[purplegamer]

Hello Everyone,

I am trying to couple (Fmoc)Arginine to an amine, using carbodiimide/NHS chemistry.
The coupling seems to work but when I separate the reaction products on RP-HPLC the coupled product seem to have different retention times when I use as carbodiimides EDC or DCC.

Does anybody know if the carbodiimides could modify arginine?

Thanks in advance, any input would be greatly appreciated. 

[Dea]

All I can think right now is that the stereochemistry of the product might have changed in comparison to the initial reagent, if you didn't use HOBt for example to conserve it. Are the differences of Rt big?

[purplegamer]

thanks  for the reply DEA


Our amine is soluble only in aqueous solutions therefore we cannot use HOBt but instead have to use NHS ester as activated intermediates.

On the RP column the unreacted amine elutes at 6ml, the EDC coupling product elutes at 7.8ml and the DCC coupling product elutes at 7.4ml.

I will estimate the concentration of MeCN at each of the retention times and post in a few hours.

[Dan]

when i saw the thread title I immediately thought of what DEA has said. If you don´t use an additive such as HOBt you risk scrambling the stereochemistry. That said, if this was the issue I would have thought you would see both diastereoisomers coming off. It seems odd that you get a different one in each case. Peptide synthesis isn´t very fresh in my mind though I must admit.

[agrobert]

I have isolated acids coupled to DCC that have not reacted with the amine.  Are you sure that these are the same products?  If they have the same MW then it probably is stereochemistry.

[kiwi]

First up, i'd take a drop of both products, and run them together in one run down the hplc, to be sure you're seeing two species.
Its possible you're seeing an N-acylurea of arginine (formed by a slow rearrangement of the active O-acylurea intermediate), can you be sure the product contains any of the original amine? And as pointed out by agrobert, you need MS data ideally.