Unpaired Electron Spin Density Research Articles

ENDOR and special triple resonance studies of chlorophyll cation radicals in photosystem 2.

Electron nuclear double resonance (ENDOR) and special triple (ST) resonance spectroscopies have been used to study the cation radicals of the primary donor, P680, and two secondary donor chlorophylls (Chl) in photosystem 2 (PS2). Two different preparations were employed, Tris-washed PS2 membranes and PS2 reaction centers (D1-D2-I-Cytb559 complex). One secondary donor Chl a cation radical, Chl1.+, was generated in the Tris-washed preparation, while the P680.+ radical cation and a further Chl a cation radical, Chl2.+, were produced in the reaction center preparation. The ENDOR spectrum of the primary donor radical cation of photosystem 1 (P700.+) is also presented for comparison. Hyperfine coupling constants for methyl groups have been measured for all three PS2 radical species and assigned by comparison with previously published spectra of Chl a radicals in vitro. Electron spin densities were calculated from these hyperfine couplings. Comparison of ENDOR spectral features with those of Chla.+ in vitro indicates similar values for Chl1.+ and Chl2.+ radicals but an apparent reduction in unpaired electron spin density for P680.+. It has been proposed from the more detailed studies of purple bacterial reaction centers that such a reduction in spin density can be interpreted as a delocalization over two Chl a molecules. Our calculations therefore suggest that P680.+ is a weakly coupled chlorophyll pair with 82% of the unpaired electron spin located on one chlorophyll of the pair at 15 K. Environmental or geometrical changes to the chlorin ring structure to give a novel monomeric primary donor are also possible.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and structure of tetracoordinated nickel(II) complexes of deprotonated chelated aminoazo ligands: X-ray crystal structure of bis[1-isopropyl-3-methyl-4-(4-methylphenylazo)-5-(4-methoxyphenylamidato)pyrazole] nickel(II)

Tetracoordinate nickel(II) complexes NiL2 derived from the deprotonated forms of aminoazoligands HL were prepared and investigated by spectroscopy and magnetic susceptibility measurements. The magnetic moments, which lie in the 3.1–3.6 B.M. range correspond to the occurrence both in solution and in the solid state of the high spin (S = 1) form and a tetrahedral configuration at the metal centre. The u.v. spectra exhibit three ligand field bands at 1020–1280 nm characteristic of high spin nickel(II) complexes. The large isotropic chemical shifts found in the 1H n.m.r. spectra are consistent with partial delocalization of unpaired electron spin density to the ligand HOMO. The X-ray single crystal structure of NiL 2 4 [L4= 1-isopropyl-3-methyl-4-(4-methylphenylazo)-5-(4-methoxyphenylamino)-pyrazole] reveals that the metal is coordinated by four nitrogen atoms in tetrahedral configuration with an angle of 90° between the N(1)NiN(2) and N(6)NiN(7) planes belonging to the different almost planar metallocycles. The rates of R⇄S interconversion of the tetrahedral configuration for NiL 2 4 and NiL 2 5 [L5 = 1-isopropyl-3-methyl-4-phenylazo-d 5-5-(4-methoxyphenylamino)pyrazole] are slow on the n.m.r. timescale. In contrast to NiL 2 1 -Ni 2 6 , NiL 2 7 ], which contains coordinated NH-groups instead of NAr-groups, is planar.

Redox-dependent 1H NMR spectral features and tertiary structural constraints on the C-terminal region of putidaredoxin.

Putidaredoxin (Pdx) is a 106-residue Fe2S2 ferredoxin which acts as the physiological reductant and effector of cytochrome P-450cam. Pdx has two accessible oxidation states, Fe+3-Fe+3 (oxidized) and Fe+3-Fe+2 (reduced), and exhibits redox-dependent binding affinities for cytochrome P-450cam, with reduced Pdx binding over 100-fold more tightly than oxidized Pdx to the oxidized cytochrome P-450cam [Hintz, M. J., Mock, D. M., Peterson, L. L., Tuttle, K., & Peterson, J. A. (1982) J. Biol. Chem. 257, 14324-14332]. The analysis of two-dimensional 1H NMR experiments has yielded sequential 1H resonance assignments for the diamagnetic regions of the reduced form of Pdx, which are compared to those of oxidized Pdx, described previously [Ye, X. M., Pochapsky, T. C., & Pochapsky, S. S. (1992) Biochemistry 31, 1961-1968]. Increased unpaired electron-spin density on the metal cluster in reduced relative to oxidized Pdx increases the number of 1H resonances which are broadened by the metal cluster, and the pattern of paramagnetic broadening provides information concerning the placement of the metal cluster within the protein. Two-dimensional exchange experiments on half-reduced samples of Pdx indicate that electron self-exchange is slow on the chemical shift time scale, with a second-order rate constant < or = 66 M-1 s-1 at 290 K. Spectral changes unrelated to increases in unpaired electron-spin density are also observed. The largest changes of this type are observed for features structurally contiguous with the C-terminal region Pro 102-Trp 106. The C-terminal residue Trp 106 has been implicated in binding to cytochrome P-450cam.(ABSTRACT TRUNCATED AT 250 WORDS)

