Density Functional Theory Calculations of Electron Paramagnetic Resonance Parameters of a Nitroxide Spin Label in Tissue Factor and Factor VIIa Protein Complex

Interplay of Intrinsic, Environmental, and Dynamic Effects in Tuning the EPR Parameters of Nitroxides: Further Insights from an Integrated Computational Approach

The role of stereoelectronic, environmental, and short-time dynamic effects in tuning the hyperfine and gyromagnetic tensors of a prototypical nitroxide spin probe has been investigated by an integrated computational approach based on extended Lagrangian molecular dynamics and discrete-continuum solvent models. Trajectories were generated in two protic solvents as well as in the gas phase for reference; structural analysis of the dynamics, and comparison with optimized solute-solvent clusters, allowed for the identification of the prevailing solute-solvent hydrogen-bonding patterns and helped to define the strategy for the computation of magnetic parameters. This was performed in a separate step, on a large number of frames, by a high-level DFT approach coupling the PBE0 hybrid functional with a tailored basis set and with proper account of specific and bulk solvent effects. Remarkable changes in solvation networks are found on going from aqueous to methanol solution, thus providing a rationalization of indirect experimentally available evidence. The computed magnetic parameters are in satisfactory agreement with the available measured values and allow for an unbiased evaluation of the role of different effects in tuning the overall EPR observables. Apart from their intrinsic interest, our results pave the route toward the development of tunable detection protocols based on specific spectroscopic signatures.

Hyperfine Coupling Tensors of the Benzosemiquinone Radical Anion from Car–Parrinello Molecular Dynamics

Based on Car-Parrinello ab initio molecular dynamics simulations of the benzosemiquinone radical anion in both aqueous solution and the gas phase, density functional calculations provide the currently most refined EPR hyperfine coupling (HFC) tensors of semiquinone nuclei and solvent protons. For snapshots taken at regular intervals from the molecular dynamics trajectories, cluster models with different criteria for inclusion of water molecules and an additional continuum solvent model are used to analyse the HFCs. These models provide a detailed picture of the effects of dynamics and of different intermolecular interactions on the spin-density distribution and HFC tensors. Comparison with static calculations allows an assessment of the importance of dynamical effects, and of error compensation in static DFT calculations. Solvent proton HFCs depend characteristically on the position relative to the semiquinone radical anion. A point-dipolar model works well for in-plane hydrogen-bonded protons but deviates from the quantum chemical values for out-of-plane hydrogen bonding.

Theoretical investigation of the EPR hyperfine coupling constants in amino derivatives

The HFCCs of the radical cations of a series of amines have been determined at different levels of approximation including the CISD, QCISD, and CCSD ab initio correlated methods and density functional theory approaches employing the B3LYP, PBE0, BHandHLYP, TPSS, and BLYP exchange-correlation functionals. Although quantitative differences with respect to experimental data have been noticed, these are mostly systematic within a given class of N and H atoms. As a consequence, these different levels of theory are reliable in most cases to account for the substituent and structure effects on the HFCCs of amines. Linear regression fits have then been performed to reach quantitative agreement between the theoretical and experimental values. This has finally been substantiated by considering the EPR signal of the recently synthesized radical cations of two derivatives of [10-(4-aminophenyl)-9-anthryl]aniline as well as in confirming a recent assignment of the EPR signal of n-propylamine.

Understanding the EPR Parameters of Glycine-Derived Radicals: The Case of N-Acetylglycyl in the N-Acetylglycine Single-Crystal Environment

As a step toward an in-depth understanding of the electron paramagnetic resonance parameters of glycyl radicals in proteins, the hyperfine tensors and, particularly, the g-tensor of N-acetylglcyl in the environment of a single crystal of N-acetylglycine have been studied by systematic state-of-the-art quantum chemical calculations on various suitable model systems. The quantitative computation of the g-tensors for such glycyl-derived radicals is a veritable challenge, mainly because of the very small g-anisotropy combined with a nonsymmetrical, delocalized spin-density distribution and several atoms with comparable spin-orbit contributions to the g-tensors. The choice of gauge origin of the magnetic vector potential, and of approximate spin-orbit operators, both turn out to be more critical than found in previous studies of g-tensors for organic radicals. Environmental effects, included by supermolecular hydrogen-bonded models, were found to be moderate, because of a partial compensation between the influences from intramolecular and intermolecular hydrogen bonds. The largest effects on the g-tensor are caused by the conformation of the radical. The density functional theory methods employed systematically overestimate both the Delta gx and Delta gy components of the g-tensor. This is important for parallel investigations on the protein-glycyl radicals. The 1H alpha and 13C alpha hyperfine couplings depend only slightly on the supermolecular model chosen and appear less sensitive probes of detailed structure and environment.

High-field (275 GHz) spin-label EPR for high-resolution polarity determination in proteins

The polarity of protein surfaces is one of the factors driving protein-protein interactions. High-field, spin-label EPR at 95 GHz, i.e., 10 times higher than conventional EPR, is an upcoming technique to determine polarity parameters of the inside of proteins. Here we show that by 275 GHz EPR even the small polarity differences of sites at the protein surface can be discriminated. To do so, four single cysteine mutations were introduced at surface sites (positions 12, 27, 42, and 118) of azurin and spin labeled. By 275 GHz EPR in frozen solution, polarity/proticity differences between all four sites can be resolved, which is impossible by 95 GHz EPR. In addition, by 275 GHz EPR, two spectral components are observed for all mutants. The difference between them corresponds to one additional hydrogen bond.

