Complete Hyperfine Research Articles

Complete heme proton hyperfine resonance assignments of the Glycera dibranchiata component IV metcyano monomer hemoglobin.

Monomer hemoglobin component IV is one of three major myoglobin-like proteins found in the erythrocytes of the marine annelid Glycera dibranchiata. Unlike myoglobin, all three of these monomer hemoglobin components lack the distal histidine, which is replaced by leucine. This substitution alters the protein's functional properties due to its proximity to the heme ligand binding site. As the initial step toward a full NMR characterization of this protein, a complete set of self-consistent proton NMR assignments for the heme and the proximal histidine of the paramagnetic, metcyano form of native component IV (metGMH4CN) is presented. These assignments relied upon a combination of one- and two-dimensional NMR spectroscopy, including nonselective spin-lattice relaxation time measurements. The metcyano form has been chosen for several reasons: (1) The heme paramagnetism acts as an intrinsic shift reagent which aids in making individual resonance assignments for the heme and neighboring amino acids in the protein's ligand binding site. (2) Heme paramagnetism also enhances proton nuclear relaxation rates, thereby allowing two-dimensional NMR experiments to be carried out at very rapid repetition rates (i.e., 5 s-1). (3) The heme proton hyperfine resonance pattern for this paramagnetic form of wild-type monomer hemoglobin component IV provides an analytical reference for the integrity of the heme active site. This is anticipated to facilitate rapid analysis of subsequently produced recombinant derivatives of this protein. (4) The cyanide-ligated protein has a heme pocket structure similar to those of the O2- and CO-ligated forms of the physiologically important, reduced form of the protein, so that the heme and proximal histidine proton assignments will serve as a basis for further assignments within the heme binding site. Complete assignments, in combination with recombinant derivatives of this monomer hemoglobin, will give further insight into local interactions that influence ligand binding kinetics and heme orientational isomerism.

The electronic structure of the primary donor cation radical in Rhodobacter sphaeroides R-26: ENDOR and TRIPLE resonance studies in single crystals of reaction centers

The electron spin density distribution of the cation radical of the primary donor, D +, a bacteriochlorophyll a dimer was determined by ENDOR and TRIPLE resonance experiments performed on single crystals of reaction centers (RCs) of Rhodobacter sphaeroides R-26. Nine isotropic proton hyperfine coupling constants (hfc's) were obtained and from the angular dependence of the hfc's in three crystallographic planes, five complete hyperfine (hf) tensors were determined. Theoretical hf tensors were calculated by the all-valence-electron SCF molecular orbital method RHF-INDO/SP using the X-ray structure data of the dimer D and its amino acid environment. A comparison of the directions of the principal axes of the experimental and calculated hf tensors enabled us to identify the hfc's with specific protons on the two bacteriochlorophyll halves D L and D M of the dimer. The result shows that the unpaired valence electron is unequally distributed over the dimer halves, favoring D L by approx. 2:1. This ratio has been obtained from the proton hfc's of rotating methyl groups, which directly reflect the π-spin densities at the corresponding positions in the two macrocycles, D L and D M. It was further confirmed by recent 15N-ENDOR experiments on RC single crystals (Lendzian, F., Bönigk, B., Plato, M., Möbius, K. and Lubitz, W. (1992) in The Photosynthetic Bacterial Reaction Center II (Breton, J. and Verméglio, A., eds.), pp. 89–97, Plenum Press, New York). The observed asymmetry of D + is attributed to the difference in energies of the highest filled molecular π-orbitals of the monomeric halves, D L and D M, which is caused by differences in the structure of the two bacteriochlorophylls and/or their environment. Possible implications of this asymmetry for the electron transfer in the RC are discussed.

High-brightness, high-purity spin-polarized cesium beam.

We describe a cesium atomic beam (${10}^{14}$ atoms ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$, 6\ifmmode\times\else\texttimes\fi{}${10}^{16}$ atoms ${\mathrm{sr}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$) spin polarized using light from two diode lasers. Of the atoms, 95% may be placed into either the 6${\mathit{S}}_{1/2}$(F,${\mathit{m}}_{\mathit{F}}$)=(3,3) or (4,4) state, with less than 2\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}4}$ of the atoms left in the depleted hyperfine level. The latter fraction rises linearly with atomic beam intensity because of reabsorption of light scattered during the optical pumping process. This and other effects limiting complete hyperfine pumping are discussed.

