2H2O quadrupolar splitting used to measure water exchange in erythrocytes

The 2H NMR resonance from HDO (D=2H) in human red blood cells (RBCs) suspended in gelatin that was held stretched in a special apparatus was distinct from the two signals that were symmetrically arranged on either side of it, which were assigned to extracellular HDO. The large extracellular splitting is due to the interaction of the electric quadrupole moment of the 2H nuclei with the electric field gradient tensor of the stretched, partially aligned gelatin. Lack of resolved splitting of the intracellular resonance indicated greatly diminished or absent ordering of the HDO inside RBCs. The separate resonances enabled the application of a saturation transfer method to estimate the rate constants of transmembrane exchange of water in RBCs. However both the theory and the practical applications needed modifications because even in the absence of RBCs the HDO resonances were maximally suppressed when the saturating radio-frequency radiation was applied exactly at the central frequency between the two resonances of the quadrupolar HDO doublet. More statistically robust estimates of the exchange rate constants were obtained by applying two-dimensional exchange spectroscopy (2D EXSY), with back-transformation analysis. A monotonic dependence of the estimates of the efflux rate constants on the mixing time, tmix, used in the 2D EXSY experiment were seen. Extrapolation to tmix=0, gave an estimate of the efflux rate constant at 15 degrees C of 31.5+/-2.2 s(-1) while at 25 degrees C it was approximately 50 s(-1). These values are close to, but less than, those estimated by an NMR relaxation-enhancement method that uses Mn2+ doping of the extracellular medium. The basis for this difference is thought to include the high viscosity of the extracellular gel. At the abstract level of quantum mechanics we have used the quadrupolar Hamiltonian to provide chemical shift separation between signals from spin populations across cell membranes; this is the first time, to our knowledge, that this has been achieved.     

Enhanced ionization of perpendicularly aligned H2 in intense laser fields

Using three-dimensional classical ensembles, we have investigated the enhancement of double ionization of perpendicularly aligned H₂ molecules by a 800 nm laser pulse with intensity ranging from 1 × 10¹⁴ W/cm² to 6 × 10¹⁴ W/cm². The simulated results show that double ionization probability of H₂ strongly depends on R and reaches a maximum at an intensity independent critical distance R_C ≈ 5 a.u. Furthermore, the enhancement of double ionization is more pronounced in the cases of weaker or stronger fields. These results, a well indication of the influence of molecular structures and laser–molecule interactions on double ionization of diatomic molecules, are analyzed in detail and qualitatively explained based on the field-induced barrier suppression model and back analysis.     

Developments in quantum information processing by nuclear magnetic resonance: Use of quadrupolar and dipolar couplings

Use of dipolar and quadrupolar couplings for quantum information processing (QIP) by nuclear magnetic resonance (NMR) is described. In these cases, instead of the individual spins being qubits, the 2 ⁿ energy levels of the spin-system can be treated as an n-qubit system. It is demonstrated that QIP in such systems can be carried out using transition-selective pulses, in CH₃CN, ¹³CH₃CN, ⁷Li (I=3/2) and ¹³³Cs (I=7/2), oriented in liquid crystals yielding 2 and 3 qubit systems. Creation of pseudopure states, implementation of logic gates and arithmetic operations (half-adder and subtractor) have been carried out in these systems using transition-selective pulses.     

Solid H2: Has the quadrupolar glass phase a well-defined transition?

We report measurements of the NMR lineshape, the second and fourth moments (M₂ and M₄), and of the longitudinal relaxation tine in H₂ single crystals. Although a sharp phase transition cannot be excluded, our results for ortho concentrations between 0.15 and 0.4 suggest rather a continuous freezing of rotational motion to a “glass state” as T decreases. In this regime; the NMR spectrum is found to be very nearly isotropic.     

Role of nuclear couplings in the inelastic excitation of weakly bound neutron-rich nuclei

We study the breakup of weakly bound systems close to the neutron drip line in collisions with heavy-ions. The process is described in terms of inelastic excitations leading to states in the continuum. The nuclear and Coulomb multipole fields induced by the reaction partner are both included in the microscopic construction of the inelastic form factors. A fraction of the dissociation cross sections is triggered by the presence of resonances at high excitation energy. The largest probabilities for breakup result, however, from strongly coupled transitions to low-lying states in the continuum. Nuclear couplings associated with inelastic excitation to these states are found to extend to unusually large distances. As a consequence, the interplay of nuclear and Coulomb excitations differs from the ordinary situation encountered in heavy-ion inelastic processes involving systems close to the stability line.     

