Native mass spectra of proteins and protein complexes often contain narrow charge-state distributions and wide peaks, which makes assigning charge states challenging and can limit the accuracy of native mass spectrometry experiments. These complications are exacerbated when samples contain mixtures of proteins or oligomeric states. Here we use selected cation to anion proton transfer (SCAPT) reactions to generate charge-reduced product ions. SCAPT is analogous to ion/ion proton transfers reactions pioneered by McLuckey and coworkers to aid in the analysis of peptides and other analytes (Chem. Commun. 2013  , 49, 947�965). Using SCAPT, we have detected the lowest charge state below m/z 100,000 for a series of protein complexes ranging from 12-468 kDa. Charge states can be assigned unambiguously using the additional charge-reduced peaks, which enables accurate oligomeric state and mass assignments. Additionally, we demonstrate the ability to separate proteins in a mixture by reacting ions selected from a congested m/z region containing contributions from multiple protein complexes. Using this approach, we observe an 8-fold increase in the resolution of two components from that mixture. The improved resolution of the SCAPT products is attributable to selecting precursor ions of similar m/z but different z; thus, the resolution increases following each proton transfer. Lastly, collision cross sections of several protein complexes were determined. These results show a reproducible, but subtle range of collision cross sections (-3% to +3%) for protein complexes as a function of charge state. SCAPT is an efficient method to manipulate the charge states of native-like ions, which we used to accurately assign charge states, separate protein mixtures, and probe the relationship between charge state and protein structure.

Determining masses, separating mixtures, and probing structures of native-like ions using selected cation to anion proton transfer (SCAPT) reactions

248th National Meeting (2014)

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