Preparation and Conjugation of dPSA

dPSA was produced by heating a solution (10 mL) of 100 mg of colominic acid (MilliporeSigma) and 10 mg sodium cyanoborohydride (MilliporeSigma) in 2 M NaOH to 100 °C in a sealed hydrolysis tube (ThermoFisher Scientific) for 40 min. The solution was cooled, neutralized with 2 M HCl, dialyzed (Spectra/Por™, Fisher Scientific) two times with 4 L of water and lyophilized. dPSA containing 35% de-N-acetyl neuraminic acid residues, as determined by a modified resorcinol assay [30 (link)], was conjugated to biotin as described previously [28 (link)]. The ELISA was performed with serial dilutions of nucleolin or SEAM 3 as in the absence or presence of 50 μg/mL of colominic acid or unmodified dPSA as described previously [28 (link)].