So i have a few peptides (synthesized by SPPS and not cleaved from resin) and I want to add a maleimide label. We have NHS-PEG-Maleimide and from literature it is evident that the free amine group (deprotected) at the N-terminus of a peptide/protein can react at the NHS end of the NHS-PEG-Mal molecule and form a Peptide-PEG-Maleimide conjugate via the formation of a peptide bond as the NHS end of the NHS-PEG-Mal molecule is an activated ester.
My question is, is it possible to carry out this conjugation to the peptide ON RESIN, using the same coupling protocols of standard Fmoc SPPS chemistry?
I understand this should be theoretically possible but I'm not able to find a reference with the reaction conditions for an ON RESIN conjugation.
Aminohexane-Sepharose 4B gel is commercially available via GE Healthcare which is used to couple ligands with either COOH groups (carbodiimide linkage) or -NH2 groups. I should think that the SPPS resin maybe considered to be similar to the AH-Sepharose 4B gel and you maybe able to use the same protocol. GE Healthcare has the protocol on their website.
I don't have any direct experience with this. But I've modified peptides on-resin with other things, and I've used NHS and Mal a lot.
You should be able to use SPPS conditions to run the reaction on-resin with reasonable efficiency. The PEG linker is going to collapse and create steric hindrance around the reactive site, so you may want to double couple, and I'm sure your reactant is expensive.
I suspect your biggest problem will be subsequent cleavage, where various nucleophiles are likely to react with the maleimide moiety and probably also PEG.
An alternate strategy is to use orthogonal side chain protectors (like 2-Cl-Z) in your peptide assembly, so after resin cleavage (and reaction workup) you can react the protected peptide N-ter with your handle molecule. And then cleave the side chains.
John Carter Thank you for that! As far as I read, just using a base like DIPEA or TEA with the ester in DMF solvent can do the job. Hoping for the best.
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