NAKED PROTEIN CONFORMATIONS : CYTOCHROME C IN THE GAS PHASE

Ion mobility measurementsl have been used to obtain direct information about the conformers present for naked cytochrome c ions in the gas phase. The relative abundance of compact native-like conformations is negligible. However, a number of well-defined, partially folded conformers that are considerably more diffuse than the native structure are observed. These may be intermediates in the folding process. The results suggest that the gas phase is an environment that will allow access to complementary information about protein conformations and the dynamics of protein folding.' The development of gentle ionization techniques' has facilitated studies of biological molecules using mass spectrometry. Accurate measurements of molecular weigh& as well as sequence information*.h are becoming commonplace, and recently there have been several attempts to use mass spectrometry to deduce information about the three-dimensional structures of biological molecules. For example, the charge distribution generated by electrospray ionization' (ESI) has been shown to depend on the conformation of the protein in s o l u t i ~ n . ~ . ~ Mass spectrometry has been used to monitor 'W'H isotope exchange for proteins in solution"' as well as in the gas phase."." Finally, ion beam scattering experiment^'^.^^ have shown that protein ions in different charge states have different collision cross sections. The ion mobility measurementsl reported here resolve protein conformers on the basis of their different collision cross sections and provide precise cross sections for the conformers