Structures and dynamics of the lowest excited triplet state, radical cation and radical anion of 1,4-diphenylbutadiyne: time-resolved resonance Raman study

Time-resolved resonance Raman spectra of the lowest excited triplet state T 1, the radical cation R + and the radical anion R − of 1,4-diphenylbutadiyne (DPB) have been measured. It has been shown that the CC symmetric stretch exhibits low-frequency shifts in the order, S o (2219 cm −1), R + (2156 cm −1), R − (2108 cm −1), and T 1 (1999 cm −1), indicating that the CC triple bond weakens in this sequence. However, the frequency shifts are not as large as those observed for diphenylacetylene (DPA), suggesting that in the case of DPB the unpaired electron (spin density) is distributed over the two CC triple bonds of the conjugated system, which makes the weakening of the CC bonds of DPB much less than that of DPA. It has also been shown that like DPA, the ionization of DPB to generate R + can occur biphotonically through the T 1 state.

Electron spin resonance spectroscopy of free radicals

AbstractSolution and powder ESR spectra of the 4‐trifluoromethyl‐1,2,3,5‐dithiadiazolyl and 4‐pentafluorophenyl‐1,2,3,5‐dithiadiazolyl free radicals were analysed. Comparison of the ESR parameters with those obtained for the protonated equivalents shows that fluorination produces a drift of unpaired electron spin density away from the nitrogen 2px orbitals towards sulphur 3px orbitals (x is the direction perpendicular to the heterocyclic ring), consistent with an increase in electronegativity of the substituent. The fluorinated radicals dimerize more readily than their protonated counterparts.

Bond-centered muonium in diamond: Resolved nuclear hyperfine structure.

The nuclear hyperfine structure of bond-centered muonium in $^{13}\mathrm{C}$ enriched diamond has been resolved using time-differential transverse-field muon spin rotation. The measured nearest-neighbor $^{13}\mathrm{C}$ hyperfine parameters are compared to theoretical values from a recent ab initio Hartree-Fock type cluster calculation. The ratios of the measured $^{13}\mathrm{C}$ hyperfine parameters and free atom values indicate that 92% of the unpaired electron spin density resides on the two nearest-neighbor atoms, in marked contrast with the situation for the analogous muonium center in Si.

Electron paramagnetic resonance of Sc3+-stabilized CO 3 3? molecule-ion in natural calcite

Electron paramagnetic resonance (EPR) spectra of CO 3 3− molecule-ions stabilized by Sc3+ in natural calcite were identified and studied at X-band frequencies and room temperature. The principal values of the g-tensor (gx′x′= 1.9997, gy′y′ = 2.0030, gz′z′ = 1.9972) and the direction cosines of the g and A tensors for CO 3 3− -Sc3+ center were found to be close to that for the well-known CO 3 3− -Y3+ center. A quantitative comparison of different impurity contents in calcite samples and analysis of the intensities of forbidden transitions were used to identify Sc3+. An estimation of the unpaired electron spin density on the nuclei of paramagnetic centers confirms that both centers, CO 3 3− -Sc3+ and CO 3 3− -Y3+, have the same nature.

Electronic structure of anomalous muonium in GaP and GaAs.

