Lee-Goldburg Cross-polarization Research Articles

The local physical structure of amorphous hydrogenated boron carbide: insights from magic angle spinning solid-state NMR spectroscopy**This paper is dedicated to the memory of James M Phillips, Professor of Physics at the University of Missouri-Kansas City from 1965 to 2009 who worked, in small part, on theoretical models of carborane adsorbates.

Magic angle spinning solid-state nuclear magnetic resonance spectroscopy techniques are applied to the elucidation of the local physical structure of an intermediate product in the plasma-enhanced chemical vapour deposition of thin-film amorphous hydrogenated boron carbide (BxC:Hy) from an orthocarborane precursor. Experimental chemical shifts are compared with theoretical shift predictions from ab initio calculations of model molecular compounds to assign atomic chemical environments, while Lee–Goldburg cross-polarization and heteronuclear recoupling experiments are used to confirm atomic connectivities. A model for the BxC:Hy intermediate is proposed wherein the solid is dominated by predominantly hydrogenated carborane icosahedra that are lightly cross-linked via nonhydrogenated intraicosahedral B atoms, either directly through B–B bonds or through extraicosahedral hydrocarbon chains. While there is no clear evidence for extraicosahedral B aside from boron oxides, ∼40% of the C is found to exist as extraicosahedral hydrocarbon species that are intimately bound within the icosahedral network rather than in segregated phases.

Intermediate motions and dipolar couplings as studied by Lee–Goldburg cross-polarization NMR: Hartmann–Hahn matching profiles

In this article, we evaluate the use of simple Lee-Goldburg cross-polarization (LG-CP) NMR experiments for obtaining quantitative information of molecular motion in the intermediate regime. In particular, we introduce the measurement of Hartmann-Hahn matching profiles for the assessment of heteronuclear dipolar couplings as well as dynamics as a reliable and robust alternative to the more common analysis of build-up curves. We have carried out dynamic spin dynamics simulations in order to test the method's sensitivity to intermediate motion and address its limitations concerning possible experimental imperfections. We further demonstrate the successful use of simple theoretical concepts, most prominently Anderson-Weiss (AW) theory, to analyze the data. We further propose an alternative way to estimate activation energies of molecular motions, based upon the acquisition of only two LG-CP spectra per temperature at different temperatures. As experimental tests, molecular jumps in imidazole methyl sulfonate, trimethylsulfoxonium iodide, and bisphenol A polycarbonate were investigated with the new method.

13C−1H heteronuclear dipolar correlation studies of the hydrogen bonding of the quinones inRhodobacter sphaeroides R26 reaction centers

The photosynthetic reaction center (RC) of the photosynthetic bacteriumRhodobacter sphaeroides R26 contains two quinones, QA and QB. Solid-state heteronuclear (1H−13C) dipolar correlation spectroscopy has been used to study the binding of the quinones in the ground state for RCs reconstituted with l-13C ubiquinone-10. Lee-Goldburg cross-polarization buildup curves are recorded to determine distancesrCH between the l-13C carbon labels and the protons involved in the polarization transfer. The l-13C of both QA and QB have intermolecular correlations with protons that resonate downfield, in the region of hydrogen-bonding protons. The distances between the carbon labels and the correlated protons are short, 0.21±0.01 nm. Hence the nuclear magnetic resonance provides evidence for strong hydrogen-bonding interactions at the l-C=O of both QA and QB for RCs in the ground state. The environment of the l-13C of the QB is structurally heterogeneous compared to that of the QA. The data can be reconciled with a strong H-bonding interaction of the l-C=O of QA with Ala M260 NH, and with complex hydrogen bonding involving NH of Ile-L224 and of Gly-L225, and possibly the Ser-L223 hydroxyl group of the l-C=O of the QB, in the proximal site.

