4 years ago

Highly Charged Protein Ions: The Strongest Organic Acids to Date

Highly Charged Protein Ions: The Strongest Organic Acids to Date
Fangtong Zhang, Toby T. Funston, Muhammad A. Zenaidee, Michael G. Leeming, William A. Donald
The basicity of highly protonated cytochrome c (cyt c) and myoglobin (myo) ions were investigated using tandem mass spectrometry, ion–molecule reactions (IMRs), and theoretical calculations as a function of charge state. Surprisingly, highly charged protein ions (HCPI) can readily protonate non-polar molecules and inert gases, including Ar, O2, and N2 in thermal IMRs. The most HCPIs that can be observed are over 130 kJ mol−1 less basic than the least basic neutral organic molecules known (tetrafluoromethane and methane). Based on theoretical calculations, it is predicted that protonated cyt c and myo ions should spontaneously lose a proton to vacuum for charge states in which every third residue is protonated. In this study, HCPIs are formed where every fourth residue on average is protonated. These results indicate that protein ions in higher charge states can be formed using a low-pressure ion source to reduce proton-transfer reactions between protein ions and gases from the atmosphere. Protein ions in extremely high charge states are sufficiently acidic that noble gases can be protonated in ion–molecule reactions. The results are not consistent with the charge residue model for the formation of highly charged protein ions in electrospray ionization (ESI) because the protein ions are more than 375 kJ mol−1 less basic than the least volatile components of ESI solutions. The chain ejection model is more likely.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/anie.201702781

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