Adrian R. Jaszewski

Influence of Pb(II) ions on the EPR properties of the semiquinone radicals of humic acids and model compounds: high field EPR and relativistic DFT studies.

X-band (9.76 GHz) and high field (416.00 GHz) electron paramagnetic resonance spectroscopy (EPR) was used to study the interactions between Pb(II) ions and semiquinone radicals of natural humic acids and their simple models. The EPR experiments were performed on powder samples. The formation of Pb(II) complexes with the radicals was accompanied by a significant decrease of g parameters as compared to those observed for parent radicals. Two types of complexes were identified depending on the initial concentration of Pb(II) ions. For one of them the anisotropic hyperfine coupling with the (207)Pb nucleus was observed. Systematic DFT calculations were carried out for complexes with different forms of radical ligands (L(2)(-*), HL(-*), and H(2)L*) derived from 3,4-dihydroxybenzoic acid representing different ligation schemes. The g parameters calculated for the structure characterized by a significant accumulation of spin density on the Pb atom are strongly deviated from the values observed experimentally. Moreover, a decrease of the spin population on all oxygen atoms as a result of complexation of Pb(II) via carboxyl oxygens and protonation of hydroxyl oxygens is required to reproduce the experimental g parameters.

Hybrid density functional approach to the isotropic and anisotropic hyperfine couplings with 14N and 1H nuclei in the blue copper proteins

We report UB1LYP hybrid density functional studies on the hyperfine parameters of wild-type azurin from Pseudomonas aeruginosa and M121Q mutant of azurin from Alcaligenes denitrificans. The large models of the copper site used in the calculations give quantitative insight into the spin density distribution and confirm highly delocalized character of the unpaired electron. Theoretically predicted isotropic and anisotropic hyperfine couplings are compared to the available EPR data and the assignment of the hyperfine splittings is verified.

Hybrid density functional approach to the structures and EPR parameters of σ-type iminoxy radicals: the case of 3-oxobutan-2-iminoxyl

Abstract Hybrid density functional calculations are used to study geometries together with 1 H , 13 C , 14 N and 17 O isotropic hyperfine coupling constants for the 3-oxabutan-2-iminoxy radical. The effect of 12 different basis sets on the calculated hyperfine parameters is investigated. Particular emphasis is placed on the performance of the B3LYP functional in the description of the unpaired electron interaction with the iminoxy 17 O nucleus. It is found that the EPR-III basis sets of Barone and TZVP of Godboud and Andzelm lead to the best quantitative agreement between theoretical and experimental hyperfine constants; the mean absolute deviation from experiment for 13 C , 14 N and 17 O is ca. 2 G.

Structural insight into an ankyrin-sensitive lipid-binding site of erythroid β-spectrin

It was recently shown that the region within β-spectrin responsible for interactions with ankyrin includes a lipid-binding site which displayed sensitivity to inhibition by ankyrin. We studied its structure by constructing a series of single and double spin-labeled β-spectrin-derived peptides and analyzing their spin-spin distances via electron paramagnetic resonance spectroscopy and the Fourier deconvolution method. The results indicate that the whole ankyrin-sensitive lipid-binding site of β-spectrin exhibits a helical conformation revealing a distinct 310-helix contribution at its N-terminus. The start of the helix was located five residues upstream along the sequence compared to the theoretical predictions. A model based on the obtained data provides direct evidence that the examined lipid-binding site is a highly amphipathic helix, which is correlated with the specific conformation of its N-terminal fragment.

EPR spectroscopy and hybrid density functional studies of dialkoxyphosphinyl-phenyl iminoxy radicals

Abstract The dimethoxyphoshinyl-phenyl and 13 C enriched diisopropoxyphosphinyl-phenyl iminoxy radicals have been prepared by reaction of corresponding oximes with AgO. The magnetic properties of the radicals have been studied by a combined experimental and theoretical approach. The results of B3LYP and B1LYP computations are in excellent agreement with the EPR parameters determined on the basis of simulation of the experimental spectra. A difference observed in experimental hyperfine coupling with the iminoxy 13 C nucleus on going from Z to E isomer of the radical, 16.9 and 24.2 G, respectively, is discussed on the basis of calculated spin density distribution and natural bond orbitals analysis.

