Cross-polarization Method Research Articles

Light-emitting-diode-induced fluorescence detection of fluorescent dyes for capillary electrophoresis microchip with cross-polarization method

A cross-polarization scheme is presented to filter out the excitation light from the emission spectrum of fluorescent dyes using green light emitting diodes as a light source and a linear charge coupled device as an intensity detector. The excitation light was linearly polarized and was then used to illuminate the fluorescent dyes in the microchannels of a capillary electrophoresis microchip. The detector was shielded by the second polarizer, oriented perpendicular to the excitation light. The fluorescent signals from Rhodamine B dyes were measured in a dilution series with resulting emission signals and four different concentrations of fluorescent dyes were detected simultaneously with the same excitation source and detector. A limit-of-detection of 1 μM was demonstrated for Rhodamine B dye under the optimal conditions.

Quantitative cross-polarization at magic-angle spinning frequency of about 20 kHz

We study the QUantitative Cross-Polarization (QU-CP) method proposed by Hou et al. (Chem. Phys. Lett. 421 (2006) 356) under the moderate MAS speed of 23 kHz, re-examining its two building blocks, namely, the CP polarization transfer from 1H to 13C, and the thermal equalization of the 13C magnetizations. We show that the nuclear-integrated cross-polarization (NI-CP) scheme is conveniently used for 1H– 13C polarization transfer, because of its simplicity, robustness to rf-mismatch, and compatibility with fast sample spinning. In the mixing part, in addition to dipolar-assisted rotational-resonance (DARR) recoupling, we examine the Phase-Alternated Recoupling Irradiation Schemes (PARIS and PARIS xy ), and Second-order Hamiltonian among Analogous Nuclei Generated by Hetero-nuclear Assistance Irradiation (SHANGHAI) sequences, and show that SHANGHAI gives the best performances in equalizing the 13C magnetizations.

Optimal (2)H rf Pulses and (2)H-(13)C Cross-Polarization Methods for Solid-State (2)H MAS NMR of Perdeuterated Proteins.

We present a novel concept for rf pulses and optimal control designed cross-polarization experiments for quadrupolar nuclei. The methods are demonstrated for (2)H CP-MAS and (2)H multiple-pulse NMR of perdeuterated proteins, for which sensitivity enhancements up to an order of magnitude are presented relative to commonly used approaches. The so-called RESPIRATION rf pulses combines the concept of short broad-band pulses with generation of pulses with large flip angles through distribution of the rf pulse over several rotor echoes. This lead to close-to-ideal rf pulses, facilitating implementation of experiments relying on the ability to realize high-performance 90 and 180° pulses, as, for example, in refocused INEPT and double-to-single quantum coherence experiments, or just pulses that provide a true representation of the quadrupolar powder pattern to extract information about the structure or dynamics. The optimal control (2)H → (13)C CP-MAS method demonstrates transfer efficiencies up to around 85% while being extremely robust toward rf inhomogeneity and resonance offsets.

Solid-State Molecular Rotors with Perdeuterated Stators: Mechanistic Insights from Biphenylene Rotational Dynamics in Ordered and Disordered Crystal Forms

Samples of 4,4'-bis(3,3,3-tri-d(5)-phenylpropynyl)biphenyl 2, 9,10-bis(3,3,3-tri-d(5)-phenylpropynyl)anthracene 3, 1,4-bis(3,3,3-tri-d(5)-phenylpropynyl)naphthalene 4, and 4,4'-bis(3,3,3-tri-d(5)-phenylpropynyl)-1,1'-binaphthyl 5 were prepared via a Sonogashira coupling of 3,3,3-tri-d(5)-phenylpropyne 7 and the appropriate aryl dibromide. Single crystal X-ray diffraction structures were obtained for an o-xylene clathrate of 2 and for solvent-free crystals of 3. All four molecular rotors were characterized by CPMAS (13)C NMR experiments with varying contact times in order to determine whether the carbon signals of the central rotator group could be selectively enhanced and studied without interference or overlap of signals from the deuterated stator, which is insensitive to the {(1)H}-(13)C cross-polarization method. It was shown that the (13)C signals of the natural abundance rotator group can be selectively observed with short contact times (ca. 50 micros) without interference from other (13)C signals in the molecule. Variable-temperature CPMAS (13)C NMR studies with a crystalline o-xylene solvate of biphenylene rotor 2 suggested a 2-fold flipping process in the fast exchange regime, even at temperatures as low as 199 K (-74 degrees C). Indirect support for this was obtained by studies carried out with a disordered, solvent-free solid, obtained by fast precipitation from hexanes and dichloromethane, which displayed slower dynamics within the same temperature range with an activation energy of 8.7 kcal/mol and a pre-exponential factor of 4.9 x 10(9) s(-1). Confirmation of an exchange process in the megahertz regime for the crystalline solvate was obtained by variable-temperature quadrupolar echo (2)H NMR data acquired with samples prepared with a deuterated biphenylene rotator and a natural abundance stator. Although rotational exchange occurs in the solvated samples with a slightly lower barrier of 7.4 kcal/mol, the main difference with the precipitated solid comes from the pre-exponential factor, which is nearly 3 orders of magnitude greater with a value of 2.5 x 10(12) s(-1). On the basis of these differences, we speculate that efficient rotational motion in the solvated crystals may take advantage of long-range lattice vibrations that couple with molecular modes and that the lack of long-range order may be responsible for the low pre-exponential factor observed in the disordered crystals.

