Anisotropic Hyperfine Coupling Research Articles

Hydrogenases in the "active" state: determination of g-matrix axes and electron spin distribution at the active site by 1H ENDOR spectroscopy.

Hydrons and electrons are substrates for the enzyme hydrogenase, but cannot be observed in X-ray crystal structures. High-resolution 1H electron nuclear double resonance (ENDOR) spectroscopy offers a means to detect the distribution of protons and unpaired electrons. ENDOR spectra were recorded from frozen solutions of the nickel-iron hydrogenases of Desulfovibrio gigas and Desulfomicrobium baculatum, in the "active" state ("Ni-C" EPR signal) and analyzed by orientationally selective simulation methods. The experimental spectra were fitted using a structural model of the nickel-iron centre based on crystallographic results, allowing for differences in electron spin distribution as well as the spatial orientation of the g-matrix ( g-tensor), and anisotropic and isotropic hyperfine couplings of the protons nearest to the nickel ion. ENDOR signals, detected after complete deuterium exchange, were assigned to six protons of the cysteines bound to nickel. The assignment took advantage of the substitution of a selenium for a sulfur ligand, which occurs naturally between the [NiFeSe] and [NiFe] hydrogenases from Dm. baculatum and D. gigas, respectively, and was found to affect just two signals. The four signals with the largest hyperfine couplings, including isotropic contributions from 4.5 to 13.5 MHz, were assigned to the beta-methylene protons of the two terminal cysteine ligands, one of which is substituted by seleno-cysteine in [NiFeSe] hydrogenase. The electron spin is delocalized onto the nickel (50%) and its sulfur ligands, with a higher proportion on the terminal than the bridging ligands. The g-matrix was found to align with the active site in such a way that the g1- g2 plane is nearly coplanar (18.3 degrees) with the plane defined by nickel and three sulfur atoms, and the g2 axis deviates by 22.9 degrees from the vector between nickel and iron. Significantly for the reaction of the enzyme, direct evidence for the binding of hydrons at the active site was obtained by the detection of H/D-exchangeable ENDOR signals.

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.

Model calculations of radiation induced damage in 1-methylthymine:9-methyladenine

Detailed electron paramagnetic resonance and electron nuclear double resonance experiments on the co-crystalline complex of 1-methylthymine:9-methyladenine (MTMA) have revealed that the major radiation induced products at low temperatures (10 K) are MTMA1, a radical formed by net hydrogen abstraction from the C5 methyl group on thymine, and MTMA2, a radical formed by net hydrogen abstraction of the N1 methyl group on thymine. The following four minor products were also observed: MTMA3, the C4–OH protonated anion of thymine, MTMA4, the C6 H-addition product of thymine, and MTMA5 and MTMA6, radicals formed by net H-addition to C2 and C8 of the adenine base. The geometries, energetics and hyperfine properties of all possible radicals of MT and MA, the primary anions and cations, as well as radicals formed via net hydrogen atom abstraction (deprotonated cations) or addition (protonated anions) were investigated theoretically. All systems were optimized using the hybrid Hartree–Fock density functional theory functional B3LYP, in conjunction with the 6-31G(d,p) basis set of Pople and co-workers. Calculations of the anisotropic hyperfine couplings for all the radicals observed in MTMA are presented, and are shown to compare favorably with the experimentally measured hyperfine couplings. The calculated ionizations potentials indicate that MA would be the preferred oxidation site. However, in MTMA neither the adenine cation nor its N4-deprotonated derivative were observed. The adenine cation in MTMA is not stabilized by deprotonation, and is thus likely subject to recombination. The calculated electron affinities indicate that MT would be the preferred reduction site. Reduction of MT is believed to result in protonation of the anion at C4O. The calculated hyperfine couplings for the MT anion are very similar to those of the C4–OH protonated anion, and therefore, the theoretical calculations are not useful in deciding the actual protonation state of this reduction product.

