1H Decoupling Research Articles

Thallium(III) coordination compounds: chemical information from 205Tl NMR longitudinal relaxation times

Abstract205Tl longitudinal relaxation rate measurements were performed on several thallium(III) complexes with the composition Tl(OH)n(H2O)6−n(3−n)+ (n = 1,2), Tl(Cl)n(H2O)m−n(3−n)+, Tl(Br)n(H2O)m−n(3−n)+ (m = 6 for n = 1–2, m = 5 for n = 3, m = 4 for n = 4), Tl(CN)n(H2O)m−n(3−n)+ (m = 6 for n = 1–2, m = 4 for n = 3–4) in aqueous solution, at different magnetic fields and temperatures. 13C and 2D isotopic labelling and 1H decoupling experiments showed that the contribution of the dipolar relaxation path is negligible. The less symmetric lower complexes (n < 4) had faster relaxation rate dominantly via chemical shift anisotropy contribution which depended on the applied magnetic field: T1 values are between 20 and 100 ms at 9.4 T and the shift anisotropy is Δσ = 1000–2000 ppm. The tetrahedral complexes, n = 4, relax slower; their T1 is longer than 1 s and the spin–rotation mechanism is probably the dominant relaxation path as showed by a temperature dependence study. In the case of the TlCl4− complex, presumably a trace amount of TlCl52− causes a large CSA contribution, 300 ppm. Since the geometry and the bond length for the complexes in solution are known from EXAFS data, it was possible to establish a correlation between the CSA parameter and the symmetry of the complexes. The relaxation behaviour of the Tl–bromo complexes is not in accordance with any known relaxation mechanism. Copyright © 2002 John Wiley & Sons, Ltd.

NMR local range investigations in amorphous starchy substrates: II-Dynamical heterogeneity probed by 1H/ 13C magnetization transfer and 2D WISE solid state NMR

In the preceding paper, we have investigated the structural heterogeneous character of a series of amorphous samples prepared from various starchy substrates (native potato starch, amylopectin and amylose) following different techniques of preparation (casting, freeze drying and solvent exchange). Spectral decompositions of the C1 resonances of the 13C CP-MAS (Cross Polarization and Magic Angle Spinning) spectra under 1H decoupling have shown the existence of five main types of α(1-4) linkages. In this part, 2D solid state NMR WISE experiments and the 13C/ 1H magnetization transfer in CP as a local probe for both structures and dynamics were used. The 13C CP magnetization curves versus contact time of each C1 component in each recorded spectrum were fitted with an analytic function taking into account two 1H reservoirs. Interpretation of the characteristic times derived from fitting yields some improvements on the knowledge of the heterogeneity of the samples and on the water molecules distribution.

Background Signal in Solid State 13C NMR Spectra of Soil Organic Matter (SOM)—Quantification and Minimization

13C background signal, obtained for an empty rotor, was shown by spin counting experiments to be equivalent to 1 mg of observable carbon for cross-polarization (CP) spectra and 69 mg of observable C for Bloch decay (BD) spectra. The BD background was mainly due to Kel-F in the stator, with minimal signal detected from the Kel-F end-caps. The CP background was attributed to non-Kel-F components of the stator, probe, or probe supports. The BD background signal was eliminated by using a modified dipolar dephasing pulse sequence in which the absence of 19F decoupling (rather than the absence of 1H decoupling) causes selective elimination of the Kel-F signal.

A general strategy for the NMR observation of half-integer quadrupolar nuclei in dilute environments.

