Peptide Modification
Modification turns a linear sequence into a molecule with the stability, binding behaviour or detection handle a project needs. This hub covers the chemistry we run most often, the constraints each modification places on synthesis, and how to specify a modified peptide so that the analytical release criteria still make sense.
Cyclisation: head-to-tail, disulfide and side-chain bridges
Head-to-tail cyclisation joins the N- and C-termini and is common for short sequences where conformational restraint improves selectivity or protease resistance. Disulfide cyclisation uses two cysteine residues and depends on selective oxidation, so the number and pairing of cysteines must be designed deliberately. Lactam and thioether bridges offer alternatives when a disulfide would be unstable in the assay buffer.
Stapled peptides and helix stabilisation
Stapling introduces a covalent cross-link between two non-natural residues, usually on the same face of a helix, to raise helicity and proteolytic stability. The staple positions, linker chemistry and stereochemistry all change the synthesis route and the analytical profile. Stapled products are released with the same HPLC and mass-spectrometry evidence as linear peptides, plus confirmation that the cross-link is present.
PEGylation and half-life extension
PEGylation attaches a polyethylene-glycol chain to improve solubility or reduce clearance. The PEG length and the attachment site matter: N-terminal, lysine side-chain and cysteine-directed chemistries give different product distributions. Branched PEGs increase the analytical challenge, so the release method is validated against the specific conjugate rather than a generic linear-peptide method.
Labelling: biotin, fluorophores, isotopes and click handles
Biotinylation supports pull-down and surface capture, usually through a lysine side chain or an N-terminal position. Fluorescent labels such as FITC, FAM, rhodamine and FRET pairs make the peptide detectable, but the dye can change solubility and should be specified with its linker. Isotope labelling with deuterium or carbon-13 supports quantitative work, and azide or alkyne handles enable later click chemistry without imposing a bulky group early in the synthesis.
Post-translational modifications
Phosphorylation, glycosylation and sulfation are frequently requested for signalling and epitope work. Phosphoserine, phosphothreonine and phosphotyrosine are supplied as pre-formed building blocks because post-assembly phosphorylation is not selective. Glycosylation is more demanding: the glycan, the attachment residue and the anomeric configuration must all be fixed in the specification before synthesis begins.
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Frequently asked questions
Which modification should I choose for a cell assay?
Start from the detection method. Biotin suits streptavidin-based capture, a fluorescent dye suits imaging or flow cytometry, and a stable isotope label suits mass-spectrometry quantification. For stability rather than detection, cyclisation or stapling is usually the better first choice.
Can a modified peptide still be purified by HPLC?
Yes, but the gradient and column chemistry may need to change. PEGylated, glycosylated and dye-labelled peptides often elute differently from the parent sequence, so the release method is adjusted to the conjugate rather than assumed from the unmodified peptide.
Does cyclisation always improve stability?
Not automatically. Cyclisation can improve protease resistance and binding affinity, but it can also reduce solubility or lock an unproductive conformation. The effect depends on the sequence and the ring size, so a small pilot batch is a sensible first step.
Can you add a fluorescent label at a specific lysine?
Site-specific labelling requires the reactive residue to be unique or selectively protected. If several lysines are present, either the sequence is redesigned with a single attachment site or an orthogonal chemistry such as cysteine maleimide labelling is used.
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