Novel method for [M + Cu] + ion formation by matrix-assisted laser desorption ionization

Abstract

Direct deposition of a MALDI sample onto a copper sample stage and irradiation with UV light (337 nm) produces copper adduct ions of both the matrix and analyte molecules. This technique for introducing Cu + into the gas-phase avoids suppression of ion signal that accompanies addition of metal salts to the sample solution. We observe good correlation between the number of basic residues in peptides and the number of Cu + ions that add to the peptide. For example, the peptide KRQHPG contains three basic residues and forms ions with up to three Cu + adducts. Postsource decay experiments demonstrate that for arginine containing peptides, arginine anchors the Cu + ion. That is, all metastable ions contain the arginine complexed to Cu + and the only immonium ion observed is that of arginine–Cu +. In addition, preliminary calculations indicate that guanidine has the highest Cu + ion affinity followed by histidine.