A general strategy for the observation of low gamma half-integer quadrupolar nuclides in biological systems is presented. The methodology combines low-temperature (4-100 K) techniques with cross-polarization (CP) experiments while employing a so-called Carr-Purcell-Meiboom-Gill spin-echo sequence (CPMG). This combined approach is termed CP/QCPMG. Also discussed are data processing issues that are unique to the induced signals that result from the QCPMG pulse sequence. Central to this strategy is the development of a stable low-temperature (4 to 250 K) NMR double-resonance probe. The probe is robust enough to handle multiple contact experiments and long acquisition periods with 1H decoupling. This approach is illustrated with low-temperature solid-state 67Zn and 25Mg NMR CP/QCPMG experiments on model compounds. The conclusion reached is that the strategy affords sufficient sensitivity to examine Zn2+ and/or Mg2+ binding sites in metalloproteins.

13C NMR spectra of halocarbons

Abstract13C NMR chemical shifts and 13C–19F and 13C–1H coupling constants are reported for 103 halocarbons. Methods used for analyzing complex spectra (e.g. simultaneous 1H and 19F broadband decoupling, 2D INADEQUATE/FRED, HMQC, 35,37Cl isotope effects) are briefly illustrated. Longitudinal relaxation times for CF3CCl2CH2CHClF are reported. Copyright © 2001 John Wiley & Sons, Ltd.

Radio-frequency probe for 1H decoupled 31P MRS of the head and neck region

For optimal performance of 31P MRS at 1.5 Tesla, the use of a double resonant probe is essential to enable the application of 1H decoupling and Nuclear Overhauser Enhancement. This note describes the design, evaluation and safety validation of a versatile and compact probe optimized for 1H decoupled 31P MRS studies of tumors close to the surface of the body, in particular the head and neck region.

New structural information on a humic acid from two-dimensional 1H-13C correlation solid-state nuclear magnetic resonance.

New information on the chemical structure of a peat humic acid has been obtained using a series of two-dimensional 1H-13C heteronuclear correlation solid-state NMR (HETCOR) experiments with different contact times and with spectral editing by dipolar dephasing and 13C transverse relaxation filtering. Carbon-bonded methyl groups (C-CH3) are found to be near both aliphatic and O-alkyl but not aromatic groups. The spectra prove that most OCH3 groups are connected directly with the aromatic rings, as is typical in lignin. As a result, about one-third of the aromatic C-O groups is not phenolic C-OH but C-OCH3. Both protonated and unprotonated anomeric O-C-O carbons are identified in the one- and two-dimensional spectra. COO groups are found predominantly in OCHn-COO environments, but some are also bonded to aromatic rings and aliphatic groups. All models of humic acids in the literature lack at least some of the features observed here. Compositional heterogeneity was studied by introducing 1H spin diffusion into the HETCOR experiment. Comparison with data for a synthetic polymer, polycarbonate, indicates that the separation between O-alkyl and aromatic groups in the humic acid is less than 1.5 nm. However, transverse 13C relaxation filtering under 1H decoupling reveals heterogeneity on a nanometer scale, with the slow-relaxing component being rich in lignin-like aromatic-C-O-CH3 moieties and poor in COO groups.

Examination of an Internal Standard Substance for the Quantitative Analysis of Sarin Using 31P-NMR.

The toxic solution used at the Tokyo Subway Sarin Incident was purified by vacuum distillation. We were thus able to obtain sarin with high purity. We conducted a quantitative analysis of sarin using 31P-NMR spectrometry. High-purity sarin was examined by trimethyl phosphate as an internal standard. In comparison with 1H nondecoupling, the relative intensity between sarin and the internal standard did not change in 1H decoupling. Trimethyl phosphate is a good internal standard substance. A 31P-NMR chemical shift of trimethyl phosphate is separate from those of sarin and related compounds. Trimethyl phosphate has one phosphorus atom in the molecule, as does sarin. Since it is advantageous in the conversion of quantitative value, the molecular weight of trimethyl phosphate (m.w. 140.08) is close to that of sarin (m.w. 140.09). By using trimethyl phosphate as an internal standard, it is possible to conduct a quantitative analysis of sarin through 1H decoupled 31P-NMR spectrometry.

TROSY NMR with partially deuterated proteins.