Density Functional Theory Study of the β-Carotene Radical Cation

The hybrid density functional theory method B3LYP is employed to study the β-carotene radical cation. The radical is characterized by means of its geometry, spin distribution, and isotropic and anisotropic hyperfine coupling constants. It is shown that the spin is delocalized over the whole π-conjugated system, including the double bonds of the headgroups. This delocalization results in methyl hyperfine coupling constants lower than 9 MHz, in excellent agreement with recent experimental couplings of the carotene radical in Photosystem II and in vitro, but in conflict with previous theoretical calculations. It is also demonstrated that rotation of the headgroups can affect the properties of the radical, in particular the spin delocalization to the ring.

ENDOR study of the chlorinated thiocytosine radical in a crystal matrix

Interaction of protons with the unpaired electron in the σ2σ*1 three-electron S∴Cl bond has been investigated by electron nuclear double resonance (ENDOR) spectroscopy. The S∴Cl bond in the chlorinated thiocytosine radical, imbedded in the crystal lattice of cytosine hydrochloride, interacts significantly with at least seven protons. Five of these interactions have been analyzed. From the nature of the coupling tensors, it is concluded that three couplings are due to protons being constituents of the chlorine–thiocytosine radical, and the remaining two are due to protons belonging to neighboring cytosine molecules. The nature of the dipolar component of the coupling tensors is not consistent with the location of the entire electron spin density in the S∴Cl bond. From isotropic as well as anisotropic hyperfine coupling tensor components, it is concluded that the unpaired electron is somewhat delocalized over the radical, with a major portion being concentrated in the C(2)–S–Cl region but with a significant fraction occupying a π* molecular orbital extending over the cytosine ring atoms. Density functional theory molecular orbital calculations support such a kind of spin distribution. The data also suggest that the radical remains protonated at both the N(1) and N(3) positions.

Probing magnetic properties of the reduced [2Fe-2S] cluster of the ferredoxin from Arthrospira platensis by 1H ENDOR spectroscopy.

The 1H electron nuclear double resonance (ENDOR) spectra in frozen solutions of the reduced [2Fe-2S] cluster in ferredoxin from Arthrospira (Spirulina) platensis have been measured at low temperatures (5-20 K) and simulated using orientational selection methods. The analysis confirmed the existence of a single paramagnetic species with iron valence states II and III connected uniquely to the cluster irons. The experimental ENDOR spectra were fitted to a model including the spin distribution on the centre, the orientation of the g-matrix, and the isotropic and anisotropic hyperfine couplings of the nearest protons in the crystallographically determined structure. In order to partially simulate ENDOR line shapes, a statistical distribution of the corresponding torsion angles between the Fe(III) centre and one of the beta-CH2 protons was introduced. From the analysis, four of the larger hyperfine couplings found were assigned to the cysteine beta-protons near the Fe(III) ion of the cluster, with isotropic hyperfine couplings ranging from 1.6 to 4.1 MHz. The spin distribution on the two iron ions was estimated to be +1.85 for the Fe(III) ion and -0.9 for the Fe(II) ion. The Fe(III) ion was identified as being coordinated to the cysteine ligands Cys49 and Cys79, confirming previous NMR results. The direction of the g-tensor with respect to the cluster was deduced. The g1-g2 plane is parallel to the planes through each iron and its adjacent cysteine sulfurs; the g2-g3 plane is nearly perpendicular to the latter planes and deviates by 25 degrees from the FeSSFe plane. The g1 direction is dominated by the bonding geometry of Fe(II) and does not align with the Fe(II)-Fe(III) vector.

Evaluation of MNDO approximation in quantum-chemical calculations ofg-tensors of free radicals

The MNDO approximation was tested for applicability in calculations of theg-tensors of free radicals. The representative test set included 15 free radicals for which the isotropic and anisotropic hyperfine coupling constants were calculated previously in the framework of the MNDO approach.

Hybrid Density Functional Studies of Pheophytin Anion Radicals: Implications for Initial Electron Transfer in Photosynthetic Reaction Centers

The electronic structure of the pheophytin a anion radical is calculated using hybrid density functional calculations at the B3LYP level. 1H, 14N, 17O, and 13C isotropic and anisotropic hyperfine couplings are calculated. Experimentally determined hyperfine couplings for 14N and 1H positions are in excellent agreement with theoretically calculated values. Comparison of the spin density distribution of the pheophytin radical with the related bacteriopheophytin form shows that the pheophytin radical exhibits a greater asymmetry in the distribution of spin around the ring system compared with its bacterial counterpart. In addition the delocalization of spin density onto the 3-acetyl group of bacteriopheophytin is greater than the delocalization calculated on to the corresponding vinyl group of pheophytin. This decreased delocalization for pheophytin is proposed to be responsible for the increase in electron-transfer time from the primary acceptor bacteriochlorophyll, BA, to the secondary electron acceptor,HA , when pheophytin replaces bacteriopheophytin in the HA site of bacterial reaction centers. Optimal orbital overlap between BA and HA facilitates rapid electron transfer from BA and may be essential in the prevention of charge recombination between BA and the primary donor bacteriochlorophyll, D.

