A peptide library is a controlled set, not a loose collection of sequences. One missing member, shifted plate position or weakly filled well can distort an entire screen. When qualifying a custom peptide library synthesis service, define design logic, member-level quality control and physical layout before asking for the lowest price per peptide.
Freeze the design before synthesis
For overlapping libraries, specify source sequence, peptide length, offset, terminal treatment and whether the final window is shortened or omitted. Alanine scans need a rule for native alanine positions. Truncation sets need a clear direction and minimum length. Include wild-type, scrambled and assay controls explicitly.
Assign a stable member ID independent of the plate well. Plate coordinates can change during replating; the member ID should connect sequence, synthesis record, QC, vial or well and any resupply.
Use purity tiers deliberately
Crude material may suit an early screen, but crude members do not have uniform impurity burdens. Difficult hydrophobic sequences and short highly charged sequences can behave very differently. A tiered plan often works better: economical synthesis for the first pass, then purified and fully characterized confirmation material for hits.
Do not compare a crude-library quote with a purified-library quote as if they are equivalent. State which members receive analytical HPLC, which receive MS identity only and what happens when a member fails.
Per-member QC must remain traceable
Pooled or representative QC saves cost but cannot prove every address. For critical libraries, request a member-specific mass result and a clear status flag. HPLC data should identify the corresponding member and method. A folder of spectra without IDs is not usable.
Define acceptance for low signal, multiple masses and incomplete synthesis. Repeated testing until a plausible peak appears should not replace an investigation.
Plate format creates practical risk
Specify 96- or 384-well layout, dry or solution format, amount basis, sealing system and orientation marks. Edge wells can dry differently, and low microgram fills are vulnerable to static loss. Ask how dispensing uniformity is checked and whether the quoted amount is gross material or peptide equivalent.
If the library is supplied in solution, define solvent, concentration, mixing, evaporation control and compatibility with the assay. DMSO can improve solubility but does not suit every workflow.
Pooling needs an auditable recipe
For screening pools, require the member list, contribution basis and pooling record. Equal gross weights do not produce equal molar concentrations when peptide content and molecular weight differ. A marker peptide may help confirm pool identity, but it does not prove every member is present at the target level.
Plan resupply at the start
Hit confirmation often fails because the original sequence table, salt form or terminal chemistry cannot be reconstructed. Lock the project file, naming convention and analytical methods. Agree whether resupply uses the original route and whether retained crude or purified reference material is available.
- Controlled sequence list and design algorithm.
- Member IDs linked to wells and QC files.
- Purity and identity requirements by tier.
- Fill basis, plate seal and storage conditions.
- Failure, replacement and resupply rules.
How much QC does a library need? The answer depends on the cost of a false hit or missed hit. Send the screening format, sequence set, purity tier and confirmation plan with the RFQ. A capable supplier should identify difficult members before production rather than hide them in a final pass-rate percentage.