Electrospray ionization mass spectrometry of highly heterogeneous protein systems: protein ion charge state assignment via incomplete charge reduction.

Correct mass and charge assignment for large highly heterogeneous macromolecular ions (e.g., large glycoproteins with significant carbohydrate content) presents a great challenge in native electrospray ionization mass spectrometry (ESI MS). A new approach to this problem combines complexity reduction (mass-selection of a narrow distribution of ionic species from a heterogeneous mixture) and gas-phase ion chemistry (electron-transfer reactions) to induce partial reduction of the ionic charge. The resulting spectra are devoid of complexity and are easy to interpret, leading to correct mass assignment. The new method is tested using several glycoproteins and their complexes, for which standard deconvolution approaches do not work.