Dipole-dipole Contribution Research Articles

Evidence of slow motions by cross-correlated chemical shift modulation in deuterated and protonated proteins.

Cross-correlated fluctuations of isotropic chemical shifts can provide evidence for slow motions in biomolecules. Slow side-chain dynamics have been investigated in (15)N and (13)C enriched ubiquitin by monitoring the relaxation of C(alpha)-C(beta) two-spin coherences (Frueh et al., 2001). This method, which had hitherto been demonstrated only for protonated ubiquitin, has now been applied to both protonated and deuterated proteins. Deuteration reduces the dipole-dipole contributions to the DD/DD cross-correlation, thus facilitating the observation of subtle effects due to cross-correlation of the fluctuations of the isotropic (13)C chemical shifts. The decays of double- and zero-quantum coherences are significantly slower in the deuterated protein than in the protonated sample. Slow motions are found both in loops and in secondary structure elements.

NMR study of the dissolution of laser-polarized xenon

NMR of laser-polarized xenon is used to probe the dissolution behaviour of the noble gas in different liquids. The dissolution and self-relaxation rates are extracted via a macroscopic model, and comparison of the decay rate of the xenon magnetization in deuterated and non-deuterated solvent pairs allows the determination of the pure dipole-dipole contribution to relaxation. A transient convective effect, tentatively assigned to the xenon concentration gradient, is observed and characterized by diffusion encoding MRI experiments. The flow of xenon penetrates inside the solvent near the walls of the NMR tube, the longitudinal images showing a “ ” shape, the transverse ones a “O” shape. This convection effect has implications for delivery conditions of laser-polarized xenon in continuous flow experiments and magnetic resonance imaging.

Analysis of the electron paramagnetic resonance spectrum of a radical intermediate in the coenzyme B(12)-dependent ethanolamine ammonia-lyase catalyzed reaction of S-2-aminopropanol.

The structure of the steady-state radical intermediate in the deamination of S-2-aminopropanol catalyzed by ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium has been probed by electron paramagnetic resonance (EPR) spectroscopy using isotopically labeled forms of the substrate and of the adenosylcobalamin cofactor. Electron spin-spin coupling between the radical, centered on the carbon skeleton of the substrate, and the low-spin Co(2+) in cob(II)alamin (B(12r)) produces a dominant splitting of the EPR signals of both the radical and the Co(2+). Analysis of the exchange and dipole-dipole contributions to the spin-spin coupling indicates that the two paramagnetic centers are separated by approximately 11 A. Experiments with (13)C- and with (2)H-labeled forms of S-2-aminopropanol show that the radical is centered on C1 of the carbon skeleton of the substrate in agreement with an earlier report [Babior, B. M., Moss, T. H., Orme-Johnson, W. H., and Beinert, H., (1974) J. Biol. Chem. 249, 4537-4544]. Experiments with perdeutero-S-2-aminopropanol and [2-(15)N]-perdeutero-S-2-aminopropanol reveal a strong hyperfine splitting from the substrate nitrogen, which indicates that the radical is the initial substrate radical created by abstraction of a hydrogen atom from C1 of S-2-aminopropanol. The strong nitrogen hyperfine splitting further indicates that the amino substituent at C2 is approximately eclipsed with respect to the half-occupied p orbital at C1. Experiments with adenosylcobalamin enriched in (15)N in the dimethylbenzimidazole moiety show that the axial base of the cofactor remains attached to the Co(2+) in a functional steady-state reaction intermediate.