ENDOR Spectra of 2-Methyl-Naphthalene-TCNB Triplet Trap in Naphthalene-TCNB Single Crystals

Abstract Naphthalene-TCNB charge transfer crystals doped with 2-methyl naphthalene illuminated by visible light give rise to triplet traps which have been identified by EPR and ENDOR spectroscopy as 2-methyl naphthalene-TCNB CT complexes in their first excited triplet state. The complete hyperfine tensors of all the 2-methyl naphthalene protons have been obtained by analysis of the variation of the ENDOR frequencies with the crystal orientation in the magnetic field. The structure of the CT complex, the spin distribution and the CT character are discussed.

ENDOR Spectra of Pyrene Excited Triplet State in Naphthalene-TCNB Crystals

Abstract Triplet state traps, observed on optically excited single crystals of Naphthalene-TCNB doped with Pyrene, have been studied by EPR and ENDOR spectroscopy. The magnetic resonance lines recorded at low temperature (20 K) are shown to be due to triplet excitations localized almost completely on a Pyrene molecule. The complete hyperfine tensors of all the protons have been determined. The spin distribution is shown to be distorted in such a way that the triplet Pyrene molecule lacks the in-plane symmetry axes of the ground state molecule. The orientation of the triplet molecule in the crystal lattice and the relative orientation of Pyrene and TCNB are also discussed.

On Mössbauer Spectrum Analysis for Spin Orientation in Fe2As

AbstractThe reports of the analysis of 57Fe Mössbauer spectra of Fe2As and the derived spin orientation are inconsistent. To understand the reasons for this inconsistency, the spectra are recorded at various temperatures between 4.2 and 375 K and least squares fitted by solving the complete nuclear hyperfine Hamiltonian matrix using different sets of fitting parameters. The trade off in the fitting of the spectra corresponding to the two iron sites is responsible for this inconsistency. The incorporation of magnetically induced asymmetry parameter in the fitting procedure as suggested in a recent study, did not produce consistent results at all temperatures. The possible reasons for the reported discrepancies are discussed.

Hyperfine Interactions in the Lowest Triplet State of Pyridine

AbstractResults are reported of nitrogen and deuterium hyperfine studies of the triplet state of pyridine‐d5 as a guest in a single crystal of benzene at 1.2 K. Through the use of electron spin echo (ESE) techniques complete hyperfine tensors have been obtained: for nitrogen from the ESE detected EPR spectra and for the para‐deuterium atom from the modulations of the ESE envelope. These tensors unambiguously show that the molecule is no longer planar in the lowest triplet state T0. Pyridine's structure considerably changes upon excitation, but the mirror plane perpendicular to the molecule is preserved. Experimental and theoretical evidence is presented for the fact that this deformation is an intrinsic property of T0 and is not induced by the crystal field. About fourty percent of the triplet electron spin density is found to be localised in the π‐orbital of nitrogen, about ten percent in the lone pair orbital of nitrogen and somewhat more than thirty percent in the π‐orbital of the para‐carbon atom.

Frozen solution proton endor spectroscopy of solvated Mo(V) oxochloro complexes

Proton ENDOR spectra of randomly oriented [MoOCl 4(S)] − complexes, with S = H 2O, CH 3OH, C 2H 5OH, and CH 3CHO, have been measured at ≈15 K. Complete hyperfine coupling parameters have been obtained for the hydroxyl and aldehydic protons, as well as for some of the alkyl hydrogens. Structural information concerning coordination of the solvent molecules has been obtained in the limit of the point dipole approximation. Resonance lines ascribable to the presence of an ordered secondshell solvation sphere have been detected in the spectrum of the methanol solvate.

Proton endor spectra of phenanthrene-TCNB triplet traps in single crystals of naphthalene-TCNB

The ENDOR spectra of triplet traps of phenanthrene-TCNB in doped single crystals of naphthalene-TCNB have been recorded and the complete hyperfine tensors of all the protons have been measured. The spin densities on the carbon atoms are obtained from the isotropic as well as from the anisotropic part of the tensors. The spin density distribution on phenanthrene inphenanthrene-TCNB differs from that of the pure phenanthrene in its first excited triplet state, due to a polarizing effect of the acceptor. The CT degree is obtained from the reduction of the total spin density on the donor molecule and it results to be 40%. The structure of the trap is also discussed.

