Amidination imidoester

The amidine bond formed is quite stable at acid pH however, it is susceptible to hydrolysis and cleavage at high pH. A typical reaction condition for using imidate crosslinkers is a buffer system consisting of 0.2 M triethanolamine in 0.1 M sodium borate, pH 8.2. After conjugating two proteins with a bifunctional imidoester crosslinker, excess imidoester functional groups may be blocked with ethanolamine. 

Figure 4.12 Imidoesters react with amine groups to form amidine bonds, which are positively charged at physiological pH.

Dimethyl pimelimidate (DMP) is a homobifunctional crosslinking agent that has imidoester groups on either end (Thermo Fisher). The imidoesters are amine reactive to give stable amidine linkages with target molecules. The 7-atom bridge created by DMP crosslinks is non-cleavable and... 

Imidoesters react with primary amino groups to form amidine bonds (Scheme 7). The amidine bonds are protonated and carry a positive charge at neutral and physiological pH. In addition, they are relatively stable at low pH but prone to hydrolysis at elevated pH. 

Imidoesters (11) react with various amines in nonaqueous solutions or in aqueous solution at low temperatures and at pH near neutrality to yield the corresponding amidines (12) (reaction 9). 

Another route is the amidination of lysine by means of imidoesters and imidothiolanes (9). The resulting amidine still carries a positive charge and the overall charge of the protein is recovered [106]. 

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