Heavy quark production in photon-nucleon and photon-photon collisions

We discuss several mechanisms of heavy quark production in (real) photon-nucleon and (real) photon - (real) photon collisions. In particular we focuse on application of the Saturation Model. We discuss how to generalize the formula from virtual photon - proton scattering and analyze threshold effects. We discuss a possibility to measure the cross section for $\gamma \gamma \to 2 c 2 \bar c$. In addition to the main dipole-dipole contribution included in a recent analysis, we propose how to calculate within the same formalism the hadronic single-resolved contribution to heavy quark production. At high energies this yields a sizeable correction of about 30-40 % for inclusive charm production and 15-20 % for bottom production. We consider a subasymptotic component to the dipole-dipole approach. We get a good description of recently measured $\sigma(\gamma \gamma \to c \bar c X)$. Adding all possible contributions to $e^+ e^- \to b \bar b X$ together removes a huge deficit observed in earlier works but does not solve the problem totally. Whenever possible, we compare the present approach to the standard collinear one. We propose how to distinguish different mechanisms by measuring heavy quark-antiquark correlations.

A tight-binding representation of electron-hole exchange interaction in semiconductors

The electron-hole exchange interaction in semiconductors is analyzed in the framework of the empirical tight-binding method. It is demonstrated that intra-atomic and interatomic contributions to the long-range exchange interaction enter in an inequivalent way. In particular, for the Γ6×Γ7 exciton in a spherical nanocrystal with a cubic lattice, the dipole-dipole contribution associated only with the intra-atomic (or intra-site) transitions does not lead to singlet-triplet splitting of the exciton level. The interatomic transitions, for example, anion-to-cation transitions between the nearest neighbors in binary semiconductor compounds, determine the so-called monopole-monopole contribution to the exchange splitting of the Γ6×Γ7 exciton, and this contribution does not vanish in a spherical nanocrystal.

Vibrational energy-transfer processes in the [formula omitted] system

Dipole-dipole coupling in the 12 CO Cu(100) system results in a v CO frequency at 2086 cm −1, which is a shift of + 43 cm −1. The chemical effect (measured here as the difference between the frequency of the singleton at low coverage minus the singleton at high coverage), was found to be −32 cm −1. Dilution of 12CO with Ar at 23 K revealed that at high concentrations, argon underwent island growth. At lower dilutions, two different interactions shifted the observed ν CO frequency; charge transfer between Ar and the surface combined with an additional dipole-dipole coupling contribution caused by the dipole induced in Ar by the CO gave a maximum shift of −10 cm −1 to 2066 cm −1 at 0.25 ML CO coverage. The temperature dependence of the FWHM and peak position is observed for the compressed (7√2 × √2)R45° structure in the temperature range 23–120 K. No significant temperature dependence of dipole-dipole coupling was observed over this temperature range. The FWHM temperature dependence is conclusively assigned to anharmonic coupling of the CO stretch with the frustrated translation. The total lifetime at 0 K of the ν CO mode is determined to be ∼ 2 ps.

Towards a theory of coexistence and criticality in real molten salts

To improve our understanding of the liquid-gas phase separation occurring in ordinary molten salts a variety of model ionic fluids is investigated extending from the restricted primitive model (RPM) (composed of hard ions with equal size) to model reactive fluids in which a chemical reaction between ionic and covalent species takes place. In searching to improve the existing linearized theories for the RPM, it is shown that the mean-field approach of Debye-Hückel (DH) theory is very sensitive to successive improvements (ion pairing, hard sphere contribution, dielectric constant), whereas the mean-spherical approximation (MSA) exhibits, under similar conditions, less sensitive behaviour. In particular, it is shown that the good agreement obtained by M. E. Fisher and Y. Levin (1993, Phys. Rev. Lett., 71, 3826) with computer simulation data in implementing the ion-dipole interaction into DH theory with ion pairing worsens significantly if the dielectric medium composed of ion pairs is properly taken into account in a self-consistent way. However, our improved MSA incorporating at the same time ion pairing, ionic solvation and dipole-dipole contributions shows only qualitative agreement with the available simulation data. In order to describe more realistically the world of molten salts a primitive model is introduced of a reactive fluid composed of ionic and covalent (polar or non-polar) species which can interact in accordance with the chemical equilibrium In this model fluid the phase separation is governed mainly by ΔE the energy difference between molecules (C, D) and ions (A+, B-) in their ground states. When ΔE ≳ 0 the concentration of molecules in the fluid is so small that it behaves like the RPM discussed previously. On the other hand, when ΔE < 0 the concentration of molecules becomes significant in both phases (liquid and gas) and the critical point shifts towards higher densities (as compared with the RPM critical point) while the coexistence curve adopts a peculiar arched shape, these features being all the more pronounced when ΔE is more negative. In spite of the key role played by the covalent species, the phase transition is still driven by the ionic population. Thus at the high temperatures of the onset of the phase transition, the molecules behave mostly like ideal species. The theory is next specialized to two highly conducting molten salts, namely NaCl and NH4Cl, as well as a weakly dissociated liquid, water, which is known to ionize under pressure. The calculation permits one to understand why molten salts, like alkali halides, exhibit a very low critical density and low vapour pressure in contrast to other molten salts, where covalent species are in a much more significant concentration, and for which the critical density and the vapour pressure practically match those found with regular liquids. Finally, the difficulty of obtaining a quantitative agreement with experimental data is emphasized and a route for further improvements is indiated.