A Model for Photoreceptor-Based Magnetoreception in Birds

A large variety of animals has the ability to sense the geomagnetic field and utilize it as a source of directional (compass) information. It is not known by which biophysical mechanism this magnetoreception is achieved. We investigate the possibility that magnetoreception involves radical-pair processes that are governed by anisotropic hyperfine coupling between (unpaired) electron and nuclear spins. We will show theoretically that fields of geomagnetic field strength and weaker can produce significantly different reaction yields for different alignments of the radical pairs with the magnetic field. As a model for a magnetic sensory organ we propose a system of radical pairs being 1) orientationally ordered in a molecular substrate and 2) exhibiting changes in the reaction yields that affect the visual transduction pathway. We evaluate three-dimensional visual modulation patterns that can arise from the influence of the geomagnetic field on radical-pair systems. The variations of these patterns with orientation and field strength can furnish the magnetic compass ability of birds with the same characteristics as observed in behavioral experiments. We propose that the recently discovered photoreceptor cryptochrome is part of the magnetoreception system and suggest further studies to prove or disprove this hypothesis.

Evaluation of MNDO approximation in quantum-chemical calculations of anisotropic hyperfine coupling tensors in free radicals

The MNDO approximation was tested for applicability in quantum-chemical calculations of anisotropic hyperfine coupling tensors using the same set of 17 free radicals as that used previously in evaluating a new procedure for quantum-chemical estimates of constants of isotropic hyperfine coupling with protons in the framework of the same approach.

Hybrid density functional studies of a bacteriopheophytina model and its anion radical form: Geometry, spin densities, and hyperfine couplings

Hybrid density functional calculations are performed on models of the bacteriopheophytin a molecule, which acts as an electron acceptor in the initial electron transfer events of bacterial photosynthesis. The geometry of the neutral and one-electron reduced free radical form are calculated. Little variation in the main π framework geometry is predicted to occur on one-electron reduction. The electron density of the SOMO for the anion free radical form is also investigated, and the unpaired spin density distribution is also analyzed. Anisotropic and isotropic hyperfine couplings are calculated and discussed on the basis of the unpaired spin density distributions. The electron density of the SOMO is shown to extend on to the carbonyl group attached to ring A; the extent of this delocalization being dependent on the twist angle, this groupmakes with the main ring plane with the magnitude of the 1H hyperfine couplings of the 2methyl group strongly dependent on the value of this twist angle. Good agreement is observed between calculated and experimentally measured isotropic hyperfine couplings. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1292–1298, 1999

A quantum mechanical periodic ab initio approach to materials science: the CRYSTAL program

The capabilities of the present version of the CRYSTAL program (CRYSTAL98) in the description of the structural, electronic and magnetic properties of perfect and defective crystalline systems will be illustrated with reference to three examples: (i) The relative stability of the ferromagnetic and anti-ferromagnetic structures of a transition metal insulator; we will illustrate how different magnetic states can be obtained and evaluate the corresponding superexchange coupling constant J. (ii) The relative stability of the different acidic sites obtained by inserting an Al atom at (and by linking the accompanying H atom to) non-equivalent zeolite framework positions has been evaluated; the geometry of the various Si(OH)Al groups and the frequency of the OH stretching are compared. (iii) The physico-chemical features of a trapped hole formed when a monovalent alkali metal ion is substituted for a divalent cation in bulk alkaline earth oxides; the properties computed and compared with available experimental data are: the degree of localization of the hole, the relaxation of the defect ion and its neighbours, the electric field gradient, the Fermi contact term and the anisotropic hyperfine coupling constant.

