Strong Scalar Coupling Research Articles

Roles of Strong Scalar Couplings in Maximizing Glutamate, Glutamine and Glutathione Pseudo Singlets at 7 Tesla

For the H4 protons of glutamate (Glu), glutamine (Gln), and the glutamyl moiety of glutathione (GSH), the effect of the internal strong scalar coupling between the two nonequivalent H4 protons is far greater than that of the external couplings between the H3 and H4 protons. In this work, the roles of the internal and external scalar coupling terms in the dependence of Glu, Gln, and glutamyl GSH H4 peak amplitudes on the placement of the refocusing pulses of the point resolved spectroscopy sequence were investigated by full density matrix simulations. These strong coupling effects allowed practical and approximately simultaneous maximization of the sensitivity of the spectrally resolved Glu, Gln, and glutamyl GSH H4 pseudo singlets for spatially localized in vivo detection of Glu, Gln, and GSH in the human brain using magnetic resonance spectroscopy at the magnetic field strength of 7 Tesla.

Off-resonance rotating-frame relaxation dispersion experiment for 13C in aromatic side chains using L-optimized TROSY-selection

Protein dynamics on the microsecond–millisecond time scales often play a critical role in biological function. NMR relaxation dispersion experiments are powerful approaches for investigating biologically relevant dynamics with site-specific resolution, as shown by a growing number of publications on enzyme catalysis, protein folding, ligand binding, and allostery. To date, the majority of studies has probed the backbone amides or side-chain methyl groups, while experiments targeting other sites have been used more sparingly. Aromatic side chains are useful probes of protein dynamics, because they are over-represented in protein binding interfaces, have important catalytic roles in enzymes, and form a sizable part of the protein interior. Here we present an off-resonance R 1ρ experiment for measuring microsecond to millisecond conformational exchange of aromatic side chains in selectively 13C labeled proteins by means of longitudinal- and transverse-relaxation optimization. Using selective excitation and inversion of the narrow component of the 13C doublet, the experiment achieves significant sensitivity enhancement in terms of both signal intensity and the fractional contribution from exchange to transverse relaxation; additional signal enhancement is achieved by optimizing the longitudinal relaxation recovery of the covalently attached 1H spins. We validated the L-TROSY-selected R 1ρ experiment by measuring exchange parameters for Y23 in bovine pancreatic trypsin inhibitor at a temperature of 328 K, where the ring flip is in the fast exchange regime with a mean waiting time between flips of 320 μs. The determined chemical shift difference matches perfectly with that measured from the NMR spectrum at lower temperatures, where separate peaks are observed for the two sites. We further show that potentially complicating effects of strong scalar coupling between protons (Weininger et al. in J Phys Chem B 117: 9241–9247, 2013b) can be accounted for using a simple expression, and provide recommendations for data acquisition when the studied system exhibits this behavior. The present method extends the repertoire of relaxation methods tailored for aromatic side chains by enabling studies of faster processes and improved control over artifacts due to strong coupling.

Critical Line Back-Bending Induced either by Finite <i>N<sub>c</sub></i> Corrections or by a Repulsive Vector Channel

We analyze the two flavor version of the Nambu-Jona-Lasinio model with a repulsive vector coupling (GV), at finite temperature and quark chemical potential, in the strong scalar coupling (Gs) regime. Considering GV = 0, we review how finite Nc effects are introduced by means of the Optimized Perturbation Theory (OPT) which adds a term to the thermodynamical potential. This 1/ Nc suppressed term is similar to the contribution obtained at the large-Nc limit when GV ≠ 0. Then, scanning over the quark current mass values, we compare these two different model approximations showing that both predict the appearance of two critical points when chiral symmetry is weakly broken. By mapping the first order transition region in the chemical potential-current mass plane, we show that, for low chemical potential values, the first order region shrinks as μ increases but the behavior gets reversed at higher values leading to the back-bending of the critical line. This result, which could help to conciliate some lattice results with model predictions, shows the important role played by finite Nc corrections which are neglected in the majority of the works devoted to the determination of the QCD phase diagram. Recently the OPT, with GV = 0, and the large-Nc approximation, with GV ≠ 0, were compared at zero temperature and finite density for one quark flavor only. The present work extends this comparison to finite temperatures, and two quark flavors, supporting the result that the OPT finite N

