Need Glu/Asp-modification at pH7

[Marcus Macht]

Hello !

Does anyone know a method for modification of Asp/Glu-sidechains under neutral
conditions (pH 6-7.5) in salt buffers at 37 degree (reaction time doesn't 
really care) ? 
I want to perform a peptide-sidechain modification of a peptide bound to an 
monoclonal antibody while leaving the antigen-antibody bond intact. In our 
group there were tried modifications with carbodiimides, but they seemed to 
work specific only at pH around 4.5. The modification should deliver a 
recognizable mass shift (i think more then 20 amu will do it) for mass 
spectrometric identification of the modifcated residues.

If you know a suitable method for this reaction please mail it to me 
(macht at chclu.chemie.uni-konstanz.de) or post it. Yours sincerly

Marcus Macht