Impact of selective excitation on carbon longitudinal relaxation: Towards fast solid-state NMR techniques

The effect of selective pulses on the apparent carbon longitudinal relaxation is investigated in three fully 13C-labeled systems, histidine as a model system and two proteins MerP and YajG. It is shown that the longitudinal relaxation of a selectively excited carbon spin is greatly enhanced, mainly because of fast spin-diffusion. This relaxation enhancement allows reducing the time necessary for polarization recovery between two experiments. This effect can be exploited either to improve the sensitivity of NMR experiments or to reduce the experimental time. Using selective carbon excitation combined with fast pulsing on fully 13C-labeled proteins, a sensitivity improvement of 20–45% over standard cross-polarization methods is predicted from the measured relaxation times.

Structural Analysis of Transmembrane Halobacterial Tranceducer pHtrII by Multi-Dimensional High-Resolution Solid-State NMR

°Transmembrane protein pHtrII is a transducer which binds to phoborhodopsin. The light excitation of phoborhodopsin is transmitted into the cytoplasm through pHtrII to promote negative photoaxis. We studied uniformly 13 C, 15 N labeled 159residues pHtrII by high-resolution solid-state NMR. The pHtrII (1-159) was econstructed into the deuterated DMPC membranes. High-resolution solid-state 2D NMR was measured under magic-angle spinning at the 1 H resonance frequencies of 500, 600 and 700 MHz . At first, we have assigned 13 C signals of pHtrII (1-159) by using 13 C- 13 C spin diffusion under DARR at -40 o C.Intraresidue correlations across 1-2 bonds were observed at short mixing time. At a long mixing time of 200 ms, intra-residue C α correlations across 3-bonds were obtained. Most amino acids except Tyr and Pro were assigned. The DARR spectra were observed for immobile region of pHtrII. All 13 C signals could not be observed in these dipolar-coupling-based cross-polarization methods because of large-amplitude molecular motions at room temperature. We performed Jcoupling-based HC-INEPT and HCC-TOCSY experiments in order to obtain signals of mobile protein segments. Mobile protein segments make up about 70% of pHtrII (1-159). Many signals in mobile regions were observed in HC and HCC 2D spectra. 13 C and 1 H signals were assigned to amino acid groups were successful in mobile regions. The C α and CO chemical shifts indicate that pHtrII (1-159) mainly forms α -helix structure in lipid bilayer environment.

Studying Organic Matter Molecular Assemblage within a Whole Organic Soil by Nuclear Magnetic Resonance

This work shows the applicability of two-dimensional (2D) (1)H-(13)C heteronuclear correlation (HETCOR) nuclear magnetic resonance (NMR) spectroscopy to the characterization of whole soils. A combination of different mixing times and cross polarization (CP) methods, namely Lee-Goldberg (LG)-CP and Ramp-CP are shown to afford, for the first time, intra- and inter- molecular connectivities, allowing for molecular assemblage information to be obtained on a whole soil. Our results show that, for the brackish marsh histosol under study, two isolated domains could be detected. The first domain consists of O-alkyl and aromatic moieties (lignocellulose material), while the second domain is comprised of alkyl type moieties (cuticular material). The role of these domains is discussed in terms of hydrophobic organic compound sorption within soil organic matter (SOM), including the possible effects of wetting and drying cycles.

A Morphological Study of Poly[bis(trifluoroethoxy)phosphazene] Using High Resolution Solid-State 1H, 19F, 31P and 13C NMR Spectroscopy

High-resolution 19F, 1H, 31P and 13C solid-state NMR methods were assessed to ascertain their suitability for studying the morphological behavior in the crystalline domain of phosphazene polymers with partially fluorinated side-chains. Poly[bis(trifluoroethoxy)phosphazene] (PBFP) was used as a sample system. Fast magic angle spinning (MAS), along with simultaneous 19F and 1H decoupling using the xy-16 sequence, were employed, as this has proven to greatly improve resolution in 13C spectra of perfluorinated materials. Information obtained from Discrimination Induced by Variable Amplitude Minipulses (DIVAM) nutation experiments and cross-polarization (CP) methods aided the deconvolution analysis used to identify all components in the 1H, 19F and 13C signals. DIVAM nutation experiments were also used to discriminate between signals from the amorphous and crystalline domain. The crystallinity in the solvent-cast PBFP was determined to be approximately 70%, which was seen to increase to approximately 80% in the heat-treated material, for all nuclei studied. A preliminary assignment was made for the crystalline signals in the 1H and 13C spectra to the α-, β- and γ-phases. Therefore, high-resolution 13C and 1H methods are valuable tools for morphological investigations into this class of polymer.

