Enhancing electrospray ionization efficiency of peptides by derivatization

With the advent of electrospray ionization mass spectrometry, the world was given a new way to look at complex peptide mixtures. Identification of proteins via their signature peptides requires ionization of a representative portion of the peptides derived from proteins by proteolysis. Unfortunately, matrix effects prohibited electrospray ionization of many peptides. This paper describes the development of a new labeling reagent that simultaneously adds a permanent positive charge to peptides and increases their hydrophobicity to enhance their ionization efficiency. The labeling agent is preactivated with N-hydroxysuccinimide to react with primary amines to form a peptide bond. In the most dramatic case, ionization efficiency of the peptide ADRDQYELLCLDNTRKPVDEYK increased 500-fold after derivatization as opposed to other peptides where ionization efficiency was impacted little. Ionization efficiency of peptides was enhanced roughly 10-fold in general by derivatization. Peptides of less than 500 Da experienced the greatest increase in ionization efficiency by derivatization. Poor ionization efficiency of native peptides was found to be due more to their inherent structural properties than the matrix in which ionization occurs.