N-ter / Internal / C-ter modification
Peptides can be modified at various positions including the free N-terminal amine group, side chains of some normal or modified amino acids or C-terminal carboxylic group.
The N-terminal extremity allows many modifications quite easily. The C-terminal extremity is generally more challenging to modify than the N-terminal extremity. The easiest alternative is to add an extra Lysine in C-terminal position, that can be labeled in side chain. Internal modifications consist generally in adding the conjugate of interest on the side chain of lysine, asparagine, serine, threonine or modified amino acids.
In any cases, the modification of a peptide can modify its properties and may for example decrease its bioactivity. This is hardly predictable and has to be tested.
List of classical peptide modifications
| Modification | N-ter | C-ter | Internal |
| Acetylation | x | Lys side chain | |
| Amidation | x | ||
| Pyroglutamic acid | x | ||
| Flurorescent dye | x | Lys side chain | Lys side chain |
| Affinity tag | x | Lys side chain | Lys side chain |
| Fatty acid(1) | x | Lys side chain | Lys side chain |
| Phosphorylation | Thr, Ser, Tyr side chain | ||
| Spacer(2) | x | x | x |
| Glycation(3) | Ser, Thr, Asn side chain | ||
| Alcyne functionalization | x | Lys side chain, Pra side chain | |
| Azide functionalization | Lys side chain, Aha side chain | ||
| DOTA/NOTA labeling | x | x | Lys side chain |
| Cytotoxic payloads (peptide-drug conjugate) | x | x | x |
(1) Fatty acid
- Stearic acid (C18)
- Palmitic acid (C16). Improves the peptide penetration
- Myristic acid (C14)
- Capric acid (C10)
- …
(2) Spacer
- Ahx
- PEG
- O2Oc
(3) Sugar
- Glucopyranosyl
- Glucopyranoside
- Galactopyranosyl
- Mannopyranosyl
- Fucopyranosyl
- Xylopyranosyl