Observation of Solid-State 103Rh NMR by Cross-Polarization

Using 103Rh[1H] cross-polarization (CP) methods, we have obtained solid-state 103Rh NMR spectra for diamagnetic Rh(III) compounds. The isotropic chemical shift and chemical shift anisotropy (CSA) are reported for a crystalline form of the dihydroxy-bridged Rh(III) dimer and for a salt of the oxo-centered acetate-bridged Rh(III) trimer, from analysis of conventional CP/MAS spectra. Comparison of the CP kinetics of the dimer with new crystal structure data suggests a strategy for predicting 103Rh CP/MAS properties in solids.

The Study of Australian Coal Maturity: Relationship between Solid-State NMR Aromaticities and Organic Free-Radical Count

Samples of a range of Australian bituminous coals from Pelican-5, which is a petroleum drill site in the Bass Basin, located in Australia offshore between Victoria and Tasmania, were studied by 13C solid-state nuclear magnetic resonance (13C NMR) spectroscopy and electron spin resonance (ESR) spectroscopy. As expected, the fraction of carbon that is aromatic in these coals, measured using a single-pulse (Bloch decay) method, was higher than values obtained by the cross-polarization (CP) method, because some carbon observed using the single-pulse method is not observed by the CP method. Loss of signal through rapid spin−spin relaxation due to inorganic paramagnetics has been excluded as a source of differences in the aromaticity measurements; however, the organic free-electron content correlates well with observed spin−lattice relaxation times in the rotating frame and the difference between aromaticities measured by the Bloch decay and CP methods decreases as the electron count and rank increase. Some aro...

67Zn solid-state and single-crystal NMR spectroscopy and X-ray crystal structure of zinc formate dihydrate.

The crystal structure, quadrupole coupling parameters, and the orientation of the electric field gradient tensors for each site of zinc formate dihydrate have been determined. There are two distinct sites in the asymmetric unit: one containing four in-plane waters with two bridging formats, the other containing six bridging formates. The solid-state NMR lineshapes have been assigned to their respective sites by using isotopic labeling and cross-polarization methods. The hydrated site corresponds to the lineshape having a quadrupole coupling constant (Cq) of 9.6 MHz and the anhydrous site has a Cq of 6.2 MHz. The absence of chemical shielding contributions to the observed lineshapes has been verified with a high-field solid-state NMR experiment performed at 18.8 T.

Excitation-and-collection geometry insensitive fluorescence imaging of tissue-simulating turbid media

We present an imaging technique for the correction of geometrical effects in fluorescence measurement of optically thick, turbid media such as human tissue. Specifically, we use the cross-polarization method to reject specular reflection and enhance the diffusive backscattering of polarized fluorescence excitation light from the turbid media. We correct the nonuniformity of the image field caused by the excitation-and-collection geometry of a fluorescence imaging system by normalizing the fluorescence image to the cross-polarized reflection image. The ratio image provides a map of relative fluorescence yield, defined as the ratio of emerging fluorescence power to incident excitation, over the surface of an imaged homogeneous turbid medium when fluorescence excitation-and-collection geometries vary in a wide range. We investigate the mechanism of ratio imaging by using Monte Carlo modeling. Our findings show that this technique could have a potential use in the detection of early cancer, which usually starts from a superficial layer of tissue, based on the contrast in the tissue fluorescence of an early lesion and of the surrounding normal tissue.

Correction of geometrical effects on fluorescence imaging of tissue

A cross-polarization method has been used to eliminate the specular reflection and enhance the diffusive backscattering of fluorescence excitation light from tissue-like phantoms and skin tissue. We have demonstrated that the non-uniformity of excitation and collection of a fluorescence imaging system can be corrected by normalizing fluorescence signal to the cross-polarized reflection signal recorded from the same site. The ratio image of fluorescence vs. cross-polarized reflection provides a map of relative fluorescence yield over the tissue surface. This ratio imaging technique may hold the potential to detect early cancer, which usually starts from the superficial layer of tissue, based on the contrast in the fluorescence yield between early lesion and surrounding normal tissue.

