Cleavable ester linked magnetic nanoparticles for labeling of solvent exposed primary amine groups of peptides/proteins

Covalent labeling of solvent exposed amino acid residues using chemical reagents/crosslinkers followed by mass spectrometric analysis can be used to determine the solvent accessible amino acids of a protein. A variety of chemical reagents containing cleavable bonds were developed to label abundantly found lysine residues on the surface of protein. To achieve efficient separation of labeled peptides prior to mass spectrometric analysis, magnetic nanoparticles can be decorated with amino acid reactive functional groups and utilized for quick recovery of labeled peptides. [1] In this work, iron oxide magnetic nanoparticles (Fe(3)O(4) MNPs) were synthesized by thermal decomposition method and coated with silica (SiO(2)@Fe(3)O(4) MNPs) by reverse micro emulsion approach. The Fe(3)O(4) MNPs and SiO(2)@Fe(3)O(4) MNPs were characterized by TEM and XRD. The SiO(2)@Fe(3)O(4) MNPs were further coated with amine groups and conjugated to N-hydroxysuccinimidyl (NHS) ester groups via a cleavable ester bond. Fluorescence based qualitative analysis of ester linked NHS ester modified SiO(2)@Fe(3)O(4) MNPs was performed to confirm the presence of NHS ester group. The active NHS ester sites on the surface of SiO(2)@Fe(3)O(4) MNPs were determined by depletion approach and found to be 694 active sites per 1 mg of SiO(2)@Fe(3)O(4) MNPs. Free amine groups of a small peptide, ACTH (4–11) were labeled by ester linked, NHS ester modified SiO(2)@Fe(3)O(4) MNPs under physiological conditions. Superparamagnetic nature of SiO(2)@Fe(3)O(4) MNPs allowed quick and efficient magnetic separation of labeled peptides from the solution. The ester bond was further cleaved to separate labeled peptides followed by mass spectrometric analysis. The ester linked, NHS ester modified SiO(2)@Fe(3)O(4) MNPs introduced a mass shift of 115.09 Da on amine groups of ACTH (4–11), which was confirmed by mass spectrometry.