Computational Studies of EPR Parameters for Paramagnetic Molybdenum Complexes. II. Larger Mo^V Systems Relevant to Molybdenum Enzymes

Abstract

The careful validation of modern density functional methods for the computation of electron paramagnetic resonance (EPR) parameters in molybdenum complexes has been extended to a number of low-symmetry MoV systems that model molybdoenzyme active sites. Both g and hyperfine tensors tend to be reproduced best by hybrid density functionals with about 30-40% exact-exchange admixture, with no particular spin contamination problems encountered. Spin-orbit corrections to hyperfine tensors are mandatory for quantitative and, in some cases, even for qualitative agreement. The g11 (g||) component of the g tensor tends to come out too positive when spin-orbit coupling is included only to leading order in perturbation theory. Compared to single-crystal experiments, the calculations reproduce both g- and hyperfine-tensor orientations well, both relative to each other and to the molecular framework. This is significant, as simulations of the EPR spectra of natural-abundance frozen-solution samples frequently do not allow a reliable determination of the hyperfine tensors. These may now be extracted based on the quantum-chemically calculated parameters. In a number of cases, revised simulations of the experimental spectra have brought theory and experiment into substantially improved agreement. Systems with two terminal oxo ligands, and to some extent with an oxo and a sulfido ligand, have been confirmed to exhibit particularly large negative Deltag33 shifts and thus large g anisotropies. This is discussed in the context of the experimental data for xanthine oxidase.

Relativistic two-component calculations of electronic g-tensor for oxo-molybdenum(V) and oxo-tungsten(V) complexes: The important role of higher-order spin-orbit contributions

“…That is, not only should W(V) complexes and tungsten enzyme sites exhibit overall larger g-anisotropy than a given Mo(V) homologue [19][20][21][22], but we expect that the errors made by an only perturbation-theoretical inclusion of spin-orbit coupling will also be larger. As this will be of substantial importance for future quantum chemical work on tungsten enzyme EPR, we extend here our previous study of Mo(V) complexes [26,27] to DFT g-tensor calculations for a number of small to medium-sized W(V) species. We will compare oneand two-component approaches to evaluate the importance of higher-order SO effects (including spin polarization in both cases).…”

“…DFT calculations have been done either at the generalized-gradient-approximation (GGA) level, with the Becke exchange and Perdew correlation functionals (BP86) [30][31][32], or employing Becke's three-parameter hybrid functional with Perdew-Wang GGA correlation (B3PW91) [33][34][35][36]. As in most of our previous studies on EPR parameters of transition metal complexes [26,27,[37][38][39][40][41][42][43][44] the best agreement with experimental data for both hyperfine and g-tensors was obtained with hybrid functionals containing approximately 30-40% Hartree-Fock exchange, we have also evaluated the use of the oneparameter BPW91-40HF functional of the form E hybrid…”

“…In case of Mo V and W V sites, metal and ligand hyperfine couplings, certain ligand nuclear quadrupole couplings, and the electronic g-tensor, are the most important parameters to be considered to describe the EPR spectra [14][15][16]25]. We have recently validated state-of-the-art quantum chemical approaches based on density functional theory (DFT) for the calculation of g-tensors and Mo hyperfine tensors in a series of Mo(V) complexes ranging from small models to larger complexes of low symmetry that resemble some of the enzyme active sites [26,27]. It turned out that hybrid functionals with an exact-exchange admixture between 30% and 40% provided the best agreement with experimental g-and hyperfine tensors, including the orientations of these tensors relative to each other and to the molecular framework (as compared to single crystal EPR data with 95 Mo and 97 Mo isotope-enriched samples) [26].…”

“…It turned out that hybrid functionals with an exact-exchange admixture between 30% and 40% provided the best agreement with experimental g-and hyperfine tensors, including the orientations of these tensors relative to each other and to the molecular framework (as compared to single crystal EPR data with 95 Mo and 97 Mo isotope-enriched samples) [26]. In cases of X-band powder spectra without isotope enrichment, the hyperfine tensors of model complexes and enzymes frequently cannot be determined accurately by experiment, and it was shown that assistance by quantum chemical calculations may allow improved spectra simulations in such cases [27].…”

Which functional groups of the molybdopterin ligand should be considered when modeling the active sites of the molybdenum and tungsten cofactors? A density functional theory study

“…In theoretical studies, the ligand is typically modeled with rather simple systems: ethenedithiol (edt; S 2 C 2 H 2 2− ) [20], maleonitrile [mnt; S 2 C 2 (CN) 2 2− ] [21–23], 1,2-dimethyldithiolene (S 2 C 2 Me 2 2− ) [21, 24–26] or benzenedithiol (bdt; S 2 C 6 H 4 2− ) [22, 24–27]. Functional groups of molybdopterin, besides the dithiolene function that binds to the metal, were ignored, probably to reduce the computational load.…”