Custom Peptide Synthesis: How It Works, Pricing, and Choosing a CRO

A peptide sequence on paper is free. Turning it into a vial costs anywhere from $80 to tens of thousands.

The process that bridges that gap, Solid Phase Peptide Synthesis, is a 60-year-old technology that still drives the entire custom peptide market. Here is how it actually works, what determines the price, and who you should buy from.

How SPPS Works

Imagine building a chain one link at a time, but you can only add links at one end.

That is SPPS. The peptide C-terminus is anchored to an insoluble polystyrene bead. Each amino acid is added one at a time in a repeating cycle: deprotect, wash, couple, wash, cap. A 30-amino-acid peptide goes through roughly 30 complete cycles.

The steps:
Resin. Beaded polystyrene crosslinked with 1% divinylbenzene.
Deprotect. Remove the temporary blocking group from the growing chain.
Couple. Activate and attach the next amino acid.
Cap. Acetic anhydride blocks any unreacted sites to prevent wrong sequences.
Cleave. Detach the completed peptide from the resin and strip all protecting groups.
Recover. Precipitate and freeze-dry the crude peptide.

Imperfections at any step, like incomplete coupling or side reactions, produce truncated sequences that must be removed during purification. This is where most of the cost comes from.

Fmoc or Boc?

Two chemistries dominate. The choice determines the entire chemical environment.

Fmoc is the commercial standard. Uses mild base (20% piperidine) for deprotection, trifluoroacetic acid for final cleavage. Milder conditions produce higher quality and yield. Resins: Wang, Rink amide, 2-chlorotrityl. Use this for 95% of research peptides.

Boc is older, harsher, and more specialized. Uses acidic deprotection and corrosive hydrogen fluoride for cleavage. Reserved for complex or base-sensitive sequences that degrade under Fmoc conditions. Resins: Merrifield, PAM, MBHA.

For most projects, choose Fmoc. If your sequence contains base-labile modifications, you may need Boc.

Purification and QC

Crude peptide contains truncation sequences, deletion peptides, and isomers. RP-HPLC removes them using a C18 column and a water/acetonitrile gradient. Polar contaminants elute first; the target peptide elutes as the organic phase increases. Detection at 210-220 nm.

For hydrophobic peptides that bind irreversibly to C18, temporary hydrophilic tags can be added. MCSGP (multicolumn countercurrent purification) reduces solvent use by 30% and increases yield by 10%.

QC is two methods: ESI mass spectrometry confirms molecular weight (identity). Analytical HPLC determines purity percentage. A reputable CRO defines purity as target sequence relative to all impurities at 220 nm, explicitly excluding moisture and counter-ions.

Purity Grades

Crude (>70%): Desalted only. Screening use.
Standard (>80%): ELISA, antibody production.
>95%: Standard research grade. Most common.
>98%: Structural studies, in vivo work.
>99%: Clinical reference standards.

Each step up costs significantly more because yield drops with each HPLC pass.

What Drives the Price

Length. A 36-mer costs 10-20x more than a 5-mer at the same purity. Over 150 residues, chemical synthesis becomes prohibitively expensive. Switch to recombinant expression.

Sequence difficulty. Hydrophobic or aggregation-prone sequences cost more. Cysteine, methionine, and tryptophan are side-reaction-prone. Charged, hydrophilic sequences are cheapest.

Modifications. N-terminal acetylation or C-terminal amidation: small fee. Isotopic labeling (13C, 15N): major price jump. Phosphorylation, cyclization, fluorescent tags: substantial cost. Counter-ion exchange (TFA to HCl or acetate): adds to the bill.

Scale. Total cost goes up with quantity but per-mg price goes down.

Purity. More HPLC runs = more cost. 99% costs far more than 95% because you lose most of your material during extended purification.

Base pricing: Biomatik from $3/amino acid. GenScript from $3.20 with volume discounts. Bachem charges more but handles PEGylation, counter-ion exchange, and de novo protocols that commodity CROs cannot.

Lead Times

Standard: 2-3 weeks for peptides under 30 amino acids. Rush: 5-day guarantee at a premium.

Longer sequences, difficult synthesis, multiple modifications, high purity (>98%), or gram-scale quantities all extend lead times.

Which CRO to Pick

Commodity CROs (GenScript, Biomatik, Thermo Fisher): Straightforward sequences under 30 AA, research scale, competitive pricing, standardized QC. Right choice for most academic labs.

Specialized manufacturers (Bachem): Complex sequences, GMP production, modifications requiring de novo protocol development. Serves pharma customers.

Clinical CDMOs (CordenPharma, Lonza): Process development, scale-up, regulatory filing under GMP. For peptides entering clinical trials.

Ask before ordering: What purity definition do they use? Do they include MS and HPLC with every peptide? What is their success rate for your sequence properties? What is their resynthesis policy for failed sequences?