K-Shell Excitation and Ionization of a Gas-Phase Protein: Interplay between Electronic Structure and Protein Folding

Understanding the correlation between proteins' tertiary and electronic structures is a great challenge, which could possibly lead to a more efficient prediction of protein functions in living organisms. Here, we report an experimental study of the interplay between electronic and tertiary protein structure, by probing resonant core excitation and ionization over a number of charge-state selected precursors of electrically charged proteins. The dependence of the core ionization energies on the protein charge state shows that the ionization of a protonated protein is strongly correlated to its tertiary structure, which influences its effective Coulomb field. On the other hand, the electronic core-to-valence shell transition energies are not markedly affected by the unfolding of the protein, from compact to totally elongated structures, suggesting that frontier protein orbitals remain strongly localized. Nevertheless, the unfolding of a protein seems to influence the cross section ratio between different resonant electronic transitions.