Magnetization transfer via residual dipolar couplings: application to proton-proton correlations in partially aligned proteins

A novel three-dimensional NMR experiment is reported that allows the observation of correlations between amide and other protons via residual dipolar couplings in partially oriented proteins. The experiment is designed to permit quantitative measurement of the magnitude of proton-proton residual dipolar couplings in larger molecules and at higher degree of alignments. The observed couplings contain data valuable for protein resonance assignment, local protein structure refinement, and determination of low-resolution protein folds.     

Accurate measurement of small spin-spin couplings in partially aligned molecules using a novel J-mismatch compensated spin-state-selection filter

The accurate measurement of small spin-spin coupling constants in macromolecules dissolved in a liquid crystalline phase is important in the context of molecular structure investigation by modern liquid state NMR. A new spin-state-selection filter, DIPSAP, is presented with significantly reduced sensitivity to J-mismatch of the filter delays compared to previously proposed pulse sequences. DIPSAP presents an attractive new approach for the accurate measurement of small spin-spin coupling constants in molecules dissolved in anisotropic solution. Application to the measurement of 15N-13C' and 1H(N)-13C' coupling constants in the peptide planes of 13C, 15N labeled proteins demonstrates the high accuracy obtained by a DIPSAP-based experiment.     

CP/MAS of quadrupolar S = 3/2 nuclei.

The spin dynamics of Hartmann-Hahn cross-polarization from I = 1/2 to quadrupolar S = 3/2 nuclei is investigated. A density-matrix model applicable to cases where the quadrupole frequency vQ is much larger than the rf amplitude v1S of the S spins, predicts the time development of the spin state of an isolated I, S spin pair in static situations and in three distinct cases of magic-angle-spinning speed vR. These cases are characterized as slow, intermediate, and fast, depending on the magnitude of the parameter alpha = v1S2/vQvR relative to the intermediate value of 0.4. The model predictions are supported by numerical simulations. The polarization transfer from I to S is efficient in the limits of slow and fast sample spinning. When alpha < 1, the Hartmann-Hahn condition is shifted over once or twice vR. When the spinning rate is intermediate, poor spin-locking of the quadrupolar spins prevents the accumulation of a cross-polarization signal and, in addition, depletes the spin-locked I magnetization. Experimental CP/MAS data obtained in NaOH show that the concepts developed for isolated spin pairs are also applicable to cross-polarization in a strongly coupled multi-spin system.     

An optimal strategy for recovering the deuterium (2H) quadrupolar interaction under magic-angle spinning NMR

By exploiting the homology in the form of the truncated high-field homonuclear dipole–dipole and quadrupole coupling Hamiltonians, we have previously demonstrated that a simple adaptation of a rotor-synchronized pulse sequence (DRAMA) used for the recovery of dipole–dipole couplings can also be used to resurrect quadrupole couplings (QUADRAMA). In the canonical implementation of these recovery pulse sequences, the couplings are not significantly scaled down from their static sample values. While such minimal scaling is of course desirable in the recovery of typical homonuclear dipolar couplings ( ≤ 2 kHz) and small quadrupole couplings, it is clearly not ideal for the recovery of the much larger quadrupole couplings (20–200 kHz) often encountered in solid-state ²H NMR. In such a case, some prior knowledge of the order of magnitude of the coupling is required to optimize the experimental conditions for QUADRAMA. In order to overcome this drawback, in this study, we have developed a general and optimized strategy for implementing the QUADRAMA technique which does not require any knowledge of the size of the coupling ν_Q. Experimental tests of the optimized protocol demonstrate that by judicious choices of a combination of scaling factors and recoupling times, ²H quadrupole couplings ranging over an order of magnitude from 3 to 42 kHz can be measured. Since this optimized protocol can reliably be used to recover couplings over a broad range, it expands the range of systems accessible to study by ²H NMR into a realm where static sample NMR and simple MAS NMR may fail.     