Mapping the fluorescence yield on turbid media

In this letter, we introduce a combined polarization and fluorescence imaging technique for the measurement of fluorescence yield on the surface of turbid media. We use the cross-polarization method to reject the specular reflection and enhance the diffusive backscattering from the turbid media. It has been found that the ratio image of fluorescence versus cross-polarized reflection is not sensitive to the geometry of fluorescence excitation and illumination, and provides a map of fluorescence yield on the surface of imaged subject. The technique reported in this letter may potentially solve the problem for imaging of early cancers which usually start from the superficial layer of tissue and have the fluorescence yield lower than surrounding normal tissue.

Estimation of charcoal (char) in soils

A method for the semiquantitative estimation of charcoal (char) in soils is reported. The technique, performed on the <53 μm fraction of soils, utilizes the highly aromatic nature of char and its relative stability to two hours of high energy ultraviolet photo‐oxidation compared to other soil carbon fractions. The proportion of char in the <53 μm fraction is estimated as the aromatic carbon (C) content measured by solid‐state nuclear magnetic resonance spectroscopy using cross polarization and magic angle spinning. Corrections are made for the underestimate of aryl C by the cross polarization method in highly condensed aromatic char structures and for the small lignin content that occurs in some samples. Evidence is also provided for the occurrence of >90% of soil char in the <53 μm fraction. Because of the assumptions used, estimates of char by this approach must be considered to be semiquantitative, but are conservative and provide estimates of the minimum char content of a soil.

Polarized 129Xe solid for polarizing unstable nuclei

A polarized 129Xe solid has been developed as a host material in order to polarize implanted unstable nuclei ( 11Be) through the cross polarization method. Up to now, 1% of polarization has been realized with about 80 mg of 129Xe solid. An experiment to confirm the feasibility of implanting 11Be atoms from high-energy heavy ion reaction in the Xe stopper has been successfully carried out.

Measurement of Hartmann-Hahn cross-polarization dynamics with quenching of proton T1 ρ relaxation dependence

A simple modification of the standard cross-polarization method designed for quenching the proton T 1 ρ dependence when studying polarization transfer is presented. It is demonstrated that by using this simple procedure, new and subtle details of cross-polarization dynamics, previously hidden by the T 1 ρ ( 1H) effect, can be observed in dipolar-coupled spin systems.

Optimized Spectral Editing of 13C MAS NMR Spectra of Rigid Solids Using Cross-Polarization Methods

Abstract Combinations of 13C magic-angle spinning (MAS) NMR experiments employing cross polarization (CP), cross polarization-depolarization (CPD), and cross polarization-depolarization-repolarization are analyzed quantitatively to derive simple and general procedures for optimized spectral editing of 13C CP/MAS NMR spectra of rigid solids by separation of the 13C resonances into CHn subspectra (n = 0, 1, 2, and 3). Special attention is devoted to a differentiation by CPD/MAS of CH and CH2 resonances since these groups behave quite similarly during spin lock under Hartmann-Hahn match and are therefore generally difficult to distinguish unambiguously. A general procedure for the design of subexperiments and linear combinations of their spectra to provide optimized signal-to-noise ratios for the edited subspectra is described. The technique is illustrated by a series of edited 13C CP/MAS spectra for a number of rigid solids ranging from simple organic compounds (sucrose and l-menthol) to complex pharmaceutical products (calcipotriol monohydrate and vitamin D3) and polymers (polypropylene, polyvinyl alcohol, polyvinyl chloride, and polystyrene).

J Cross-Polarization Methods for Localized NMR without Magnetic Field Gradients

This work predominantly refers to 13C in vivo spectroscopy and has two objectives. First, heteronuclear editing is considered for proton-detected NMR spectroscopy of rare nuclei. The second aspect refers to localized heteronuclear NMR spectroscopy implying the editing option. Both purposes are achieved by employing J cross polarization (or coherent rotating-frame polarization transfer). The spatial selectivity is due to different in-homogeneities of the superimposed double-resonance radiofrequency fields of the J cross-polarization experiment. A 13C surface coil, for instance, was combined with a 1H birdcage resonator, so that J cross polarization is restricted to the region where the Hartmann/Hahn condition is approached. Heteronuclear editing of NMR lines for proton-detected 13C spectroscopy was performed by the aid of a cyclic cross-polarization pathway. Test experiments demonstrating the volume selectivity of the transfer process and the editing efficiency of the cyclic cross-polarization procedure are reported.

The study of the phase transitions in mixed crystals of NH4HSeO4group

Abstract The study of the mixed system Mex(NH4)1-xHSeO4, where Me = K, Rb, Cs and of the system NH4H(SxSe1-x)O4 have been carried out using NMR cross-polarization method, dielectric and X-ray measurements. The region of x where the structure of ammonium hydrogen selenate (AHSe) is maintained for different substitutes was determined. The temperature shift of the transition points T i and T c have been found in the mixed crystals. The selective sensitivity of the different type of hydrogen bonds in the monoclinic structure of AHSe to the substitutions and the appearance of the frozen local electric dipole moments affecting the transition temperatures T i and T c are discussed.