Locating hydrogen atoms in single crystal and uniaxially aligned amino acids by solid-state NMR

We demonstrate a novel method to locate hydrogen atoms in amino acids, which involves measuring the C αH α bond vector geometry through orientationally dependent dipolar coupling frequencies measured by Lee–Goldburg cross polarization (LGCP). A 2D LGCP experiment is used to measure the polar angle of the C αH α bond vector in a single crystal of the model compound l-alanine. It is also demonstrated that by coupling the 13C α 1H α LGCP experiment to a 13C α 15N REDOR experiment, one can determine the complete three-dimensional geometry of the C αH α and C αN vectors in a single crystal. These measurements allow for location of hydrogen atoms in crystalline biological macromolecules.

Order parameters based on 13C1H, 13C1H2 and 13C1H3 heteronuclear dipolar powder patterns: a comparison of MAS‐based solid‐state NMR sequences

Order parameters describing conformational exchange processes on the nanosecond to microsecond timescale can be obtained from powder patterns in solid-state NMR (SSNMR) experiments. Extensions of these experiments to magic-angle spinning (MAS) based high-resolution experiments have been demonstrated, which show a great promise for site-specific probes of biopolymers. In this study, we present a detailed comparison of two pulse sequences, transverse Manfield-Rhim-Elleman-Vaughn (T-MREV) and Lee-Goldburg cross-polarization (LGCP), using experimental and simulation tools to explore their utility in the study of order parameters. We discuss systematic errors due to passively coupled (13)C or (1)H nuclei, as well as due to B(1) inhomogeneity. Both pulse sequences can provide quantitative measurements of the order parameter, but the LGCP experiment is capable of greater accuracy provided that the B(1) field is highly homogeneous. The T-MREV experiment is far better compensated for B(1) inhomogeneity, and it also performs better in situations with limited signal.

Molecular Motion of Isolated Linear Alkanes in Nanochannels

The mobility of a series of linear alkanes in their inclusion compound with tris(o-phenylenedioxy)spirotriphosphazene is studied by high-resolution carbon-proton magic-angle spinning solid-state NMR spectroscopy. Two different carbon-proton dipolar recoupling experiments are compared with respect to their ability to yield precise site-specific, motion-averaged dipolar coupling constants. The most accurate results are obtained by analysis of extrema positions in Lee-Goldburg cross-polarization build-up curves. We present a comprehensive collection of coupling constants, which evidence a rotational motion of the all-trans chains around the channel axis, with some further averaging due to additional fluctuations, as previously found for alkanes in other host matrices such as urea. The order parameter increases toward the inner parts of the chains, and is largely independent of chain length. Notably, chains in a TPP host are not more ordered than in urea, even though the average TPP channel diameter is reported to be smaller. Significantly decreased order is found for highly filled short-alkane samples, which is interpreted in terms of an increased rate of mutual collisions. From residual dipolar couplings as well as carbon chemical shifts, we derive similar amounts of gauche conformers. Translational motions along the channels are further studied by proton double-quantum spectroscopy, which probes guest-host dipolar couplings. The extent of local-scale lateral motion is again correlated with the sample filling, and is a weak function of temperature, as expected from a case in which highly restricted single-file diffusion should dominate the mobility. Characteristic effects of sample aging are apparent in all our experiments.

Spectral editing based on selective excitation and Lee-Goldburg cross-polarization under magic angle spinning

We show that a Gaussian-shaped pulse can be used to excite selected 1H signals in hydroxyapatite, monetite and H–Y zeolite loaded with trimethylphosphine oxide (TMPO). This selective excitation method can be incorporated into Lee-Goldburg (LG) cross-polarization to obtain useful spectral editing opportunity. This new strategy has been applied to identify the Brønsted and the Lewis acid sites in H–Y zeolite using TMPO as the probe molecule.

Dynamics of Collagen in Articular Cartilage Studied by Solid‐State NMR Methods

AbstractFor Abstract see ChemInform Abstract in Full Text.