Calculating order parameter profiles utilizing magnetically aligned phospholipid bilayers for 2H solid-state NMR studies

Solid-state deuterium NMR spectroscopy was used to study the structural and dynamic properties of stearic acid-d(35) in magnetically aligned phospholipid bilayers as a function of temperature. Magnetically aligned phospholipid bilayers or bicelles are model systems, which mimic biological membranes for magnetic resonance studies. Paramagnetic lanthanide ions (Yb(3+)) were added to align the bicelles such that the bilayer normal is colinear with the direction of the static magnetic field. The corresponding order parameters of the stearic acid-d(35) probe were calculated and compared with values obtained from unoriented samples in the literature. The addition of cholesterol to the bicelle system decreases the fluidity of the phospholipid bilayers and increases the ordering of the acyl chains of stearic acid-d(35). This study demonstrates the feasibility of utilizing magnetically aligned bicelles for calculating 2H order parameter profiles for non-biological systems such as polymer-grafted membranes and Schiff's base complexes.     

Homogeneous broadenings in 2D solid-state NMR of half-integer quadrupolar nuclei

The question of the homogeneous broadening that occurs in 2D solid-state NMR experiments is examined. This homogeneous broadening is mathematically introduced in a simple way, versus the irreversible decay rates related to the coherences that are involved during t1 and t2. We give the pulse sequences and coherence transfer pathways that are used to measure these decay rates. On AlPO4 berlinite, we have measured the 27Al echo-type relaxation times of the central and satellite transitions on 1Q levels, so that of coherences that are situated on 2Q, 3Q, and 5Q levels. We compare the broadenings that can be deduced from these relaxation times to those directly observed on the isotropic projection of berlinite with multiple-quantum magic-angle spinning (MAS), or satellite-transition MAS. We show that the choice of the high-resolution method, should be done according to the spin value and the corresponding homogeneous broadening.     

Accurate measurement of H(N)-H(alpha) residual dipolar couplings in proteins.

A method for accurately measuring H(N)-H(alpha) residual dipolar couplings is described. Using this technique, both the sign and magnitude of the coupling can be determined easily. Residual dipolar coupling between H(N)(i)-H(alpha)(i) and H(N)(i)-H(alpha)(i-1) were measured for the FK506 binding protein complexed to FK506. The experimental values were in excellent agreement with predictions based on an X-ray crystal structure of the protein/ligand complex, suggesting that these residual dipolar couplings will provide accurate structural constraints for the refinement of protein structures determined by NMR.     

4f-electric quadrupolar anomalies in PrCu2Si2 and its pseudoternary solid solutions

Experimental observations for possible presence of 4f quadrupolar Kondo effect in PrCu₂Si₂ are critically discussed. A comparison of the low temperature properties of the alloys, Pr _x Y_1–x Cu₂Si₂ (x=0.05, 0.2 and 0.4), seems to signal the existence of 4f electric quadrupolar glass phenomenon around 4K forx=0.2.     

Enantiomeric excess measurements in weakly oriented chiral liquid crystal solvents through 2D H selective refocusing experiments

In this article, a simple and robust method is proposed for simplifying the analysis of proton spectra of molecules dissolved in weakly oriented chiral media. The NMR approach investigated is based on the use of proton selective refocusing 2D experiments (SERF) to measure proton–proton dipolar couplings from unresolved lines. This technique is applied to the case of enantiomers dissolved in chiral polypeptide liquid crystals. It is shown that an accurate determination of enantiomeric excess is possible within a short experimental time.     

Parameter maps of 1H residual dipolar couplings in tendon under mechanical load

Proton multipolar spin states associated with dipolar encoded longitudinal magnetization (DELM) and double-quantum (DQ) coherences of bound water are investigated for bovine and sheep Achilles tendon under mechanical load. DELM decay curves and DQ buildup and decay curves reveal changes of the 1H residual dipolar couplings for tendon at rest and under local compression forces. The multipolar spin states are used to design dipolar contrast filters for NMR 1H images of heterogeneous tendon. Heterogeneities in tendon samples were artificially generated by local compression parallel and perpendicular to the tendon plug axis. Quotient images obtained from DQ-filtered images by matched and mismatched excitation/reconversion periods are encoded only by the residual dipolar couplings. Semi-quantitative parameter maps of the residual dipolar couplings of bound water were obtained from these quotient images using a reference elastomer sample. This method can be used to quantify NMR imaging of injured ordered tissues.     

Temperature dependence of the hyperfine quadrupolar interaction in K2ZrF6

The thermal dependence of the hyperfine quadrupolar interaction of¹⁸¹Ta at Zr sites in K₂ZrF₆ has been studied by TDPAC between RT and 511 °C. Four phases of ever increasing symmetry have been found in the range RT-437 °C. The possible existence of a healing mechanism with an activation energy of 0.187±0.040 eV is discussed for the lattice recovery in the range 270 °C–335 °C. The results at temperatures above 437 °C can be understood assuming a thermal decomposition of the compound.     