More accurate 1JCH coupling measurement in the presence of 3JHH strong coupling in natural abundance

J couplings are essential for measuring RDCs (residual dipolar couplings), now routinely used to deduce molecular structure and dynamics of glycans and proteins. Accurate measurement of 1JCH is critical for RDCs to reflect the true structure and dynamics in the molecule of interest. We report noticeable discrepancies between 1JCH values measured with HSQC type pulse sequences in the 1H dimension from those measured in the 13C dimension for 17 sugars and show that these discrepancies arise from strong scalar coupling. In order to determine how to minimize errors in measuring 1JCH, we analyze the strong coupling effects in detail using the product operator-formalism and spectral simulations based on the solution of the Liouville equation (not considering relaxation effects) in the presence of strong coupling. We report that the apparent 1JCH measured with 2D HSQC-based sequences in either dimension can be in error by up to 4Hz and that the values measured in the 1H dimension can disagree with those in the 13C dimension by up to 7Hz. We demonstrate that spectral simulations can reproduce the errors induced by strong coupling and that these can be used to extract true 1JCH values. We find that the 1JCH values measured using a modified Z-filtered coupled HSQC are still affected by strong coupling. We conclude that spectral simulation yields accurate 1JCH with errors as low as 1% in the presence of strong coupling.

Stabilization of molecular LiF and LiFHF inside metallamacrocyclic hosts.

Complexes of molecular LiF and LiFHF were synthesized using the metallamacrocyclic receptors [(cymene)Ru(C(5)H(3)NO(2))](3) (1), [CpRh(C(5)H(3)NO(2))](3) (2), and [CpIr(C(5)H(3)NO(2))](3) (3). LiBF(4) complexes of 1-3 were prepared and subsequently treated with F(-) or FHF(-) to give the desired products in an anion-exchange reaction. All complexes were characterized by multinuclear NMR spectroscopy ((1)H, (13)C, (19)F, (7)Li). Strong scalar coupling between (7)Li and (19)F is observed for the LiF and the LiFHF complexes ((1)J(LiF) = 91-103 Hz). The LiFHF adduct of 1 displays fluxional behavior with fast exchange of the two fluorine atoms. The structures of the complexes 1.LiBF(4), 2.LiBF(4), 1.LiF, 2.LiF, 1.LiFHF, and 3.LiFHF were determined by single-crystal X-ray analysis. Li-F bond lengths between 1.77 and 1.81 A were found. The LiFHF complexes show a hydrogen difluoride anion coordinated in a bent fashion via one fluorine atom to the lithium ion.

Analysis of Dipolar-Coupling-Mediated Coherence Transfer in a Homonuclear Two Spin-[formula omitted] Solid-State System

Homonuclear dipolar-mediated coherence transfer (DCT), a through-space transfer of magnetization between like spins, can yield otherwise difficult-to-obtain structural information for macromolecules by measuring the internuclear distances between two sites of interest. The behavior of a spin-12 system under DCT is analyzed in detail by computing the time development of the density matrix using the product operator formalism. The effect of coherence transfer (CT) via the homonuclear isotropic scalar coupling on DCT is examined. Analytical and computational results that yield useful information on the frequencies, first-maxima, and first-zero of CT for a uniaxially oriented or a single-crystal solid-state system are presented. The results predict that the evolution of the spin angular momentum operators under the homonuclear dipolar coupling Hamiltonian leads to “cylindrical mixing” unlike “isotropic mixing” due to the strong scalar coupling Hamiltonian. These results will find relevance in both the design of RF pulse sequences for the structural studies of uniaxially oriented biological solids and the interpretation of solution NMR results from proteins embedded in partially oriented bicelles.

Dipolar Cross Correlation and the Nuclear Overhauser Effect in Systems with Strong Scalar Coupling

Dipolar Cross Correlation and the Nuclear Overhauser Effect in Systems with Strong Scalar Coupling

One-dimensional homonuclear polarization transfer. Studies of parameters which affect intensities in the SPI NMR technique

A study is presented of the effects of strong scalar coupling and of the angle of the nonselective observing pulse, as well as of the effects of selective pulses, in one-dimensional selective population inversion (SPI) experiments of homonuclear spin systems. Density matrix computer simulations, which also take into account relaxation effects, of {A}-B SPI experiments, and for comparison also of {A}-X, reproduce different aspects of the experimental spectra. An insight is therefore offered into the parameters which affect intensities in the SPI one-dimensional NMR technique for homonuclear polarization transfer.

