Binding of CO, NO, and O₂ to Heme by Density Functional and Multireference ab Initio Calculations

Abstract

Using the CASSCF/CASPT2 approach, along with several DFT methods (PBE0, B3LYP, BP86, OLYP), we have investigated the bonding of CO, NO, and O2 molecules to two model heme systems: an iron(II) porphyrin with and without an axial imidazole ligand. The experimentally available binding energies are best reproduced by the CASPT2 method and with the OLYP functional. The other functionals considered perform much worse, either severely overbinding (BP86) or underbinding (B3LYP, PBE0). Significant discrepancies between the different density functionals are observed, not only for the energetics but sometimes also for structure predictions. This confirms our viewpoint that a balanced treatment of the electronic exchange and correlation is vital to describe the weak metal-ligand bond between heme and CO, NO, or O2. The binding energies DeltaEb were split into two contributions: the so-called spin-pairing energy DeltaE sp and the "inherent" binding energy DeltaEb0, and both contributions were analyzed in terms of method and basis set effects. We have also investigated the spin density distributions resulting from the bonding of the NO molecule (a noninnocent ligand) to heme. Our analysis at the DFT and CASSCF level shows that, while various density functionals predict qualitatively very different spin distributions, the CASSCF spin populations most closely correspond to the results obtained with the pure BP86 or OLYP rather than with the hybrid functionals.

A DFT and multi-configurational perturbation theory study on O2 binding to a model heme compound via the spin-change barrier

“…For a comprehensive explanation, please see previous studies. 2,13,15,17,44,45 As in the previous studies, the ground state of Fe(O 2 )PorIm obtained in the present study is a mixed Pauling and Weiss model. The most important component that has the largest weight is…”

Spin in density‐functional theory

“…In particular, the complete active space self-consistent field method, usually in combination with second-order perturbation theory (CASPT2), has been employed to study transition metal complexes (for examples, see Refs. [13][14][15][16][17][18]). However, the factorial scaling with the size of the active space puts rather severe limits on the size of the active space, which prevents most applications to polynuclear transition metal complexes and clusters.…”

On the Electronic Structure ofmer,trans‐[RuCl3(1H‐indazole)2(NO)], a Hypothetical Metabolite of the Antitumor Drug Candidate KP1019: An Experimental and DFT Study

“…However, this notation leaves the actual physical and formal oxidation state [23] of the metal center and nitrosyl ligand unclear, and therefore it remains a subject of notable interest. [24][25][26][27][28][29][30][31][32][33][34][35][36][37] This study is an attempt to describe such a {Ru(NO)} 6 -containing system in more detail.…”