Abstract: A reagent includes a magnetic nanoparticle, a cleavable linker, and a primary amine reactive group. A first end of the cleavable linker may conjugate to a surface of the magnetic nanoparticle. A second end of the cleavable linker may conjugate to a primary amine reactive group. A method of using the reagent includes adding the reagent to a native protein mixture such that magnetic nanoparticles may conjugate to the protein with exposed primary amine groups in the native protein mixture. The method also includes separating the proteins with exposed primary amine group from other components of the native protein mixture under a magnetic field. The method further includes removing the magnetic nanoparticles from the proteins with the exposed primary amine groups from at cleavable linkers. The method finally includes identifying sequences and/or sites of the exposed primary amine groups in the proteins.