Highly Ordered Interstitial Water Observed in Bone by Nuclear Magnetic Resonance

NMR was used to study the nanostructure of bone tissue. Distance measurements show that the first water layer at the surface of the mineral in cortical bone is structured. This water may serve to couple the mineral to the organic matrix and may play a role in deformation. The unique mechanical characteristics of bone tissue have not yet been satisfactorily connected to the exact molecular architecture of this complex composite material. Recently developed solid-state nuclear magnetic resonance (NMR) techniques are applied here to the mineral component to provide new structural distance constraints at the subnanometer scale. NMR dipolar couplings between structural protons (OH(-) and H(2)O) and phosphorus (PO(4)) or carbon (CO(3)) were measured using the 2D Lee-Goldburg Cross-Polarization under Magic-Angle Spinning (2D LG-CPMAS) pulse sequence, which simultaneously suppresses the much stronger proton-proton dipolar interactions. The NMR dipolar couplings measured provide accurate distances between atoms, e.g., OH and PO(4) in apatites. Excised and powdered femoral cortical bone was used for these experiments. Synthetic carbonate ( approximately 2-4 wt%)-substituted hydroxyapatite was also studied for structural comparison. In synthetic apatite, the hydroxide ions are strongly hydrogen bonded to adjacent carbonate or phosphate ions, with hydrogen bond (O-H) distances of approximately 1.96 A observed. The bone tissue sample, in contrast, shows little evidence of ordered hydroxide. Instead, a very ordered (structural) layer of water molecules is identified, which hydrates the small bioapatite crystallites through very close arrangements. Water protons are approximately 2.3-2.55 A from surface phosphorus atoms. In synthetic carbonated apatite, strong hydrogen bonds were observed between the hydroxide ions and structural phosphate and carbonate units in the apatite crystal lattice. These hydrogen bonding interactions may contribute to the long-range stability of this mineral structure. The biological apatite in cortical bone tissue shows evidence of hydrogen bonding with an ordered surface water layer at the faces of the mineral particles. This structural water layer has been inferred, but direct spectroscopic evidence of this interstitial water is given here. An ordered structural water layer sandwiched between the mineral and the organic collagen fibers may affect the biomechanical properties of this complex composite material.

Heteronuclear dipolar recoupling in solid-state nuclear magnetic resonance by amplitude-, phase-, and frequency-modulated Lee–Goldburg cross-polarization

This paper presents a theoretical, numerical, and experimental study of phase- and frequency-switched Lee-Goldburg cross-polarization (FSLG-CP) under magic-angle spinning conditions. It is shown that a well-defined amplitude modulation of one of the two radio-frequency (rf) fields in the FSLG-CP sequence results in highly efficient heteronuclear dipolar recoupling. The recoupled dipolar interaction is gamma-encoded and, under ideal conditions, the effective spin Hamiltonian is equivalent to that in continuous-wave Lee-Goldburg CP. In practice, however, FSLG-CP is less susceptible to rf field mismatch and inhomogeneity, and provides better suppression of (1)H spin diffusion. The performance of FSLG-CP is experimentally demonstrated on liquid-crystalline samples exhibiting motionally averaged dipolar couplings.

Geometry of multiple-spin systems as reflected in 13C–{ 1H} dipolar spectra measured at Lee-Goldburg cross-polarization

