Toward the understanding of the environmental effects on core ionizations

Experimental X‐ray absorption spectra are extensively used to determine electronic structure of small molecules but remain difficult to exploit for proteins due to the large number of peaks within their spectra. For such complex systems, theoretical tools like quantum mechanics/molecular mechanics methodology can greatly ease the assignment of the spectra. This study presents a systematic methodology to evaluate core‐ionization energies (Eion) in proteins with the help of the asymptotic projection approach (Glushkov and Tsaune, Z. Vichislit. Matem. Mat. Fiz. 1985, 25, 298; Glushkov, Chem. Phys. Lett. 1997, 273, 122; Glushkov, Chem. Phys. Lett. 1998, 287, 189; Glushkov, J. Math. Chem. 2002, 31, 91; Glushkov, Opt. Spectrosc. 2002, 93, 15). An in‐depth inspection of Eion of systems of increasing complexity is considered, going from amino acids to polyglycine and to glycine in human serum albumin (HSA). Computational analysis can help to better understand experimental data and to discriminate environmental effects by tracing them back to individual and collective electrostatic contributions. In the present work, it was found that Eion of alpha carbon of glycine residues in HSA ranges from 285 to 295 eV depending on their surroundings. © 2014 Wiley Periodicals, Inc. A systematic methodology is used to evaluate core ionization energies of amino acids in the gas phase or in part of the human serum albumin protein. This is performed with the help of QM/MM tools and, more precisely, with the asymptotic projection formalism, accounting for orthogonality constraints between states. This work shows that environment effects are not trifling when computing core ionization energies, as they might be as large as 10 eV.