The nuclear hyperfine structure of anomalous muonium in GaP has been resolved using muon level-crossing resonance. We find that 43% of the unpaired electron spin density resides on the nearest-neighbor Ga and 35% on the nearest-neighbor P on the 〈111〉 axis of symmetry. The s and p character of the unpaired spin density indicates that the Ga and P are displaced 0.23 and 0.54 \AA{} away from the bond center. The now complete set of measured muon and nearest-neighbor nuclear hyperfine parameters for anomalous muonium in GaP and GaAs allows a detailed comparison of the two compounds, showing that the distribution of spin density among the group-III and group-V nearest neighbors in GaP is almost exactly the opposite of that in GaAs. This is contrary to what one would expect from a simple model of anomalous muonium in compound semiconductors involving an account of bonding characteristics.

Electronic Constitution of Paramagnetic 3d Complexes and Positronium Conversion Rate Constants

Ortho-positronium may be converted into para-positronium (and vice versa) by interacting with paramagnetic compounds such as solvated electron 3d and 4f complexes, etc. The rate constants of orthointo para-Ps conversion reactions, kSE, depend on the particular paramagnetic compound considered, e.g. the kSE's of ammoniated electron (e-am), 3d and 4f aquoions are in the ratio 10:1:0.01 [1, 2]. This finding suggested that the kSE 's may depend on the spin density of unpaired electrons at the collision site between Ps and paramagnetic compound. In order to support the hypothesis, a systematic investigation was carried out, in particular, the kSE of the aquoion Cr(III) complexes listed in Table I and III were checked or measured for the first time.

ESR spectroscopy of some dicyano-, pyrazino- and quinoxalino-1,3,2-dithiazolyl radicals

Several symmetrical 1,3,2-dithiazol-2-yl radicals have been examined that have in common nitrogen-containing substituents at the 4- and 5-positions, namely 4,5-dicyano-1,3,2-dithiazol-2-yl, [15N2]-1,3,2-dithiazolo [4,5-b] pyrazin-2-yl and 1,3,2-dithiazoleo [4,5-b] quinoxalin-2-yl. The ESR spectrum of the 4,5-dicyano radical does not reveal any 14N splittings from the cyanide groups; however, the pyrazine and the quinoxaline derivatives do have multi-lined well resolved spectra arising from substantial hyperfine interactions with these nuclei. The interactions are sufficiently large to be observed in the powder spectra of the latter. The presence of proton hyperfine interactions shows that unpaired-electron spin density is delocalized over all the rings. It has been found that the radicals attached to pyrazine and quinoxaline can be incorporated into single crystals of naphthalene and phenazine, respectively.

Synthesis and NMR spectroscopy of metallocenium ions. Support for a new ferromagnetic coupling mechanism in decamethylmetallocenium tetracyanoethenides

A strategy has been developed to prove negative unpaired electron spin density in the Cp A orbitals of ferrocenium ions which is crucial for the McConnell-Kollmar-Kahn (MKK) mechanism of ferromagnetic coupling within (Cp*2Fe)+(TCNE)- stacks and-after extension to other sandwich cations-for the design of organometallic ferromagnets in general. To this end, the paramagnetic metallocenium ions ((EtMe,Cp),M)+ (M = Cr, Mn, Fe, Ni), ((iPrC~)~Fel+, and ((tBuCp),Fe)+ have been prepared as (PFJ salts and investigated by NMR spectroscopy. At ambient temperature the I3C and IH signal shifts cover ranges of almost 1300 and 170 ppm, respectively, with shift signs depending on the position of the nucleus under study and on the metal. For the metallocenium ions with M = Cr, Mn, and Ni the signs of the signal shifts show that the spin density in the Cp K orbitals is positive for Ni and negative for Cr and Mn. By contrast, the shift sign criterion fails for Fe. The different dihedral angles between the Ca-CO bonds and the Cp K orbitals in ((EtMelCp)ZFe)+ and the two ((RCp),Fe)+ derivatives probe the sign of the K spin density selectively via hyperconjugation; it is found to be negative. The experiments support the MKK mechanism in all cases, regardless whether the intrastack coupling in (Cp*ZM)+(TCNE)- be ferromagnetic or anti- ferromagnetic. The NMR data determine the spin distribution within a substituted Cp ligand which should be helpful for the optimization of the magnetic interaction. The restrictions for a quantitative application of the NMR method are discussed.