Hydrogen bonding in systems containing imidazoline nitroxide radicals

Hydrogen bonding between imidazoline nitroxide free radicals and hydrogen donors of varying strength [acetone, chloroform, trifluoroacetic acid (TFA)] has been examined using magnetic resonance techniques. Low-field dynamic nuclear polarization and spin–lattice relaxation measurements have been used to determine absolute scalar and dipolar coupling strengths. Dipolar coupling was found to increase in the order acetone <CHCl3<TFA, while only TFA systems showed substantial scalar coupling. For acetone, observed dipolar relaxation rates are consistent with translational modulation and little, if any, complexation. With CHCl3, both translational and rotational modulation are required; the data suggest that CHCl3 forms weak hydrogen bonds with the radical chromophore with little spin density transferred to the bridge proton. In contrast, the protons of TFA show strong scalar coupling with the various radicals which results in positive proton NMR enhancements or strong hyperfine coupling (AH?5 G). The 1H scalar relaxation rates are consistent with strong hydrogen bond formation at the imine site and complex lifetimes of ∼10−9–10−8 sec. The 19F nuclei of TFA show much weaker, radical-independent scalar coupling and little rotational dipolar modulation consistent with, at best, weak, nonstereospecific complex formation.

Relaxation of fluorine nuclei by collisions with free radicals

Intermolecular nuclear relaxation during liquid-state collisions has been examined for 16 combinations of four free radicals and four fluorocarbons. The spectra of electron-induced scalar relaxation transitions are resolved into high- and low-frequency components. All samples are found to have a spectral component with a short correlation time of around 1×10−11 sec, corresponding to fast diffusion-controlled collisions. Those samples which show stronger scalar hyperfine coupling show a second component with time up to 44×10−11 sec, corresponding to sluggish stereospecific collisions or transient complexation. Strong scalar coupling is found to occur with a proportionally strong dipolar coupling, which is augmented well beyond that expected from feasible molecular approach distances. Monopoly of spin-rich sites by the fluorocarbon might account for part of the dipolar augmentation but does not satisfactorily account for the entire range of observations. An induced anisotropic hyperfine coupling, after the manner of the scalar coupling, is proposed as an alternative explanation. The existence of an augmented dipolar coupling poses certain new considerations in theoretical interpretation of diffusion-controlled dipolar relaxation, and in computation of atomic distances in spin-labeling studies.

Dynamic Nuclear Polarization by Nitroxide, Perhalocarbon, Semiquinone, and Verdazyl Radicals

Dynamic nuclear polarization has been examined in liquid solutions of 16 different radicals. Hexafluorobenzene was used to gauge the intermolecular hyperfine interaction. The scalar coupling reveals transient complexation between radical and receptor and various bond-forming tendencies. Perhalocarbon radicals show very weak scalar coupling with fluorine nuclei. Verdazyls show strong scalar coupling, with negligible effects from substituents on the outer phenyl rings. Phenoxyl and pyrylyl radicals show a wide range of coupling which is only partly explained by planar complexation. Semiquinones show increasing scalar coupling with increasing chlorination. DNP with aliphatic nitroxide radicals is dominated by the NO group, but aromatic nitroxides show a strong ring substituent effect, exceeding intramolecular hyperfine variation in ESR. It is suggested that DNP can provide unique signatures of substituents on phenyl rings.

Dynamic polarization in fluorocarbons: Effect of scalar correlation time

Multi-field nuclear-electron double resonance experiments with octafluoronaphthalene (OFN) solutions containing organic free radicals have led to the conclusion that independent fluorine scalar and dipolar correlation times are necessary to characterize these systems. Scalar correlation times were longer than dipolar times for systems with strong scalar coupling and shorter than dipolar times for systems with weak scalar coupling. This trend may be associated with the availability of the odd electron at the radical edge in conjunction with the ability of particular radical-solvent pairs to form transient complexes. Further, it signifies that a large part of the variation in F signal enhancement arises from changes in scalar correlation time. Largest observed F enhancements were + 390 for OFN with 2,3,5,6-tetrachlorosemiquinone in acetone at 1 gauss and + 435 for OFN with 2,4,6-triphenyl pyrylyl in acetone at 74 gauss.