Peptide Modification & Labeling

Peptide modification is how a linear sequence becomes an assay reagent: a biotinylated bait, a fluorescent probe, a PEGylated analogue or a cyclic binder. ChinaPeptides treats modification as part of custom peptide synthesis, not as a sticker applied after the fact.

The request should name the chemistry, the residue or terminus, any spacer, and the detection or conjugation method. Those four items determine whether the label survives cleavage and whether the peptide remains soluble enough to purify.

N-terminal and C-terminal modification

Acetylation is the most common N-terminal cap. Fatty-acid and other acyl groups are used when membrane association is part of the study. C-terminal amidation is requested when the native peptide is an amide or when a free carboxylate would change charge. Esters and functionalized C-termini are evaluated case by case.

Acetylation and amidation look like small checkboxes on a form and they are not. An acetylated N-terminus removes a charge that a protease, an HPLC method and a mass calculation all see. An amide at the C-terminus changes the molecular weight by −1 Da relative to the free acid and is the native form of many endocrine and neuropeptide sequences. If you copy a literature sequence, copy the termini as well. Custom peptide modification starts with that pair of decisions, then adds labels.

Biotinylation, PEGylation and phosphorylation

Biotin (with or without an Ahx or PEG spacer) supports streptavidin capture. PEGylation is used to change hydrodynamic size or reduce non-specific binding in vitro. Phosphopeptides on Ser, Thr or Tyr are prepared for kinase and phosphatase work; we confirm that the phospho group is compatible with the purification pH.

Design notes for biotin and PEG are expanded in biotinylated peptides and peptide PEGylation.

Fluorescent labeling and FRET

FITC, 5-FAM, TAMRA, Cy3 and Cy5 are the dyes we discuss most often. Placement is usually the N-terminus or a dedicated lysine, with a spacer when the dye would otherwise quench or interfere with binding. FRET peptides need a matched donor–acceptor pair and a cleavage or conformational sequence specified by the user. See fluorescent peptides and FITC vs TAMRA vs Cy5.

Lipidation, cyclization and isotope labels

Palmitoyl and myristoyl groups are typical lipidations. Cyclization may be a disulfide, a lactam or a head-to-tail amide; methods are compared in disulfide vs head-to-tail cyclization and on the cyclic peptides page. Stable-isotope residues are introduced for mass-spectrometry quantitation when the labeled amino acids are available for the requested positions.

Linkers, protein conjugation and click chemistry

Ahx, short PEG and cleavable linkers are used to distance a label from the epitope. Protein conjugation (KLH, BSA, OVA) is aligned with antigen design and antibody programs. Azide/alkyne click handles support later conjugation in the customer laboratory.

Click chemistry is useful when the peptide must stay unmodified through purification and the cargo is attached later. It is the wrong tool when you already know the final conjugate and can make it in one project. Name the handle (azide, alkyne, cysteine, maleimide) and the cargo so we do not install two competing reactive groups. Peptide labeling and peptide conjugation are related but not identical: a dye on lysine is a labeling project; KLH on a C-terminal cysteine is a conjugation project. Both belong on this page because the synthesis route has to know about them before the first residue is loaded.

Phosphorylation, lipidation and staples each add a purification constraint. A phosphopeptide can lose phosphate in harsh cleavage; a lipopeptide can stick to plastic and to reversed-phase columns; a staple changes both mass and HPLC retention. Those are reasons to read the comparison table and the supporting articles rather than stacking every modification on one short peptide “because the catalog of options exists.”

Modification comparison

Quality documentation for labeled lots follows the same HPLC and LC-MS path described under peptide quality control. Dye and lipid lots can show additional peaks; we discuss those before you lock a purity number.