Measurement of the reactive and available solid-supported carboxylic, N-hydroxysuccinylated carboxylic, and aldehydo groups

We describe new colorimetric methods for the determination of the reactive and available solid-supported carboxylic, N-hydroxysuccinylated carboxylic, and aldehydo groups under conditions usually applied for the coupling of biomolecules. The methods involve the reaction of the solid-supported functional groups with tyramine or cysteine, and the subsequent titration of the ligand coupled onto the solid supports using the commercially available bicinchoninic acid/copper protein assay reagent (BCA). The titration is based on the ability of these ligands to reduce Cu2+ to Cu+, which forms a chelate complex with bicinchoninic acid absorbing at 562 nm. The quantitation is finally carried out through standard curves obtained using tyramine or cysteine solutions of known concentrations. The values obtained by the assays developed for several solid supports carrying carboxylic, NHS-ester, and aldehydo groups were well correlated with those obtained by other literature methods or provided by the manufacturers. All of the proposed methods are simple, more sensitive than other relevant literature methods, and require only commericially available reagents.