Density Functional Study of EPR Parameters and Spin-Density Distribution of Azurin and Other Blue Copper Proteins

Abstract

Modern density functional methods have been used to study spin-density distribution, g tensors, as well as Cu and ligand hyperfine tensors for azurin models, for two more blue copper proteins plastocyanin and stellacyanin, and for small model complexes. The aim was to establish a consistent computational protocol that provides a realistic description of the EPR parameters as probes of the spin-density distribution between metal and coordinated ligands in copper proteins. In agreement with earlier conclusions for plastocyanin, hybrid functionals with appreciable exact-exchange admixtures, roughly around 50%, provide the best overall agreement with all parameters. Then the bulk of the spin density is almost equally shared by the copper atom and the sulfur atom of the equatorial cysteine ligand, and the best values are obtained for copper, histidine nitrogen, and cysteine beta-proton hyperfine couplings, as well as for g(parallel). Spin-orbit effects on the EPR parameters may be appreciable and have to be treated carefully to obtain agreement with experiment. Most notably, spin-orbit effects on the (65)Cu hyperfine coupling tensors in blue copper sites are unusually large compared to more regularly coordinated Cu(II) complexes with similar spin density on copper. In addition to the often emphasized high covalency of the Cu-S(Cys) bond, the characteristically small A(parallel) component of blue copper proteins is shown to derive to a large part from a near-cancellation between negative first-order (Fermi contact and dipolar) and unusually large positive second-order (spin-orbital) contributions. The large spin-orbit effects relate to the distorted tetrahedral structures. Square planar dithiolene complexes with similar spin density on copper exhibit much more negative A(parallel) values, as the cancellation between nonrelativistic and spin-orbit contributions is less complete. Calculations on a selenocysteine-substituted variant of azurin have provided further insight into the relations between bonding and EPR parameters.

Four-Component Relativistic Density Functional Theory Calculations of EPRg- and Hyperfine-Coupling Tensors Using Hybrid Functionals: Validation on Transition-Metal Complexes with Large Tensor Anisotropies and Higher-Order Spin–Orbit Effects

“…The effects on the isotropic HFCs are less obvious, as SO contributions may exhibit the same or opposite sign as compared to the Fermi-contact-type terms (with sometimes dramatic consequences). 86 With these considerations in mind, we have used a test set of 17 small 4d 1 A-tensor components), consistent with the above analyses and previous experience at oneand two-component levels. 30-31, 40, 48, 54-55 This is particularly notable for the metal HFC components (for some rhenium complexes the percentage error exceeds 60% at the GGA level; cf.…”

Predicting the NMR Spectra of Paramagnetic Molecules by DFT: Application to Organic Free Radicals and Transition‐Metal Complexes

“…[7] For open-shell species, most computational investigations have focused on the prediction of EPR quantities, such as hyperfine couplings, g tensors, and zero-field splittings (ZFSs); this topic has been reviewed [8] and continues to be a lively area of investigation. [9][10][11][12][13] The parameters that determine the appearance of an NMR spectrum (chemical shifts and spin-spin couplings) can also be predicted and DFT methods have been successfully applied to a wide variety of organic, organometallic, and inorganic molecules, thus providing an invaluable aid in structure determination. [7,[14][15][16][17] Of course, most such investigations have concerned diamagnetic molecules.…”

DFT investigation of the effect of spin-orbit coupling on the NMR shifts in paramagnetic solids

“…1. As a result of the limitations of the available computational methods for solid-state DFT, to the best of our knowledge the calculation of the hyperfine relativistic corrections (A FC,2 , A dip,2 ) are not currently possible, and so we do not comment further on terms (b) and (f) of Table I, which are expected to be small for ligand hyperfine couplings, except for nuclei directly bonded to truly heavy centres [48,49]. The overall isotropic paramagnetic shift for an OC is calculated as the sum of the contact terms (a) and (c) and the rank-zero component of term (h).…”