Length is only the first difficulty in a long peptide project. A 55-residue soluble sequence may be routine, while a 35-residue hydrophobic or beta-sheet-forming sequence can stall on resin, precipitate after cleavage and resist preparative purification. When selecting a custom long peptide synthesis service, the useful question is not whether the supplier accepts a residue count. Ask how the proposed route addresses the actual sequence and how success will be measured.
Request a sequence-level feasibility assessment
Provide the exact sequence, termini, disulfides, non-natural residues, labels and desired salt form. The chemist should flag long hydrophobic runs, repeated beta-branched residues, difficult Asp-Gly or Asn motifs, oxidation-prone residues and potential aspartimide formation. Net charge and predicted solubility matter after cleavage; resin-bound aggregation matters during assembly.
A quotation that promises 98% purity before discussing these features is commercial, not technical. A sound proposal distinguishes likely synthesis yield, purification recovery and final isolated quantity. These are different numbers.
Control aggregation during solid-phase synthesis
Long chains can develop secondary structure on resin, restricting reagent diffusion and producing deletion sequences. Options include lower-loading or more solvating resin, stronger solvent systems, double coupling, altered temperature, backbone-protecting groups and pseudoproline or Dmb building blocks. No single tactic is universal. Each adds cost and may introduce its own cleavage or purification burden.
Ask the supplier to retain a crude analytical sample. Comparing crude HPLC and LC-MS with purified material reveals whether the challenge arose during chain assembly or downstream isolation. A clean final chromatogram alone conceals how fragile the process may be at scale.
Know when stepwise SPPS is no longer the best route
For very long or exceptionally difficult targets, fragment condensation, native chemical ligation or another convergent strategy may be more defensible. The junction must be selected around sequence chemistry, ligation handles and purification. Desulfurization or auxiliary removal adds steps that require identity and impurity control.
A route change is not automatically a problem. Silent route changes are. Require approval before the manufacturer substitutes a resin, protected building block, fragment boundary or ligation chemistry that can alter the impurity profile.
Purification recovery can determine commercial feasibility
Closely related deletion sequences often track the parent on reversed-phase HPLC. A hydrophobic peptide may precipitate during dilution, concentration or buffer exchange even when the analytical injection appears clear. Preparative loading, stationary phase, ion-pair reagent and collection window therefore affect both purity and recovery.
Specify whether the quoted quantity is purified peptide material, net peptide content or gross lyophilized weight. Counterion, water and residual solvent can create a large difference. For scale-up, request a mass balance from crude load through pooled fractions and final dry material.
Identity needs more than an intact mass
Intact LC-MS can confirm that the dominant component has the expected molecular weight, but isobaric substitutions and some sequence errors may escape. For a long or high-value sequence, tandem MS, enzymatic mapping or amino-acid analysis can provide additional confidence. Modified or disulfide-rich peptides may require tailored fragmentation or mapping.
Review the full mass spectrum and deconvolution, not a typed molecular-weight result. Check charge states, adducts, truncations and unexplained secondary envelopes. HPLC purity and molecular identity must come from the same controlled lot.
Scale-up should be treated as a new technical stage
A milligram feasibility lot does not prove gram-scale recovery. Larger resin beds change mixing, heat transfer, washing and cleavage. Preparative column loading and lyophilization concentration also change. Agree on an engineering or pilot stage with defined acceptance criteria before committing the full quantity.
- Lock sequence, modifications, salt and content basis.
- Define minimum isolated quantity and purity method.
- Request crude and final HPLC/LC-MS data.
- Set rules for reprocessing and route changes.
- Confirm packaging, aliquots, storage and retest period.
How long can a synthetic peptide be? There is no useful universal cutoff. Feasibility depends on sequence, modifications, purity target, scale and acceptable route. Send those details with the intended analytical use. A qualified supplier should identify the difficult region, propose a staged plan and state what data will support release.