Characterisation of Cytochrome c Unfolding by Nano-Electrospray Ionization Time-of-Flight and Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Protein charge-state distributions (CSDs) in electrospray ionization mass spectrometry (ESI-MS) represent a sensitive tool to probe different conformational states. We describe here the effect of trifluoroethanol (TFE) on cytochrome c equilibrium unfolding at different pH by nano-ESI-MS. While even low concentrations of TFE destabilize the protein native structure at low pH, a TFE content of 2.5%–5% is found to favor cyt c folding at pH ∼7. Furthermore, we perform a comparison of CSDs obtained by time-of-flight (ToF) and Fourier transform ion cyclotron resonance (FT-ICR) mass analyzers. To this purpose, we analyze spectra of cyt c in the presence of different kinds of denaturants. In particular, experiments with 1-propanol also suggest that by using FT-ICR-MS, as previously observed on an ESI-ToF instrument, CSDs do not appear to be controlled by the solvent surface tension as predicted by the Rayleigh-charge model. Moreover, there is general good agreement in conformational effects revealed by the different instruments under several buffer conditions. Nevertheless, the ToF instrument appears to discriminate better between unfolded and partially unfolded forms.