Probing the protein electronic structure by soft X-ray action spectroscopy
The motivation of this study is to see if the electronic structure of the protein can be correlated with its charge state, which defines the protein tertiary structure. Due to the Coulomb repulsion, protein precursors with higher charge states tend to unfold, or have elongated structures. Therefore, by roughly controlling the protein tertiary structure in the gas phase, we are able to investigate the influence on the protein electronic structure and give more insights into protein fundamental properties.
We used the near edge X-ray absorption fine structure (NEXAFS) method to obtain the absorption spectra from the trapped Ubiquitin and bovine pancreatic trypsin inhibitor (BPTI) protein ions, produced by an electrospray ion source (ESI). Several precursor ion charge states of these two proteins were isolated and subjected to the soft X-ray photons from PLEIADES beamline at SOLEIL synchrotron near Paris. Single ionization yields were obtained for different precursor charge states in C- and N-edge photon energy ranges. In the obtained spectra, we could clearly resolve the features corresponding to the Carbon 1s and Nitrogen 1s electron excitation into higher molecular orbitals. An example of our preliminary results obtained for BPTI protein in the C-edge energy range is presented in Figure 1.
Figure 1 – Single ionization yields for the precursor charge state 5+ of the BPTI protein in the C-edge. Two peaks at about 284.5 eV and 288 eV correspond to the C 1s electron excitation into π*aromatic and π*amide molecular orbitals. The yield is normalized to the unity area under the curve.
The main goals of this STSM were successfully accomplished and we expect to publish the results in peer review journal as soon as possible. We plan to continue the collaboration with the new studies involving NEXAFS of other biologically relevant samples in the gas phase.