Mean charge state and charge state distribution of proteins as structural probes.: An electrospray ionisation mass spectrometry study of lysozyme and ribonuclease A

Analysis by electrospray mass spectrometry of lysozyme and ribonuclease A showed that the presence of triethylammonium cations leads to a spectacular decrease of the protein charge state compared with this obtained with ammonium cations, thus confirming the role played by the presence of bases (ammonia or triethylamine) in the solvent phase. However, despite their very different compositions in basic amino acid residues, these proteins gave rise to identical charge states and charge state distributions under non-denaturing solvent conditions. Unexpectedly, the main difference between the two investigated proteins corresponded to their behaviour under denaturing solvent conditions, with lysozyme leading to narrower charge state distributions than ribonuclease A. The use of the mean charge state values and charge state distributions for structure investigation of native proteins is discussed.