g Tensor and Spin Density of the Modified Tyrosyl Radical in Galactose Oxidase:  A Density Functional Study

Abstract

The influence of ortho sulfur substitution in the modified tyrosyl radical in apo-galactose oxidase on the g tensor and the spin-density distribution has been studied by calculations on various model systems using an accurate DFT approach. Computed g tensors agree well with experimental observation, and they are intermediate between the extremely large substituent effect found in a previous DFT study and the very small changes found in MCSCF calculations. The origin of the substituent effects has been studied by fragment analyses of the spin−orbit/orbital-Zeeman g-tensor contributions, and it is discussed in relation to the spin-density distributions. The influence of hydrogen bonding on g tensors is studied by appropriate model complexes with water. Further calculations on radicals including heavier chalcogen substituents XH (X = O, S, Se, Te) predict very large g x and g y components with selenium- and tellurium-substituted systems due to large, direct substituent spin−orbit contributions. In view of an unexpectedly large gauge dependence of the g-tensor orientation in several cases, a recently developed implementation of gauge-including atomic orbitals (GIAO) has been applied.

A Tryptophan Neutral Radical in the Oxidized State of Versatile Peroxidase from Pleurotus eryngii

“…7), for two different side chain orientations. Considering a reported general ϳ10% overestimation by DFT methods (63,64) of the g-tensor shifts (⌬g jj ) from the free electron value g e , (⌬g jj ϭ g jj Ϫ g e (jj ϭ xx, yy, zz), g e ϭ 2.002319), the calculated g-tensor values are in good agreement with the experimental values. Similar g-tensor values were, however, also calculated for the tryptophan cation radical (Table 2).…”

Tryptophan radicals as reaction intermediates in versatile peroxidases: Multifrequency EPR, ENDOR and density functional theory studies

“…6), for two different side chain orientations. Considering a reported general overestimation by about 10% of the g-shifts Agjj from the free electron value ge (Agjj =gjj-g~, (/)" = xx,yy,zz), ge = 2.002319) by DFT methods [36,37], the calculated g-values are in good agreement with the experimental values. Similar g-values were, however, also calculated for the tryptophan cation radical (Table 2).…”

Calculation of EPR g‐Tensors with Density Functional Theory

“…In larger systems, all common approaches to mitigation of the gauge dependence (gauge-including atomic orbitals, GIAOs [19,20,63]; individual gauge origin for localized orbitals, IGLO [21], and gauge-including projector-augmented waves, GIPAW [25,64]) give very similar results for the g-tensors. At the same time, the individual implementations vary greatly, depending on the choice of zeroth-order Hamiltonian and treatment of the spin-orbit operators.…”

“…Practical experience with ab initio and DFT calculations so far [12,14,15,20] indicates that the gauge dependence of the g-tensors is usually small, so that the common prescription [62] of placing the gauge origin at the centroid of the electron charge is sufficient to obtain accurate results in small molecules. As noted by Kaupp and co-workers [63], an exception to this rule of thumb can occur in situations where a delocalized radical exhibits a g-tensor with two similar principal components. In this case, the orientation (but not the magnitude) of the principal components becomes quite sensitive to the choice of the coordinate origin, making a gauge-independent treatment mandatory.…”