This note describes how to label proteins of MW > 10000 using

MSD? TAG-NHS-Ester. The procedure involves a 1 hour incubation

step followed by a simple column separation to isolate the

labeled protein.

MSD TAG-NHS-Ester, or Ruthenium (II) tris-bipyridine, Nhydroxysuccinimide

is an amine-reactive label often used for

detection in assays formatted with MSD technology (Figure 1). Nhydroxysuccinimide

esters readily couple to primary amine

groups of proteins to form a stable amide bond. The reaction is

favored at slightly alkaline pH in which a nucleophilic attack by

an unprotonated amine on an ester results in amide bond

formation. The reaction is rapid and occurs under mild

conditions.

MSD TAG-labeled conjugates are stable and may be used at

low concentrations. These features minimize time, costs, and

labor since large, stable batches of conjugate can be prepared,

validated, and used for long periods of time. MSD TAG-NHSEster

is a popular conjugate for labeling molecules that contain

primary amines (e.g., lysines in proteins) since the method is

fast and easy.