Pulsed EPR studies of polycrystalline cesium hexamethyl hexacyclen sodide

The electron spin echo envelope modulation (ESEEM) technique of pulsed EPR spectroscopy has been used to measure weak133Cs hyperfine couplings to trapped electrons in polycrystalline cesium hexamethyl hexacyclen sodide. Two magnetically distinct groups of weakly coupled133Cs nuclei were found — one with an isotropic electron-nuclear hyperfine (Fermi contact) coupling of 0.34 MHz and the other with a contact coupling less than 0.1 MHz. Analysis of these data, by computer simulation, shows that the number of Cs nuclei that give rise to these interactions and their distance from the paramagnetic center is consistent with the hypothesis that the trapped electrons occupy vacant anion sites. The results indicate that the spread of unpaired electron spin density, along the axis formed by the “contact ion pair,” may be greater than that in the plane perpendicular to this axis.

Resolved nuclear hyperfine structure of muonium in CuCl by means of muon level-crossing resonance

Datailed muon level-crossing resonance measurements of Mu1 and Mu11 centres in single crystals of CuCl are presented. The hyperfine and nuclear hyperfine parameters of the closest two shells of nuclei are remarkably similar for the two centres, indicating that both are located at the same tetrahedral interstitial site with four Cu nearest neighbours and six Cl next-nearest neighbours. About 30% of the total unpaired-electron spin density is located on the muon, about 60% on the four nearest neighbours and the rest on the six next-nearest neighbours, with nothing observable for any other shell.

Tyrosine radicals in photosystem II and related model compounds. Characterization by isotopic labeling and EPR spectroscopy

Deuteration at selected positions on the phenol ring and at the beta-methylene carbon for the YD.tyrosine radical in Photosystem II in the cyanobacterium Synechocystis 6803 was achieved by growing the organism under conditions in which it is a functional aromatic amino acid auxotroph (Barry, B. A., and Babcock, G. T. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 7099-7103). A series of model tyrosine radicals, also deuterated at specific sites on the aromatic ring and the methylene group, was generated by UV irradiation of frozen solutions. The EPR spectra of the specifically deuterated in vivo radicals confirm that YD.is a tyrosine; nevertheless its spectra differ from those of the tyrosine models. By comparing the EPR spectra of the specifically deuterated radicals with those of the fully protonated, the hyperfine couplings of the various protons of both YD.and the model compound radicals were determined. For both species, the unpaired electron spin density distribution is essentially identical and follows an odd-alternant pattern with high rho values at the carbons ortho and para to the tyrosine phenol oxygen; the meta positions have low spin densities. The differences in EPR spectral characteristics for the two radicals are rationalized as arising from variations in the conformation of the beta-methylene group with respect to the phenol head group. Considering these EPR results and those reported for other model and naturally occurring tyrosine radicals, we conclude that this situation is general; there is little deviation in this class of compounds from the odd-alternant spin density distribution; variations in EPR lineshapes arise primarily from variations in beta-methylene orientation. The conformation of the -CH2- group in biologically active tyrosine radicals deviates from that observed in the models and may be functionally significant. Because the EPR spectrum of YZ., the second redox active tyrosine radical in Photosystem II, is identical to that of YD., we conclude that the two radicals are in similar protein environments, a conclusion that is supported by the protein sequences in the vicinity of the two radicals.

Structure of thermal donors (NL8) in silicon: A study with electron-nuclear double resonance.

Singly ionized thermal donors [(TD${)}^{+}$], which give rise to the NL8 ESR spectrum, were investigated with electron-nuclear double resonance (ENDOR) in B-doped Czochralski-grown silicon and float-zone silicon into which the magnetic isotope $^{17}\mathrm{O}$ was diffused. TD's were formed by annealing at 460 \ifmmode^\circ\else\textdegree\fi{}C for 2--8 h. ENDOR lines of two shells of $^{17}\mathrm{O}$ and seven shells of $^{29}\mathrm{Si}$ were observed. Five different species of ${\mathrm{TD}}^{+}$'s were identified, which are formed one after another upon annealing. The ESR spectra of these TD's were superimposed in the NL8 spectrum. The smallest ${\mathrm{TD}}^{+}$ measured by ENDOR is probably identical with ${\mathrm{TD}}_{3}^{+}$ as identified from the infrared-absorption bands. The core of the TD's contains four oxygen atoms, two on each of the two (110) mirror planes. All the five ${\mathrm{TD}}^{+}$'s investigated showed ${C}_{2v}$ point-group symmetry. A single Si atom is possibly also part of the TD core structure, while the other Si neighbors are ligands. The hyperfine interactions of all $^{29}\mathrm{Si}$ and $^{17}\mathrm{O}$ nuclei are very small, representing only about 5% of the unpaired-electron spin density. The small size of the interactions cannot be explained by present theories. The compatibility of current TD models with the ENDOR results is discussed.

