Gas-Phase Protein Inner-Shell Spectroscopy by Coupling an Ion Trap with a Soft X-ray Beamline.

C, N, and O near-edge ion yield spectroscopy of 8+ selected electrosprayed cations of cytochrome c protein (12 kDa) has been performed by coupling a linear quadrupole ion trap with a soft X-ray beamline. The photoactivation tandem mass spectra were recorded as a function of the photon energy. Photoionization of the precursor, accompanied by CO2 loss, is the dominant relaxation process, showing high photoion stability following direct or resonant photoionization. The partial ion yields extracted from recorded mass spectra show significantly different behaviors for single and double ionization channels, which can be qualitatively explained by different Auger decay mechanisms. However, the single ionization spectra reveal characteristic structures when compared to existing near-edge X-ray absorption fine structure (NEXAFS) spectra from thin films of peptides and proteins. Therefore, the present experiment opens up new avenues for near-edge X-ray spectroscopy of macromolecules in the gas phase, overcoming the radiation damage issue or the environmental effects as due to the surface, intermolecular interactions, and solvent.