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Short answer: Custom peptide synthesis is the chemical assembly of a peptide of a sequence you specify, built one amino acid at a time and delivered with quality-control data. The choices that most affect what you receive are the peptide’s length, the purity grade matched to your assay, any modifications, and the QC data supplied (HPLC and mass spectrometry).
This guide is for researchers and lab staff preparing a peptide order. It is general information, not a protocol, and it does not cover prices or rank suppliers.
Definition and scope
A custom peptide is a short chain of amino acids made to order rather than purified from a biological source. Typical research uses include antibody production, ELISA and array work, enzyme-substrate assays and receptor-ligand studies. Longer chains and full proteins are normally made by other routes; this page is about synthetic peptides. If you need to predict properties such as molecular weight or charge before ordering, see the ProtParam guide.
How synthesis works in practice
Thermo Fisher’s Pierce peptide design guide describes synthesis as a repeated cycle of deprotection and coupling, with the chain built from the C-terminus toward the N-terminus. Because each cycle is imperfect, truncated and deletion peptides accumulate, so the same guide states that the purity of crude synthesized peptide is limited by the length of the peptide. That is why purification and QC matter more as sequences get longer.
The same source gives practical length guidance: peptides of 10 to 20 residues suit antibody preparation, and while 75-mers are possible, yields are poor.
Purity grades: what the labels mean
Purity grades are defined by each supplier, not by a shared standard. As one example of how a supplier maps grades to use, GenScript’s published purity guide recommends the following:
| Grade (GenScript) | Purity | Uses listed by the supplier |
|---|---|---|
| Crude | No fixed minimum | Screening, sequence optimization |
| Immuno | >75% | ELISA, arrays, antigens |
| Biochemistry | >85% | In vitro bioassays, epitope mapping, semi-quantitative enzyme-substrate work |
| High purity | >95% | NMR, quantitative receptor-ligand studies, competitive inhibition |
| Industrial | >98% | Crystallography, GMP, cosmetic and clinical-trial use, SAR studies |
Treat these thresholds as one supplier’s convention. The same GenScript page gives more than 70% for antibody applications in its overview text and more than 75% in its table, which shows how grade boundaries can differ even within a single document. Compare the stated percentage and the method used to measure it, not the grade name.
Modifications you can specify
The Thermo Fisher guide lists modifications commonly requested for research peptides:
- Phosphorylation of Tyr, Ser or Thr.
- N-terminal acetylation and C-terminal amidation, which remove terminal charges.
- Methylation of Lys or Arg, and Acm-protected Cys.
- D-amino acids and 13C/15N isotope labeling.
- Cyclization by amide or disulfide bonds.
- Spacers (Ahx, PEG), biotin or lipid tags, and fluorescent dyes.
- Carrier conjugation to KLH, BSA or OVA, or multiple antigenic peptides (MAPs).
The guide recommends conjugating tags or dyes at the N-terminus so that only full-length peptides become labeled.
Designing for synthesizability and solubility
The Thermo Fisher guide flags sequences that are hard to make: multiple Ser or Pro residues, runs of hydrophobic residues that form beta-sheets, N-terminal Gln or Asn, and Asp-Gly, Asp-Pro or Asp-Ser motifs. For solubility it suggests that roughly one residue in five be charged. Peptides that are 50% or more hydrophobic may need an organic solvent such as DMSO, DMF or acetonitrile, and peptides with several Cys residues may need DTT. It also lists conservative substitutions such as Cys to Ser and Met to norleucine where the science allows.
A vendor-neutral specification checklist
Drawing on the two sources above, a complete order specification states:
- Sequence and length, with the termini defined (free, acetylated, amidated).
- Purity target tied to the assay, for example a higher target for quantitative binding work than for screening.
- Modifications, including where tags or dyes attach.
- Solubility plan, based on charge and hydrophobic content.
- QC deliverables: GenScript states that quality control is by HPLC and mass spectrometry, with the data delivered with the peptide. For background on those techniques see the HPLC guide and LC-MS explained.
Common misunderstandings
- “Higher purity is always better.” The supplier guidance above ties grades to applications; crude material is listed as suitable for screening.
- “A grade name is a standard.” Names and cutoffs are supplier-defined, as the 70% versus 75% wording shows.
- “Any length can be made equally well.” Crude purity is limited by length, and very long peptides give poor yield.
Related handling
For background on freeze-drying of dried materials see the lyophilization guide.
FAQ
What are the main steps of custom peptide synthesis?
Synthesis is a repeated cycle of deprotection and coupling that builds the chain from the C-terminus to the N-terminus (Thermo Fisher), followed by quality control by HPLC and mass spectrometry (GenScript).
What purity should I choose?
Match the grade to the assay. One supplier lists more than 75% for ELISA and antigens, more than 85% for bioassays, and more than 95% for NMR and quantitative receptor-ligand work. Confirm exact thresholds with your supplier.
How long can a synthetic peptide be?
Thermo Fisher notes that 10 to 20 residues suit antibody preparation and that 75-mers are possible but give poor yield.
Does this page cover cost or choosing a company?
No. It covers specification and quality questions only.
Sources
- Thermo Fisher Scientific, Pierce Protein Methods, “Peptide Design”: thermofisher.com (read 2026-11-07).
- GenScript, “Recommended peptide purity”: genscript.com (read 2026-11-07).