Low-temperature localized motion of hydrogen and electronic structure transition in hexagonal-close-packed scandium.

We report nuclear magnetic resonance (NMR) measurements of the proton (/sup 1/H) spin-lattice relaxation rate (R/sub 1/) in the hexagonal-close-packed (hcp) solid solution (..cap alpha..) phase of the Sc-H system over the temperature range 10--300 K in which hydrogen pairs are known to form. At low temperatures (10--120 K), fast localized motion of hydrogen between closely spaced tetrahedral interstitial sites in the lattice gives rise to a peak in the relaxation rate. Both the temperature and frequency dependences of the relaxation rate peak exhibit characteristics typical of amorphous and disordered systems, suggesting the formation of hydrogen pairs with little long-range order results effectively in a ''proton glass'' within the metal lattice. The measurements reveal an electronic structure transition near 170 K where the unpaired electron spin density at the proton sites decreases with increasing temperature.

The structure of the primary electron donors P865 and P700 of bacterial and plant photosynthesis based on magnetic resonance data

The unpaired electron spin density distributions in the π-systems of the radical cations of bacteriochlorophyll a, chlorophyll a and the oxidized primary electron donors P865 and P700 of bacterial and plant photosynthesis are estimated from an analysis of the hyperfine interactions between the protons and nitrogens. The analysis supports a dimeric structure for P865 and P700.

Caractéristiques des spectres de résonance paramagnétique électronique des radicaux formés à partir des tétrahydro-1,2,3,4 et octahydro-1,2,3,4,6,7,8,9 phénazine

A detailed analysis of the E. P. R. spectra of the anion and diprotonated cation radicals of partly hydrogenated phenazine derivatives is presented. Furthermore, the gyromagnetic ratios of the pyrazine, tetramethylpyrazine, quinoxaline and phenazine radicals are measured. Higher than those of the corresponding aromatic radicals, the g factors are correlated to the nitrogen hyperfine coupling constant. This shows the effect of the 2 p spin-orbit coupling of the nitrogen and of the unpaired electron-spin density on the nitrogen in this series of 1,4-diazines.

Electron-spin-resonance study of Sn+(5p1) centers of the laser-active-type structure in KCl:Sn2+ and analysis of the hyperfine structure.

It is shown through an analysis of the electron-spin-resonance spectra that the ${\mathrm{Sn}}^{2+}$(5${\mathrm{s}}^{2}$) impurities in KCl can also give rise, after x irradiation above 220 K, to the so-called ${\mathrm{Sn}}^{+}$(1) centers of the laser-active-type structure. The essential core of this center is a substitutional ${\mathrm{Sn}}^{+}$(5${\mathrm{p}}^{1}$) ion strongly perturbed by an adjoining anion vacancy along the 〈001〉 direction. The observed orthorhombic symmetry, with the three crystallographic axes as the main axes, is induced by either one or two weakly perturbing cation vacancies in the neighborhood. Their exact positions are hard to establish and several possible, subtly differing, defect models are proposed. An analysis of the hyperfine interaction of all the ${\mathrm{np}}^{1}$ (n=4,5,6) centers in KCl is presented, and it is established that these atoms and ions possess large negative unpaired electron-spin densities at their nuclei when they are free or in crystal fields possessing inversion symmetry. The strong odd field component induced by the anion vacancy invariably adds, through s mixing, a substantial positive contribution to this spin density.

Hydrogen bond formation and molecular motion in solutions of chlorofluoromethanes containing nitroxide free radicals

For liquid chlorofluoromethanes containing a nitroxide radical, the spectral density functions of the radical induced nuclear relaxation rates of 1H and 19F have been determined experimentally over an extended range of frequencies and temperatures. The dominating proton relaxation rates give strong evidence of hydrogen bond formation between the solvent proton and the NO group of the radical. Radical induced dipolar relaxation processes of both nuclei are governed by the isotropic rotational diffusion of the complex. Transfer of unpaired electron spin density via the hydrogen bond is reflected in scalar contributions to the relaxation rates of 1H and 9F at low frequencies. A structural proposal of the hydrogen bond, the lifetime of the bridge, and several parameters of the respective models of molecular motion have been derived.