Toward the understanding of the environmental effects on core ionizations

Abstract

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 (E(ion)) 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 E(ion) 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 E(ion) of alpha carbon of glycine residues in HSA ranges from 285 to 295 eV depending on their surroundings.

Initial Maximum Overlap Method Embedded with Extremely Localized Molecular Orbitals for Core-Ionized States of Large Systems

“…To this regard, although Gill and collaborators clearly indicated that the importance of orthogonality is overstated [34], here it is also worth citing single Slater determinant techniques that allow the determination of core-ionized state wavefunctions that are orthogonal to those of the corresponding ground states. They are the local self-consistent field (LSCF) [44][45][46] and asymptotic projection (AP) methods [47][48][49], where the emptied core MO is kept frozen and orthogonal to the occupied molecular orbitals throughout the SCF procedure [12,[50][51][52][53][54][55][56].…”

“…In this framework, it is worth considering that the above-mentioned LSCF and AP methods have already been coupled with molecular mechanics through the traditional QM/MM scheme to determine core-ionized states and core-ionization energies of (relatively) large (bio)molecules (e.g., extended polypeptides, polymers, and even proteins) [12,50,51]. Following these examples, in the present work, we will show that coreionized states and core-ionization energies can be respectively modeled and evaluated also through the IMOM/ELMO (initial maximum overlap method/extremely localized molecular orbital) technique, a single Slater determinant-based embedding approach recently developed for the treatment of excited states of large systems [83].…”