Theoretical and experimental investigation of (13)C relayed (2)H-(2)H-COSY 2D experiments: application to the analysis of weakly aligned solutes

We describe new NMR 2D experiments denoted DECADENCY for DEuterium CArbon DEuterium Nuclear Correlation spectroscopY dedicated to the analysis of anisotropic deuterium spectra. They belong to the class of X-relayed Y,Y-COSY 2D experiments that was initially explored in the case of a (1)H-X-(1)H fragment (I(X)=1/2) in isotropic medium. DECADENCY 2D experiments permit to correlate the quadrupolar doublets associated with two inequivalent deuterium nuclei in an oriented CD(2) fragment through heteronuclear polarization transfers. Two kinds of pulse sequences are described here using either a double INEPT-type or DEPT-type process. DECADENCY 2D experiments provide an interesting alternative to (2)H-(2)H COSY experiments when the geminal (2)H-(2)H total coupling (scalar and dipolar) is null or too small to provide visible cross-correlation peaks. Such a situation is typically observed for geminal deuteriums in prochiral or chiral molecules dissolved in chiral liquid crystals. The efficiency of these techniques is illustrated using dideuterated prochiral molecules, the phenyl[(2)H(2)]methanol and the 1-chloro[1-(2)H(2)]nonane, both dissolved in organic solutions of poly-gamma-benzyl-l-glutamate. The advantages of each sequence are presented and discussed. It is shown that the relative sign of the quadrupolar doublets can be determined.     

Protein backbone dynamics from N-HN dipolar couplings in partially aligned systems: a comparison of motional models in the presence of structural noise

Residual dipolar couplings (RDCs) provide excellent probes for the exploration of dynamics in biomolecules on biologically relevant time-scales. Applying geometric motional models in combination with high-resolution structures to fit experimental RDCs allows the extraction of local dynamic amplitudes of peptide planes in proteins using only a limited number of data points. Here we compare the behaviour of three simple and intuitive dynamic modes: the Gaussian axial fluctuation model (1D-GAF), the two-site jump model, and a model supposing axially symmetric motion about a mean orientation. The requirement of a structural model makes this kind of methodology potentially very sensitive to structural imprecision. Numerical simulations of RDC dynamic averaging under different regimes show that the anisotropic motional models are more geometrically stringent than the axially symmetric model making it more difficult to alias structural noise as artificial dynamic amplitudes. Indeed, it appears that the model extracts accurate motional amplitudes even in the presence of significant structural error. We also show that a two-site jump model, also assuming the (alpha)C(i-1)-(alpha)C(i) as rotation axis, can only be distinguished from the previously developed GAF model beyond amplitude/jumps of around 40 degrees. The importance of appropriate estimation of the molecular alignment tensor for determination of local motional amplitudes is also illustrated here. We demonstrate a systematic scaling of extracted dynamic amplitudes if a static structure is assumed when determining the alignment tensor from dynamically averaged RDCs. As an example an artificial increase of 0.14 (0.85 compared to the expected 0.71) is observed in the extracted S2 if a pervasive 20 degrees GAF motion is present that is ignored in the tensor determination. Finally we apply a combined approach using the most appropriate motional model, to complete the analysis of dynamic motions from protein G.     

Resolution enhancement in spectra of natural products dissolved in weakly orienting media with the help of 1H homonuclear dipolar decoupling during acquisition: application to 1H-13C dipolar couplings measurements

In weakly orienting media such as poly-gamma-benzyl-L-glutamate (PBLG) a polymer that forms a chiral liquid crystal in organic solvents, the spectral resolution for embedded molecules is usually poor because of numerous (1)H, (1)H dipolar couplings that generally broaden proton spectra. Therefore (1)H, (13)C dipolar couplings are difficult or impossible to measure. Here, we incorporate Flip-Flop decoupling during detection into an HSQC experiment. Flip-Flop removes the (1)H, (1)H dipolar couplings and scales the chemical shifts of the protons as well as the (1)H, (13)C dipolar couplings during detection. A resolution gain by a factor 1.5-4.2 and improved signal intensity by an average factor of 1.6-1.7 have been obtained. This technique is demonstrated on (+)-menthol dissolved in a PBLG/CDCl(3) phase.