Photoinduced Isomerization of trans-Acetylene Radical Anion to Vinylidene Radical Anion in 2-Methyltetrahydrofuran

Photoinduced reactions of trans-HC⋮CH- in a glassy 2-MTHF matrix were studied by ESR and electron absorption (EA) spectroscopy. ESR and EA spectra were interpreted with the aid of the results of molecular orbital (MO) calculations. The trans-HC⋮CH- was generated and stabilized in the 2-MTHF matrix at 77 K by ionizing irradiation. The ESR parameters observed for the radical anion are A1 = 5.3, A2 = 4.6, A3 = 4.5 mT for two equivalent 1H atoms, A⊥ = 1.5, A∥ = 4.2 mT for two equivalent 13C atoms, and g1 = 2.0009, g2 = 2.0024, g3 = 2.0029 for the g tensor. By illuminating with light of λ ≥ 430 nm, the trans-HC⋮CH- was found to be irreversibly isomerized into the vinylidene radical anion, H2CC-, in the matrix. The experimental ESR parameters of H2CC- [A1 = 5.4, A2 = 6.0, A3 = 5.7 mT for two equivalent protons, g1 = 2.0026, g2 = 2.0008, g3 = 2.0026] were fully consistent with the results obtained by semiempirical MO methods, (1) INDO and Analytical Dipolar Calculation method (ANADIP) for isotropic and anisotropic 1H hyperfine (hf) couplings, respectively, and (2) AM1 for the g tensor, for a geometrical structure optimized by ab initio method (Gaussian90/uhf/6-31++G**). The structure of the anions was in agreement with the results obtained from the EA spectroscopic study. The photoinduced isomerization was initiated by the light absorption at λ ≥ 430 nm of trans-HC⋮CH-. The EA band at λmax = 374 nm observed for H2CC- was attributed to the electron transition from the 2B2 ground state and to the 2A1 excited state.

The Electronic Structure of the Bacteriopheophytin a Anion Radical, in Vivo

Density functional calculations are used to calculate the electronic structure of the bacteriopheophytin a radical anion, φA, formed in the initial electron-transfer reactions of bacterial photosynthesis. Using the hybrid B3LYP functional together with the double ζ basis set EPR−II, 13C, 1H, 17O, and 14N isotropic and anisotropic hyperfine couplings are calculated and explained by reference to the electron density of the highest occupied molecular orbital and of the unpaired spin distribution around the radical. Good agreement is observed between calculated and experimental hyperfine couplings. Hydrogen bonding to the carbonyl group of ring E leads to minor changes in the unpaired spin density distribution and the resultant hyperfine couplings. The electronic structure of the anion radical form of the other bacteriopheophytin a molecule, φB, found along the inactive-electron-transfer (B) branch, is also studied, and the calculated electronic properties are compared with φA. It is shown that, whereas the e...

ENDOR and ESR Studies of Radical Cations of Methyl-Substituted Benzene in Halocarbon Matrices

The radical cations of methyl-substituted benzene toluene, p-xylene, o-xylene, m-xylene, and their deuterated analogues generated in CFCl3 and CF3CCl3 matrices by X-ray irradiation at 77 K were investigated by ESR and high-resolution ENDOR spectroscopy. The ESR and ENDOR spectra in these radical cations are dominated by large axially symmetric hyperfine splitting due to methyl-group protons. The anisotropic hyperfine coupling constants (hfcc) of methyl protons and ring protons in radical cations of toluene and p-xylene were measured accurately by ENDOR spectroscopy. In the case of deuterated p-xylene and o-xylene radical cations in CF3CCl3 matrix, the hyperfine coupling constants are scaled by their magnetogyric ratio (γH/γD = 6.51). Theoretical calculations of the isotropic and dipolar hyperfine coupling constants in these radical cations are found to be in good agreement with experimental results. The experimental and theoretical calculations supported the stabilization of the 2B2g type of state for the...

Cluster calculations of the hyperfine properties of copper compounds

Spin-polarized ab-initio cluster procedures are used to investigate the electronic structure of CuO2-planes in the La2CuO4 and YBa2Cu3O7 compounds. In particular, electric field gradients (EFGs) and magnetic hyperfine interaction parameters for the Cu sites were calculated with various theoretical methods. Calculations with density functional theory reproduce the experimentally determined EFGs fairly well while those at the Hartree-Fock level generally yield values which are too large by about 40%. The anisotropic magnetic hyperfine coupling can also be calculated with reasonable accuracy.

