Synthesis of isoluminol derivatives with a terminal carboxyl group for protein labelling

<i>N</i>-(4-Aminobutyl)-<i>N</i>-ethylisoluminol (ABEI) as an efficient label is widely used in automated immunoassays, yet the labelling process of ABEI onto biomacromolecules like proteins is not straightforward as the transformation of an amino group into a carboxyl group is needed. We report the synthesis of isoluminol derivatives with a terminal carboxyl group for protein labelling. And also, an <i>N</i>-hydroxysuccinimide active ester of carboxylic acid as a direct protein labelling reagent was isolated with high purity. This labelling reagent was proved to have good chemiluminescence efficiency, be stable and possess high labelling efficacy with BSA. The introduction of the propyl sulfonic group as a hydrophilic functional group increased not only water solubility but also the chemiluminescence quantum yields. In addition, non-specific binding of magnetic microparticles was also decreased notably when water soluble labelling reagents were used.