Mass and charge state assignment for proteins and peptide mixtures via noncovalent adduction in electrospray mass spectrometry

A method has been developed that takes advantage of the formation of noncovalent compounds in electrospray mass spectrometry. Mixtures of proteins and peptides are shown to produce an intense ion that corresponds to a 1:1 complex with a crown ether (18-crown-6). Although the crown ether may be added directly to the solution, for the current experiments it is introduced via the methanol liquid sheath. The spacing of these complexed species in the mass spectrum allows unambiguous determination of the charge state of the ions and their actual mass. Through constant neutral loss scans, charge state may be determined, mass assigned, spectra simplified, and chemical noise may be reduced for the analysis of complex peptide samples without chromatographic separation. Finally, the prevalence of single complexation permits mass assignments based on the mass difference of a single protein ion and its complexed form at any charge state. Ln essence, the method performs a separation based on charge state. It can be used to complement chromatographic separation and deconvolution algorithms for the electrospray mass spectrometry analysis of peptide-protein mixtures.