Theoretical calculation and analysis of 13C–{ 1H} dipolar spectra of small-size spin clusters is presented. Dipolar spectra simulated using the time-independent average Hamiltonian are compared with the dipolar profiles obtained by 2D and 3D 1H– 13C correlation experiments employing Lee-Goldburg off-resonance cross-polarization (LG-CP). It is demonstrated that the structural parameters such as interatomic distances as well as mutual orientation of internuclear vectors can be derived from the dipolar profiles of simple spin clusters. Simplified analysis of the dipolar spectra based on isolated-like spin-pair approach can be used only if interacting spin cluster is reduced to the three-spin system in which the angle between both internuclear vectors ranges from 45° to 135°. For other local arrangements of spin systems the produced dipolar spectra must be analyzed with high caution. Contributions of all interacting spins to dipolar evolution of 13C magnetization are mutually mixed and cannot be easily separated. However, simplification of the dipolar spectra is achieved by selective excitation. Enhanced selectivity of LG-CP transfer due to the initial 1H chemical-shift-evolution period makes it possible to construct the dipolar spectra from 1H– 13C cross-peak intensities for every detected 1H– 13C spin-pair. Consequently, isolated-like spin pair evolution of the detected 1H– 13C coherence dominates to the resulting dipolar profile, while the influence of other interacting spins is suppressed. However, this suppression is not quite complete and analysis of the selective dipolar spectra based on isolated-like spin-pair approach cannot be used generally. Especially evolution of long-range 1H– 13C coherence is still significantly affected by spin states of other coupled hydrogen atoms.

NMR polarization echoes in a nematic liquid crystal.

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the 1H-1H dipolar dynamics. In this way, the 13C becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P(00)(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P(00)(t) was well fitted to an exponential law with a characteristic time tau(C) approximately 310 micros. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P(00)(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible 1H-1H dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P(00)(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time tau(phi) approximately 527 micros. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that tau(phi) is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.

Solid state NMR and DFT study of polymer electrolyte poly(ethylene oxide)/LiCF 3SO 3

Abstact Solid-state 13 C and 1 H NMR spectra of poly(ethylene oxide) (PEO)/LiCF 3 SO 3 polymer electrolyte and quantum-chemical DFT calculations of 13 C and 1 H NMR chemical shifts on a diglyme/LiCF 3 SO 3 model complex show a higher shielding of PEO (and diglyme) carbons and lower shielding of PEO (and diglyme) protons in the complex with LiCF 3 SO 3 , in comparison with neat PEO (diglyme). Both 13 C and 1 H chemical shifts are the same for PEO in amorphous and crystalline phases of PEO/LiCF 3 SO 3 polymer electrolyte, showing essentially the same local structure in both phases. The effective distance between the LiCF 3 SO 3 carbon and its nearest PEO protons in the PEO/LiCF 3 SO 3 complex, determined from the Lee-Goldburg cross-polarization 1 H→ 13 C dynamics, is in accord with the X-ray crystal structure.

Dynamics of collagen in articular cartilage studied by solid-state NMR methods.

Methods for studying the fast molecular dynamics of the rigid macromolecules in cartilage are described. The strong dipolar couplings and chemical shift anisotropies of these molecules necessitate application of solid-state nuclear magnetic resonance (NMR) techniques such as magic-angle spinning, cross-polarization, and high-power dipolar decoupling to obtain resolved NMR spectra. The molecules in cartilage that are amenable to these techniques are collagen and the rigid portion of the glycosaminoglycans (mostly hyaluronan). Site-specific mobility information is obtained from scaled dipolar couplings measured in 2D NMR experiments. Motionally averaged dipolar couplings can be interpreted in terms of order parameters that provide information about the amplitudes of molecular motions. Qualitative dynamics information is obtained from the simple wideline separation experiment measuring 1H-1H widelines representing the strength of the 1H-1H dipolar coupling. Quantitative values for molecular order parameters are obtained from precise measurements of 1H-13C dipolar couplings along the C-H bond vector. Two experimental techniques, the Lee-Goldburg cross-polarization and the dipolar coupling/chemical shift experiment, are illustrated to measure these 1H-13C dipolar couplings. Unlike glycosaminoglycans in cartilage, the collagen moiety remains substantially ordered, undergoing fast small amplitude motions. As enzymes cleave the macromolecules in articular cartilage in the course of arthritis, solid-state NMR techniques are capable of characterizing the increased motions of their degradation products in diseased cartilage.

