When using DCC/HOSu or EDC instead of DCC to couple am amine with an acid, usually good bonds are obtained.
But, in the case of histidine, there are two nitrogen atoms that can play the amine role. I was wondering if the secondary amine in the heterocycle wants to react with the diimide intermediate and in general, make a mess of things. Usually the carbodiimide is used in slight excess, but I can imagine this excess making it difficult in the case of histidine. Anyone have experience with this?