Fast reactions between a linear molecule and a polar symmetric top

We present rate constant calculations for fast neutral reactions between a linear molecule and a polar symmetric top. The adiabatic capture, partial centrifugal sudden approximation (ACPCSA) is adapted in order to account for the degeneracy of the rotational states of the symmetric top molecule. We use a long range potential made from the sum of the dipole-dipole, induction and dispersion contributions. The three reactions CN + NH 3, CH + NH 3 and OH + PBr 3 are studied. ACPCSA calculations of rate constants are compared with experimental results at 300 K and analytical formulae are derived in the limit of zero temperature.

Interatomic force constants from first principles: The case of alpha -quartz.

We describe a method for calculating the interatomic force constants in crystalline insulators, from first principles, with explicit inclusion of the long-ranged anisotropic dipole-dipole interaction. Using this technique, the dynamics of α-quartz, a model for tetrahedrally bonded silica, is investigated: we examine the range of interatomic forces, their anisotropy, their longitudinal and transverse character, and the importance of the dipole-dipole contribution. These force constants provide an extensive database for testing semiempirical interatomic potentials used in silica molecular-dynamics simulations.

Spin waves and temperature-dependent behaviour of the quasi-two-dimensional antiferromagnet KFeF4

A study of the quasi-two-dimensional antiferromagnet KFeF4 below the transition temperature, TN=13 K, has been undertaken using neutron scattering techniques. The dispersion relation of the spin excitations is well described by a model Hamiltonian incorporating Heisenberg exchange interactions and uniaxial anisotropy. The anisotropy controls the energy of the zone centre mode whose frequency varies in the same way with temperature as the square of the staggered magnetization which is described by the two-dimensional Ising model. Dipole-dipole contributions are too small to account fully for the anisotropy. Different nearest-neighbour exchange energies are found along the a and b directions due to different superexchange paths, with J1=-2.18 meV along a, and J2=-2.73 meV along b.

Higher-order dipole-dipole, dipole-quadrupole and field gradient contributions to the second light-scattering virial coefficient

Theory of the second light-scattering virial coefficient Bp is extended to include in the scattered intensity terms that have not previously been considered, namely dipole-dipole intensities of the fifth power in the molecular polarizability α as well as additional contributions arising from field gradient effects and induced quadrupole moments in the molecular interactions. Numerical calculations for ten gases show that terms in α5 make a significant contribution to B ρ of up to nearly 20%. In polar molecules, terms in α2 A (where A describes the dipole induced by a field gradient, or the quadrupole induced by a field) make contributions of up to 9%. Indications are that in molecules with a centre of inversion, for which A is zero, field gradient and quadrupole effects will generally lead to contributions of less than 1%.

Dipole-dipole anisotropy in Y 2Co 17 and Gd 2Co 17

The dipole-dipole contribution to the anisotropy constant K 1 in ellipsoidal single crystals of Y 2Co 17 and Gd 2Co 17 is calculated. The results are expressed in terms of closed formulae in which demagnetization factors occur explicitly. The formulae are used to show that the relatively large difference of K 1 experimentally found in the two materials can largely be ascribed to the additional dipole-dipole interactions present in Gd 2Co 17.