Multiple-spin effects in fast magic angle spinning Lee–Goldburg cross-polarization experiments in uniformly labeled compounds

The proton–carbon polarization exchange in Lee–Goldburg cross-polarization magic angle spinning (LG-CP MAS) nuclear magnetic resonance experiments on uniformly C13-labeled compounds at high spinning frequency is studied. It is shown that the multiple carbon labels in the samples greatly influence the spin dynamics during the LG-CP mixing times. The zeroth order effective LG-CP MAS spin Hamiltonian is a sum of zero quantum dipolar interaction terms. These pairwise dipolar terms generally do not commute with each other, making it impossible to factorize the evolution operator. Consequently, the frequencies of the dipolar oscillations as well as the polarization transfer amplitudes become strongly dependent on the configuration of the spins involved in the multiple heteronuclear couplings. The strong carbon–proton couplings usually attenuate polarization transfers between weakly coupled spins. In practice, this implies that except for strongly coupled or isolated heteronuclear C13–H1 spin pairs, it is difficult to unambiguously extract structural constraints from experimental data. To better understand the complexity of the LG-CP processes, experiments on simple three- and four-spin systems are simulated and analyzed. More specifically, it is shown that in CH13–CH13 and CH213–C13 spin systems, a significant amount of the proton polarization can be transferred to both carbons, despite the fact that the individual proton–carbon heteronuclear couplings between each proton and the carbon spins are very different. The dependence of the polarization transfer on the position of the proton carrier frequency is analyzed and it is shown that by an appropriate choice of this frequency, specific polarization transfer pathways can be selected. Experimental results from [U–13C] tyrosine.HCl and [U–13C, N15] histidine.HCl.H2O samples are in satisfactory agreement with simulations.

Investigation of Molecular Motions by Lee-Goldburg Cross-Polarization NMR Spectroscopy

We demonstrate the use of Lee-Goldburg cross-polarization (LG-CP) NMR under fast magic-angle spinning (MAS) to investigate the amplitude and geometry of segmental motions in biomolecular and polymeric solids. Motional geometry information was previously available only from 2 H NMR, which, however, has limited site resolution and requires site-specific isotopic labeling. Using a 2D LG-CP technique, we resolve the 1 3 C- 1 H or 1 5 N- 1 H dipolar couplings according to the 1 3 C or 1 5 N isotropic chemical shift. Applications to systems undergoing 180° phenylene ring flips show spectral line shapes reflecting the geometry of the motion. Using this LG-CP technique, we measured the 1 3 C- 1 H and 1 5 N- 1 H dipolar couplings in the water-soluble and membrane-bound states of the colicin la channel domain. The backbone motions of the membrane-bound colicin scale both the Ca-Ha and N-H couplings similarly, thus ruling out rotation of the α-helices around their axes as a specific mechanism of motion. We also show that the sensitivity of the LG-CP spectra can be enhanced by the addition of a phase-inverted 1 H- 1 3 C cross-polarization step, and the site resolution of the 1 5 N- 1 H LG-CP spectra can be enhanced by 1 3 C indirect detection.

A 3-D Structural Model of Solid Self-Assembled Chlorophyll a/H2O from Multispin Labeling and MAS NMR 2-D Dipolar Correlation Spectroscopy in High Magnetic Field