Hyperfine sublevel correlation spectroscopy in lithium silicate glasses

The structure of γ-irradiated (Li2O)0.1(SiO2)0.9 glass has been studied with two-dimensional hyperfine sublevel correlation spectroscopy (HYSCORE). The theoretical evaluation of the HYSCORE experimental data revealed the existence of weak couplings between the electron spin and nearby Li. The anisotropic hyperfine coupling T is equal to 4.0 MHz suggesting a nonbridging oxygen—Li distance of 2.0 Å and an angle of ±60° away from the gz axis. The Aiso value is equal to 0.5 MHz, corresponding to 0.135% contribution of the Li 2s orbital to the ground state. Thus, an estimation can be made of the covalent character of the bond between the nonbridging oxygen and the alkali.

Analysis of13C−NMRspectra inC60superconductors: Hyperfine coupling constants, electronic correlation effect, and magnetic penetration depth

A C-NMR anisotropic hyperfine coupling tensor was determined as 2p(21.68, 21.68, 3.37) 310 rad/sec for C60 32 in A3C60 superconductors, where A is an alkali metal, by analyzing C-NMR spectra below 85 K. Combined with an isotropic coupling constant of (2p30.69)310 rad/sec, the 2s and 2p characters of the electronic wave functions at the Fermi level were deduced. The results were compatible with local-density-approximation band calculations. From a simulation of C-NMR spectra at superconducting state, the traceless chemical ~orbital! shift tensor and isotropic chemical shift were determined as ~67, 34, 2101! ppm and ;150 ppm, respectively. An estimated magnetic penetration depth is larger than 570 nm in K3C60. Furthermore, the modified Korringa relation, T1TK ;bS ~with Knight shift K, spin-lattice relaxation time T1 , and Korringa constant S!, clearly showed the existence of weak but substantial antiferromagnetic spin fluctuation in A3C60; b50.40– 0.58 with an error of 620%. The Stoner enhancement factor was also determined as 1–1.5 from a comparison between spin susceptibility obtained from NMR and band-calculation results. @S0163-1829~98!00542-6#

Density Functional Theory Calculations on 19-Electron Organometallic Complexes: The Mn(CO)5Cl- Anion. The Difference between Unpaired Electron Density and Spin Density Due to Spin Polarization

The anisotropic hyperfine coupling (hfc) constants for the 19-electron Mn(CO)5Cl- complex, 1, and for the 17-electron Mn(CO)4Cl- complex, 2, were calculated using density functional theory (DFT). The calculated hfc values for 1 are in good agreement with the experimental ones reported in two EPR studies, which were not able to distinguish between 1 and 2. The Mn−CO(axial) bond dissociation energy in 1 was calculated to be 19 kcal/mol, which shows that 1 is stable to loss of axial CO. These data indicate that the species observed in the EPR experiments was the 19-electron complex, 1. The unpaired electron population of the manganese dz2 orbital indicated by the SOMO of the DFT wave function (ca. 0.2−0.3) is significantly less than that obtained from the two EPR analyses (0.49, 0.63), which neglected spin polarization. The calculations show that spin polarization in 1 causes the spin density (as measured by EPR) to differ significantly from the unpaired electron density (SOMO). It is concluded that neglect ...

1H, 13C, and 17O Isotropic and Anisotropic Hyperfine Couplings for the Plastosemiquinone Anion Radical

Hybrid density functional calculations (B3LYP) are performed for the plastosemiquinone anion radical in its free form and in models to simulate its hydrogen bonding environment in alcohol solvents. 1H, 13C, and 17O isotropic and anisotropic hyperfine couplings are calculated. The effect of hydrogen bonding interactions in the alcohol solvent model is shown to lead to a redistribution of spin density from the oxygen atoms to the carbon atoms of the carbonyl groups. 1H calculated hyperfine couplings are compared with previous assignments based on ENDOR measurements. A rearrangement of the experimental assignments for the methyl group couplings is indicated and a reinterpretation of the 1H hydrogen bonding data is presented.