Separating the rhenium-hydride and proton-proton dipole-dipole contributions to the 1H NMR spin-lattice relaxation rate of the hydride ligand in mer,trans-ReH(CO)3(PPh3)2 by deuteration

Separating the rhenium-hydride and proton-proton dipole-dipole contributions to the 1H NMR spin-lattice relaxation rate of the hydride ligand in mer,trans-ReH(CO)3(PPh3)2 by deuteration

The dipole-dipole contribution to the magnetic propagator in the REBa2Cu3O7- delta compounds

The dipole-dipole interaction is calculated for antiferromagnetic configurations found in the REBa2Cu3O7- delta compounds, as a function of the spacing between ab planes. Crossover from three-dimensional to two-dimensional behaviour is demonstrated for both the ground state energy and the coefficients of the quadratic terms in the propagator.

Restricted lateral diffusion of acidic lipids in phospholipid vesicles aggregated by myelin basic protein

The effect of myelin basic protein on the lateral diffusion of spin-labelled stearic acid in dimyristoylphosphatidylcholine (DMPC) and dimyristoylphosphatidylglycerol (DMPG) vesicles has been investigated measuring bimolecular collision rates by electron spin resonance spectroscopy. In a molar fraction range of 0.02–0.15, in which the line shape is dominated by the effects of exchange narrowing and dipolar broadening, spectral simulations based on the modified Bloch equations have been used to quantitate the observed spectral changes. Best fitting simulated spectra required a combination of dipole-dipole and spin exchange contributions and least-squares optimizations were applied to obtain a unique combination of these factors. Lateral diffusion coefficients were calculated from collision frequency data in the frame of the lattice model; for pure DMPC and DMPG vesicles lateral diffusion coefficients of 1.7 · 10 −7 and 1.2 · 10 −7 cm s 2 · −1 were obtained. Upon adding myelin basic protein to these vesicles the diffusion coefficients of the negatively charged DMPG decreased to 4.1 · 10 −8 cm 2 · s −1 , whereas that of the zwitterionic DMPC remained unchanged. It is concluded that myelin basic protein must strongly restrict the lateral movement of the DMPG molecules and reduce their diffusion coefficient close to the values observed for proteins. The restricted lateral diffusion is explained by electrostatic interactions which lead to an altered molecular dynamics due to one-to-one lipid exchange and lipid-protein co-diffusion.

Prediction of proton relaxation rates from measurements of deuterium relaxation in aqueous systems

Spin-lattice relaxation for deuterium in aqueous solutions of macromolecules is dominated by quadrupolar interactions so that the rotational correlation time for D 2O and HDO may be determined by measuring the deuteron spin-lattice relaxation time T 1. This correlation time may then be used to predict the dipole-dipole contribution to proton relaxation from rotational intramolecular water interactions. By subtracting these calculated values from the directly measured proton relaxation rates, the contribution to proton relaxation due to translation and intermolecular dipolar effects may be found. In solutions of polyethylene glycol the translational contribution was estimated to be a constant 40% of the overall relaxation rate when the deuteron relaxation rate varied four-fold, consistent with the view that the translation correlation time varies in direct proportion with the rotational time. It is proposed that deuteron relaxation measurements thus offer a novel method for estimating the hydrodynamic contribution to relaxation due to water-water interactions only in solutions of macromolecules and tissues, separate from the effects of cross relaxation, and these measurements are thus useful for understanding the relative importance of different proton relaxation mechanisms in such media.