Interplay of Stereoelectronic and Enviromental Effects in Tuning the Structural and Magnetic Properties of a Prototypical Spin Probe: Further Insights from a First Principle Dynamical Approach

The nitrogen isotropic hyperfine coupling constant (hcc) and the g tensor of a prototypical spin probe (di-tert-butyl nitroxide, DTBN) in aqueous solution have been investigated by means of an integrated computational approach including Car-Parrinello molecular dynamics and quantum mechanical calculations involving a discrete-continuum embedding. The quantitative agreement between computed and experimental parameters fully validates our integrated approach. Decoupling of the structural, dynamical, and environmental contributions acting onto the spectral observables allows an unbiased judgment of the role played by different effects in determining the overall experimental observables and highlights the importance of finite-temperature vibrational averaging. Together with their intrinsic interest, our results pave the route toward more reliable interpretations of EPR parameters of complex systems of biological and technological relevance.

Calculation of Solvent Shifts on Electronic g-Tensors with the Conductor-Like Screening Model (COSMO) and Its Self-Consistent Generalization to Real Solvents (Direct COSMO-RS)

The conductor-like screening model (COSMO) was used to investigate the solvent influence on electronic g-values of organic radicals. The previously studied diphenyl nitric oxide and di-tert-butyl nitric oxide radicals were taken as test cases. The calculations employed spin-unrestricted density functional theory and the BP and B3LYP density functionals. The g-tensors were calculated as mixed second derivative properties with respect to the external magnetic field and the electron magnetic moment. The first-order response of the Kohn-Sham orbitals with respect to the external magnetic field was determined through the coupled-perturbed DFT approach. The spin-orbit coupling operator was treated using an accurate multicenter spin-orbit mean-field (SOMF) approach. Provided that important hydrogen bonds are explicitly modeled by a supermolecule approach and that the basis set is sufficiently saturated, the COSMO calculations lead to accurate predictions of isotropic g-shifts with deviations of not more than 100 ppm relative to experiment. Very accurate results were obtained by employing a recently developed self-consistent modification of the COSMO method to real solvents (COSMO-RS), which we briefly introduce in this paper as direct COSMO-RS (D-COSMO-RS). This model gives isotropic g-shifts of similar high accuracy for water without using the supermolecule approach. This is an important result because it solves many of the problems associated with the supermolecule approach such as local minima and the choice of a suitable model system. Thus, the self-consistent D-COSMO-RS incorporates some specific solvation effects into continuum models, in particular it appears to successfully model the effects of hydrogen bonding. Although not yet widely validated, this opens a novel approach for the calculation of properties which so far only could be calculated by the inclusion of explicit solvent molecules in continuum solvation methods.

Electronic g-tensors of solvated molecules using the polarizable continuum model

We present the implementation of density functional response theory combined with the polarizable continuum model (PCM), enabling first principles calculations of molecular g-tensors of solvated molecules. The calculated g-tensor shifts are compared with experimental g-tensor shifts obtained from electron paramagnetic resonance spectra for a few solvated species. The results indicate qualitative agreement between the calculations and the experimental data for aprotic solvents, whereas PCM fails to reproduce the electronic g-tensor behavior for protic solvents. This failure of PCM for protic solvents can be resolved by including into the model those solvent molecules which are involved in hydrogen bonding with the solute. The results for the protic solvents show that the explicit inclusion of the solvent molecules of the first solvation sphere is not sufficient in order to reproduce the behavior of the electronic g-tensor in protic solvents, and that better agreement with experimental data can be obtained by including the long-range electrostatic effects accounted for by the PCM approach on top of the explicit hydrogen-bonded complexes.

Influence of Conformation on the EPR Spectrum of 5,5-Dimethyl-1-hydroperoxy-1-pyrrolidinyloxyl: A Spin Trapped Adduct of Superoxide

Spin trapping, a technique used to characterize short-lived free radicals, consists of using a nitrone or nitroso compound to "trap" an unstable free radical as a long-lived aminoxyl that can be characterized by EPR spectroscopy. The resultant aminoxyl exhibits hyperfine splitting constants that are dependent on the spin trap and the free radical. Such is the case with 2,2-dimethyl-5-hydroxy-1-pyrrolidinyloxyl (DMPO-OH) and 2,2-dimethyl-5-hydroperoxy-1-pyrrodinyloxyl (DMPO-OOH) whose hyperfine splitting constants, A(N) = A(H) = 14.9 G and A(N) = 14.3 G, A(H)(beta) = 11.7 G, and A(H)(gamma) = 1.25 G, respectively, have been used to demonstrate the generation of HO(*) and O(2)(*)(-). However, to date, the source of the apparent A(H)(gamma) hyperfine splitting in DMPO-OOH is not known. We consider three possible explanations to account for the unique EPR spectrum of DMPO-OOH. The first is that the gamma-splitting arises from one of the hydrogen atoms at either carbon 3 or carbon 4 of DMPO-OOH. The second is that the gamma-splitting originates from the hydrogen atom of DMPO-OOH. The third is that the conformational properties of DMPO-R change upon going from DMPO-OH to DMPO-OOH. Experimental and theoretical chemical approaches as well as EPR spectral modeling were used to investigate which of these hypotheses may explain the asymmetric EPR spectrum of DMPO-OOH. From these studies it is shown that the 12-line EPR spectrum of DMPO-OOH results not from any proximal hydrogen, but from additional conformers of DMPO-OOH. Thus, the 1.25 G hyperfine splitting, which has been assigned as a gamma-splitting, is actually from two individual EPR spectra associated with different conformers of DMPO-OOH.