103Rh and 33s hyperfine interactions in tetraphenylarsonium-bis ( maleonitriledithiolato ) rhodate (II), (Ph 4As) 2[Rh(mnt) 2]: a single-crystal EPR-ENDOR study

The complete 103Rh and 33S hyperfine coupling tensors of the 4d 7 low-spin chelate tetraphenylarsoniumbis (maleonitriledithiolato)rhodate(II) doped into the corresponding nickel complex salt have been measured by EPR-ENDOR spectroscopy. Both interactions are small and confirm that the unpaired electron is in an “out-of-plane” π-type molecular orbital. The Rh-S bonds are less covalent than bonds in the isoelectronic Pd(III) complex.

ESR and ENDOR studies of irradiated potassium dihydrogen phosphate

Irradiated single crystals of potassium dihydrogen phosphate were studied via ESR and ENDOR techniques in order to define phosphate group structures. A precursor cation species was produced at 4.2 K which converted upon warming to a radical previously reported for 77 K irradiation. The g values and proton ENDOR results allowed detailed models for both species to be constructed. Phosphorous ENDOR resonances were also detected and provided complete hyperfine splitting tensors for both species which reflected small internal phosphate group polarization densities.

Hyperfine structure and isotope shifts in odd-parity 6snp and 6snf Rydberg series of Ba I

Hyperfine structure and isotope shifts in transitions to odd-parity 6snp and 6snf Rydberg configurations of barium have been investigated using laser-atomic-beam spectroscopy. For the 6snf Rydberg configurations with n=13-42 the complete hyperfine multiplet is observed. The data on isotope shifts have been used to test the total perturber character mixed in the 6snp and 6snf Rydberg states predicted by existing multichannel quantum defect theory (MQDT) treatments of the odd-parity J=0, 1, 2, 3 and 4 spectra. The MQDT predictions are shown to be reliable in this respect. The hyperfine structure of the singlet and triplet states is analysed in terms of singlet-triplet mixing. The extracted mixing coefficients disagree with the predictions of existing MQDT models, even when an interaction between 'singlet' and 'triplet' 6snl channels is introduced. New MQDT models have been developed in which both energies and mixing coefficients are reproduced well. Hyperfine structure data provides information on interactions between mutually perturbing channels. Finally an earlier analysis of the hyperfine structure of the 6snf 3F states in terms of effective parameters is discussed.

Determination of the complete14N quadrupole interaction tensor in a photoexeited triplet state by zero and low field optically detected ENDOR Application to NaNO2/Ag

The optically detected E.S.R. and ENDOR spectra at zero field and low fields of the triplet state of the major NO2 − trap in NaNO2 doped with AgNO2 are presented and analysed. The fine structure and complete 14N nuclear hyperfine and quadrupole interaction tensors are obtained for the trap triplet state. The results are shown to be consistent with an assignment of the trap triplet state as 3 B 1, localized on a NO2 − ion and arising from a π(b 1)←σ(a 1) orbital excitation. The cation (Ag+ and Na+) interactions are observable, but small.

Hyperfine spectra in iodine-129 at 612 nm

The complete hyperfine spectrum of a transition in 129I 2 at a wavelength of 612 nm that overlaps the spectrum of 127I 2 reported by Bennett and Cerez has been obtained, together with parts of the spectra of other transitions in 129I 2 and 127I 2. The lasers used for this work are described and the frequencies for the various hyperfine components are outlined.

14N,1H, and metal ENDOR of single crystal Ag(II)(TPP) and Cu(II)(TPP)

The results of ENDOR investigations of magnetically dilute single crystals of Ag(II) and Cu(II) tetraphenylporphyrins (TPP) are presented and discussed. The complete hyperfine tensors, and quadrupole tensor for nitrogen, have been determined for the metal ions, nitrogens and pyrrole protons. The non-first order 14N ENDOR spectra resulting from multiple equivalences among the nitrogen nuclei have been completely analysed by exact diagonalization. The standard ligand-field treatment of electron-nuclear coupling parameters can largely accommodate the 14N results; among other conclusions, it is shown that the nominally d-like odd-electron is only ⩽ 62 per cent 3d in Cu(TPP) and in fact is only ⩽ 38 per cent 4d in Ag(TPP). This treatment cannot describe the observed degree of σ-π mixing manifested in the significantly non-axial 14N hyperfine tensors observed. The 14N nuclear quadrupole couplings are analysed to obtain the total electronic distribution at nitrogen; in addition, the assignment of quadrupole tens...