Exact relativistic analogues of the non-relativistic hyperfine structure operators

Exact relativistic analogues of the standard non-relativistic operators describing the nuclear spin-electron orbit (NSEO) and nuclear spin-electron spin contributions to the hyperfine hamiltonian for the interaction of an electron with a nuclear spin are derived. These relativistic operators, which are valid only for spatially extended descriptions of both the nuclear charge and magnetization, differ from their non-relativistic analogues solely by the additional multiplication by the Dirac β matrix. For the model in which the entire nuclear magnetization resides on the surface of a sphere of non-vanishing radius (rn ) the nuclear spin-electron spin interaction is the sum of a dipoledipole (SSD) contribution arising when the electron is outside the nuclear magnetization (r≥rn ) plus a contact type (SSC) term arising only when the electron is inside this magnetization (r⪯rn ). Both these SSD and SSC operators also differ from their non-relativistic counterparts by the appearance of the β matrix. It is shown...

Thermolysin-inhibitor complexes examined by 31P and 113Cd NMR spectroscopy.

Complexes between phosphoramidon (N-(alpha-rhamnopyranosyloxyhydroxyphosphinyl)-L-leucyl-L-tryptoph an) and zinc thermolysin and between phosphoramidon or N-phosphoryl-L-leucineamide and 113Cd-substituted thermolysin have been examined by 31P and 113Cd NMR spectroscopy. 113Cd resonances are observed at 168 and 152 ppm for the phosphoramidon and N-phosphoryl-L-leucineamide complexes, respectively. There are large but different chemical shift anisotropy contributions to the 113Cd line widths for the two complexes, which reflect the known structural differences for the zinc-enzyme complexes. 113Cd-31P spin-spin coupling is also seen and differs for the two cadmium complexes, being larger, 28 Hz, for the bidentate N-phosphoryl-L-leucineamide ligand than for the monodentate phosphoramidon, 16 Hz. Large changes in chemical shift, 7.5-10.9 ppm, are seen for the 31P resonances of the inhibitors upon binding to the enzyme reflecting direct phosphoryl-metal ligation. Chemical shift anisotropy is the dominant relaxation mechanism for the 31P nuclei at 9.4 T, while the dipole-dipole contribution seems to be unaffected by a change of solvent from H2O to D2O.

Rotational motions in n-hexane phosphon c acid diethyl ester studied combining 2H, 13C, and 31P NMR. Analysis of the phosphorus-31 spin-lattice relaxation

Spin-lattice relaxation of 13C and 13P in neat H 3C(CH 2) 5P(O)(OC 2H 5) 2 and of 2H and 31P in H 3C(CH 2) 4C 2H 2P(O)(OC 2H 5) 2 and H 3C(CH 2) 5P(O)(OC 2 2H 5) 2 have been measured in the temperature range from −70 to 160°C and at different fields. By using the NOE and the selectivity deuterated compounds the contributions of different segments to the total proton-phosphorus dipole-dipole relaxation were determined. Quantitative analysis of the 31P relaxation yields the result that, at the phosphorus resonance frequencies of 36.44 and 145.76 MHz and at all temperatures studied, at least two interaction mechanisms contribute. Further, the α-CH 2 segment, the ester groups, and the hexyl residue without the α-CH 2 segment, each gives approximately one-third of the total proton-phosphorus dipole-dipole contribution. The spin-lattice relaxation of 2H, 13C, and 31P is described on the basis of the Woessner model in the whole temperature range studied with one set of motional parameters. The relaxations of the spins in the H 2CP(O)O 2 and 2H 2CP(O)O 2 segments are mainly determined by the overall reorientation whereas this motion has at most a small effect on the 13C relaxation of the other segments and on the 2H relaxation of the ester groups.

113Cd spin-lattice relaxation for Cd(acac)2

Abstract 113Cd spin-lattice relaxation for DMSO-d6 and pyridine-d5 solutions of Cd(acac)2 is analyzed as a function of temperature and at three magnetic field strengths. The 113Cd relaxation for the DMSO-d6 system is readily analyzed in terms of dipole-dipole, chemical-shift anisotropy and spin-rotation contributions to relaxation. The 113Cd relaxation for the pyridine-d5 system is more complex. The anisotropy in the 113Cd shielding tensor (assuming axial symmetry) for Cd(acac)2 is about 150 ppm in DMSO-d6 and 90–100 ppm in pyridine-d5, which contrasts with the solid-state value of 41 ppm.