Magic angle spinning (MAS) NMR with Lee–Goldburg cross-polarization (LG-CP) is used to promote long-range heteronuclear transfer of magnetization and to constrain a structural model for uniformly labeled chlorophyll a/H2O. An effective maximum transfer range dmax can be determined experimentally from the detection of a gradually decreasing series of intramolecular correlations with the 13C along the molecular skeleton. To probe intermolecular contacts, dmax can be set to ∼4.2 Å by choosing an LG-CP contact time of 2 ms. Long-range 1H–13C correlations are used in conjunction with carbon and proton aggregation shifts to establish the stacking of the chlorophyll a (Chl a) molecules. First, high-field (14.1 T) 2-D MAS NMR homonuclear (13C–13C) dipolar correlation spectra provide a complete assignment of the carbon chemical shifts. Second, proton chemical shifts are obtained from 1H–13C heteronuclear dipolar correlation spectroscopy in high magnetic field. The shift constraints and long-range 1H–13C intermolecular correlations reveal a 2-D stacking homologous to the molecular arrangement in crystalline solid ethyl-chlorophyllide a. A doubling of a small subset of the carbon resonances, in the 7-methyl region of the molecule, provides evidence for two marginally different well-defined molecular environments. Evidence is found for the presence of neutral structural water molecules forming a hydrogen-bonded network to stabilize Chl a sheets. In line with the microcrystalline order observed for the rings, the long T1's, and absence of conformational shifts for the 13C in the phytyl tails, it is proposed that the Chl a form a rigid 3-D space-filling structure. Probably the only way this can be realized with the sheets is by forming bilayers with interpenetration of elongated tails. Such a 3-D space-filling organization of the aggregated Chl a from MAS NMR would match existing models inferred from electron microscopy and low-resolution X-ray powder diffraction, while a micellar model based on neutron diffraction and antiparallel stacking observed in solution can be discarded.

Polarization transfer dynamics in Lee–Goldburg cross polarization nuclear magnetic resonance experiments on rotating solids

This paper presents a theoretical description of continuous wave (CW) high frequency Lee–Goldburg cross polarization magic angle spinning (LG–CPMAS) nuclear magnetic resonance experiments. The full time-dependent LG–CPMAS Hamiltonian is replaced by its zero order time-independent Hamiltonian in the interaction representation. Carbon signal enhancements of LG–CPMAS experiments are calculated for spin systems consisting of six H1 nuclei coupled to one C13 nucleus. These simulations are based on Floquet theory calculations, explicitly taking into account the time dependence because of magic angle spinning, and calculations based on the zero-order Hamiltonian. The good agreement between these calculations justifies the use of the zero-order Hamiltonian. The time-dependent intensities of the cross peaks in heteronuclear C13–1H correlation spectra, extracted from 3D LG–CPMAS experiments on a natural abundant DL–alanine sample with increasing CP mixing times, are in good agreement with the theoretical intensities simulated by using the zero-order Hamiltonian. The approximated LG–CPMAS Hamiltonian can be used to obtain structural information about a proton coupled to a single carbon. The simulated intensities of the carbon signals of an isolated C13–1H group and a C13–1H group that is coupled to additional protons, measured by LG–CPMAS experiments with increasing CP mixing times, are compared. This study suggests that the buildup curve of each LG–CPMAS carbon signal and its Fourier transformed CP spectrum can be interpreted in terms of a single distance between the observed C13 and its nearest proton, if the additional protons are removed from this carbon by at least 1.2 times this distance.

A Method for Measuring Heteronuclear (1H−13C) Distances in High Speed MAS NMR

Magic angle spinning (MAS) NMR structure determination is rapidly developing. We demonstrate a method to determine 1H−13C distances r CH with high precision from Lee−Goldburg cross-polarization (LG-CP) with fast MAS and continuous LG decoupling on uniformly 13C-enriched tyrosine·HCl. The sequence is γ-encoded, and 1H−13C spin-pair interactions are predominantly responsible for the polarization transfer while proton spin diffusion is prevented. When the CP amplitudes are set to a sideband of the Hartmann−Hahn match condition, the LG-CP signal builds up in an oscillatory manner, reflecting coherent heteronuclear transfer. Its Fourier transform yields an effective 13C frequency response that is very sensitive to the surrounding protons. This 13C spectrum can be reproduced in detail with MAS Floquet simulations of the spin cluster, based on the positions of the nuclei from the neutron diffraction structure. It is symmetric around ω = 0 and yields two well-resolved maxima. Measurement of CH distances is straig...