weight or net peptide, and ask for the peptide content determination on the certificate. Assuming gross weight is net is a common and expensive error.

The quality target and how it will be judged

"High purity" is not a specification. Purity is a number tied to a method, and the same peptide can read differently under different gradients, columns, and detection wavelengths. State the purity you need as a percentage, the technique used to measure it (typically analytical reversed-phase HPLC), and the detection conditions if they matter to you. A purity figure without its method is not comparable across vendors or even across two batches from the same vendor.

Decide the target against the use, not against a habit of ordering the highest grade available. Crude material may be adequate for a first solubility screen; a binding study or a structural experiment usually is not tolerant of co-eluting deletion sequences. Over-specifying purity raises cost and lead time and can occasionally be unachievable for a difficult sequence, so pair the target with a conversation about feasibility. General principles for setting specifications and choosing test procedures are laid out in ICH Q6A, and USP's chapters on synthetic peptide quality attributes describe the attributes worth naming.

• Give purity as a percentage plus the analytical method and detection conditions.
• Match the target to the experiment; do not default to maximum purity reflexively.
• Ask whether the target is realistic for this sequence before committing.

Analytics you should require on delivery

Identity and purity evidence should accompany the material, not follow on request. At minimum, ask for a mass spectrum confirming the expected molecular mass and an analytical HPLC trace supporting the purity figure. These two items answer the first questions any careful user asks — is this the right molecule, and is it as clean as claimed — and their absence should be a warning rather than a convenience to overlook.

Depending on the application, additional data earn their place: amino acid analysis or peptide content for accurate quantitation, a counter-ion or residual-solvent result where the salt form matters, and an endotoxin or water-content measurement for sensitive uses. Name the analytics you will actually use to release the material into your work, because tests you did not request will not be run and data you did not receive will delay your first experiment.

Packaging, aliquoting, and handling on arrival

How the peptide is divided and packaged affects how usable it is on the bench. A single vial of hygroscopic powder that must be weighed repeatedly invites moisture uptake and weighing error; pre-weighed aliquots avoid both at some cost. Lyophilized presentation, vial count, and fill quantity per vial are all specifiable, and the right choice depends on how you will consume the material over its life, not on the vendor's standard fill.

Storage and shipping conditions belong in the brief as well. State the temperature the material should arrive and be stored at, and note any light or moisture sensitivity that follows from the sequence or a label. A peptide that is stable dry at low temperature but fragile in solution should be documented as such so that the person who reconstitutes it does not learn the constraint by degrading a batch.

Research context, change control, and the manufacturability talk

A short statement of the intended research context helps the vendor flag mismatches before synthesis, not after. If the peptide will go into a cell assay, the TFA-salt problem surfaces early; if it is a reference standard, the emphasis shifts to characterization depth. You are not obligated to disclose proprietary aims, but a sentence about the class of use lets an experienced chemist catch a specification that will not serve it.

Two conversations turn a good brief into a repeatable supply. Change control fixes what "the same product" means: agree in writing that any change to the route, resin, counter-ion, or specification is communicated and, where it matters to you, requires your sign-off, so a quiet process change does not reach your bench as an unexplained shift in behavior. The manufacturability dialogue runs the other way — invite the vendor to tell you which parts of the brief are hard, where an aggregation-prone stretch may cap purity, and where a small change to a label position or salt form would improve yield without hurting your science. A brief that opens that exchange gets you a better peptide than one that only issues instructions. ICH Q7 frames the change-control expectations that underpin this discipline.