Radical pair formation from excited states in doped aromatic crystals. I. Epr studies of the guest-host system acridine-fluorene

Following light absorption in acridine doped fluorene single crystals formation of a triplet state complex (heteroexcimer) has been established earlier. More detailed EPR investigations presented here permit the clarification of the nature of this complex. Hyperfine structure has been resolved for three nuclear spins which could be assigned by selective deuteration experiments: the nitrogen and meso-proton spin located in the guest-constituent of the complex and one proton spin originating from the CH 2-group of a fluorene host molecule. The complete hyperfine tensors could be evaluated. The anisotropy is found to be that of a planar aromatic CH-fragment for both observed proton spins. As a consequence of the CH-fragment assignment the central CH 2-group of the fluorene constituent of the complex has to change from a sp 3 to a planar sp 2 configuration indicating a photoinduced radical pair formation involving hydrogen abstraction from a fluorene and hydrogen addition to an acridine molecule. The model is able to explain: (a) the high spin density located at the observed CH-fragments; (b) the geometry of the molecular partners involved in the complex, and (c) the fine structure tensor on the basis of a simple point charge dipole-dipole coupling of the two unpaired electron spins.

ODMR and Proton-ENDOR investigations of the symmetry and geometry of the lowest excited triplet state of quinoxaline and p-dichlorobenzene at high magnetic fields

ODMR and ODENDOR spectra were obtained for quinoxaline and p-dichlorobenzene in their excited triplet states. These molecules were studied in mixed crystals using heavy atom containing hosts. For protons the complete hyperfine tensors could be evaluated, yielding information about the symmetry and geometry of these molecules in their triplet states. For quinoxaline no deviations from planarity have been observed whereas triplet p-dichlorobenzene turned out to be non-planar. The measured π-spin densities in p-dichlorobenzene confirm the 3B 1u symmetry of this molecule in its first excited triplet state.

Optically detected magnetic resonance spectra of the lowest triplet state of 1-iodonaphthalene in durene at zero and high magnetic fields

This paper describes the results of optically detected magnetic resonance (ODMR) experiments on photoexcited 1-iodonaphthalene (1IN) in a durene host crystal at liquid helium temperatures. The structure observed in both the zero-field (zf) and high-field ODMR transitions is interpreted in terms of a spin Hamiltonian which includes the complete iodine (I=5/2) nuclear quadrupole and hyperfine interactions. Principal values of D (including the absolute signs), P, AI, and estimates of the direction cosine matrices and the components of g are reported. It is concluded from this analysis that (1) the lowest triplet state of 1IN is principally (π,π*) in character but (2) is contaminated by slight admixtures of charge-transfer configurations; (3) the zf parameters of triplet 1IN are considerably perturbed from those of 3B2u naphthalene and presumably reflect the increased importance of second-order spin–orbit contributions to D in the presence of the heavy-atom substituent; and (4) 1IN occupies two magnetically inequivalent sites in the durene lattice which are not substitutional.

Endor of a randomly oriented carbazyl radical

The ESR and ENDOR spectra of the 1, 3, 6, 8 tetra- t-butyl carbazyl radical have been determined in glassy perdeuterotoluene at 77°K. By performing the ENDOR experiment at the various turning points of the ESR spectrum,the complete hyperfine tensor of the protons at the 2 and 7 positions has been determined without any single crystal measurements.

ESR Study of the Chlorofluoroacetamide Radical in Irradiated Dichlorofluoroacetamide Single Crystals

The electron spin resonance spectra of single crystals of dichlorofluoroacetamide and N, N-dideutero-dichlorofluoroacetamide irradiated with x-rays at 77°K and later warmed to 170°K, have been assigned to · CClFCONH2 and · CClFCOND2, respectively. A tensor Hamiltonian fit to the spectra have yielded either complete or partial hyperfine splitting tensors for fluorine-19, chlorine-35, carbon-13, and nitrogen-14 in addition to the chlorine-35 quadrupole coupling and the g tensor. These results along with a CNDO/2 molecular orbital calculation indicates that the · CClFCONH2 radical is a planar π radical. An INDO molecular orbital calculation was carried out for the · CH2CONH2, · CFHCONH2, and · CF2CONH2 radicals in order to interpret the experimentally observed amide nitrogen and proton splitting variations.