Hyperfine parameters of pyridyl-containing iminoxy radicals generated from oximes by γ-irradiation and in zeolite lattice: EPR spectroscopy and density functional study

Abstract Incorporation of pyridyl oximes into the channels of thermally activated pentasil zeolite ZSM-5 gives rise to iminoxy radicals. EPR spectrum of the species derived by dehydrogenation of di-2-pyridilketone oxime is fully anisotropic and characteristic for the iminoxy σ-type neutral radical. The EPR spectra of the radicals derived from 2-pyridinealdoxime and 4-pyridinealdoxime exhibit additional splitting due to the azomethine hydrogen. The ĝ and  tensor parameters were determined on the basis of EPR spectra. In order to identify a geometric isomer (E or Z) of the iminoxyls obtained in the pores of the zeolite and within γ-irradiated samples of solid oximes, hyperfine interaction parameters of the generated radicals were compared with the parameters calculated using density functional theory (DFT); these calculations allow to characterize the iminoxy radical isomers formed in zeolite and oxime matrices.

The view on the CN–O group in the RHCNO (R=H, BH2,CH3,NH2,OH,F) iminoxy radicals by means of electron localisation function (ELF)

Abstract The topological analysis of the electron localisation function (ELF) for a variety of RHCNO iminoxy radicals shows that Z to E isomerisation results in a redistribution of electron density from the N and O lone electron pairs to the N–O and CN bonds. Substitution of R by more electronegative group is associated with increase of 〈S〉 for V1(O)∪V2(O) and decrease for V(N). The mean electron population of V(R,C) basin decreases with growing electronegativity of R. Dependence of A iso ( 1 H ) hyperfine coupling on the R substituent may be explained as a consequence of an electron density delocalisation between the C–H and C–R bonds: V(C,H)⇔V(C,R).

The effect of the carbonyl moiety on the spin density delocalization in the iminoxy radicals. Hybrid density functional studies

Abstract UB1LYP method was used to study the influence of carbonyl group in 1,2-diphenyl-1-oxoethan-2-iminoxyl and 1-oxoethan-2-iminoxyl, on radical structure and unpaired electron spin density distribution. A significant change of radical properties was found due to the second carbonyl group in 2,4-dioxopenthan-3-iminoxyl and 1,3-dioxopropan-2-iminoxyl. A specific impact of the carbonyl groups on the EPR hyperfine couplings with 1 H and 13 C nuclei (calculated at UB1LYP/EPR-III level in comparison with the experimental data) was determined for the stable isomeric forms of the radicals.

Hybrid density functional studies on the EPR parameters of heterosubstituted vinyl radicals: substituent effect on the isotropic hyperfine couplings with 1H and 13C nuclei

Isotropic hyperfine parameters of a set of vinyl radicals are investigated using the B1LYP hybrid density functional. The systems studied are RHβCβCαHα radicals, where R=H, BH2, CH3, NH2, OH and F. Theoretical results indicate that electronegativity of the substituent strongly affects the magnitude of hyperfine coupling with hydrogen nuclei as well as with . varies from −8.7 (4.9) to 17.4 G (−17.8 G) for Z (E) isomers of the radicals depending on the R group, BH2 and F, respectively. In the same order, for Z (E) isomeric forms diminishes from 40.1 (67.7) to 18.4 G (40.9 G) and – from 25.6 (24.1) to 1.5 G (1.3 G). The effect of the substituents on the spin and electron density distribution is discussed in the framework of natural population analysis and theory of atoms in molecules.

HDF and QCI studies on the heterosubstituted iminoxy radicals: substituent effect on the isotropic hyperfine couplings with 13C and 1H nuclei

Abstract Isotropic hyperfine parameters of a set of iminoxy radicals are investigated using hybrid density functional and quadratic configuration interaction methods. The systems studied are R1R2CNO radicals, where R1H, BH2, CH3, NH2, OH, F, AlH2, SiH3, PH2, SH, Cl and R2H. Theoretical results indicate that magnitude of the hyperfine coupling with iminoxy 13C nucleus due to the particular isomer of the radical is strongly affected by electronegativity of the substituents R1 and R2. A iso ( 13 C ) varies from 6.7 to 56.2 G for Z (and from 45.6 to 18.7 G for E) isomers of the iminoxyls depending on the substituent (AlH2 and F, respectively).