TROSY-type optimization of liquid-state NMR experiments is based on the preservation of unique coherence transfer pathways with distinct transverse relaxation properties. The broadband decoupling of the 1H spins interchanges the TROSY and anti-TROSY magnetization transfer pathways and thus is not used in TROSY-type triple resonance experiments or is replaced with narrowband selective decoupling. To achieve the full advantage of TROSY, the uniform deuteration of proteins is usually required. Here we propose a new and general method for 1H broadband decoupling in TROSY NMR, which does not compromise the relaxation optimization in the 15N-1H moieties, but uniformly and efficiently refocuses the 1JCH scalar coupling evolution in the 13C-1H moieties. Combined with the conventional 2H decoupling, this method enables obtaining high sensitivity TROSY-type triple resonance spectra with partially deuterated or fully protonated 13C,15N labeled proteins.

Spin density distribution of the conduction electrons in diperylene hexafluorophosphate analysed by high-resolution NMR

High-resolution 13C nuclear magnetic resonance with 1H cross polarization and 1H decoupling under magic angle spinning is measured for the quasi-one dimensional organic conductor diperylene hexafluorophosphate (including tetrahydrofurane solvent molecules) at temperatures between 160 K and 270 K. Ab initio molecular orbital calculations are used for chemical shift analysis and for assignment of Knight shifted lines and individual carbon positions. The coexistence of neutral perylene molecules and perylene radicals in the same radical cation salt is revealed. From Knight and chemical shifts we were able to distinguish two inequivalent perylene radicals within the conducting stack. The spin density distribution of the molecular electronic wave function is determined quantitatively for these radicals.

Two-Dimensional High-Speed CP/MAS NMR Spectroscopy of Polymorphs. 1. Uniformly 13C-Labeled Aspartame

Three forms of crystalline aspartame have been observed: two hemihydrate polymorphs and a dihemihydrate. The 13C CP/MAS NMR spectra of two of the forms of aspartame showed that certain carbons have up to three resonances due to different conformations/arrangements of molecules in the asymmetric unit cell. Techniques for assigning resonances based upon the number of attached protons or J couplings were not effective because the multiple resonances arise from the same carbon in the molecule. We used two-dimensional exchange experiments on uniformly 13C-labeled aspartame to assign the spectra of aspartame. Experiments performed with typical MAS rates (7 kHz) and 1H decoupling powers (63 kHz) of uniformly 13C-labeled aspartame were uninformative because 1H−13C and 13C−13C dipolar couplings significantly broadened these resonances. Increasing the spinning rate to 28 kHz and the 1H decoupling power to 263 kHz increased the resolution sufficiently to observe crystallographically inequivalent sites. Two-dimensio...

Magnitudes and orientations of NMR interaction tensors in isolated three-spin systems ABX

Possibilities and limitations for the complete determination of all orientational parameters from iterative fitting of various MAS NMR spectra of ABX three-spin systems are explored. The practical example chosen is the (31P)2 113Cd ABX spin system in solid Cd(NO3)2 · 2PMe2Ph1 where 113Cd is an X nucleus of low natural abundance. Conventional 31P and 113Cd MAS NMR spectra, plus 113Cd MAS NMR spectra obtained under conditions of either simultaneous high power 31P and 1H decoupling, or under conditions of 31P rotary resonance recoupling are considered. Considerable anisotropy of the heteronuclear J coupling 1J(113Cd, 31P) is observed.

Magnitudes and orientations of NMR interaction tensors in isolated three-spin systems ABX

Possibilities and limitations for the complete determination of all orientational parameters from iterative fitting of various MAS NMR spectra of ABX three-spin systems are explored. The practical example chosen is the (31P)2 113Cd ABX spin system in solid Cd(NO3)2 · 2PMe2Ph1 where 113Cd is an X nucleus of low natural abundance. Conventional 31P and 113Cd MAS NMR spectra, plus 113Cd MAS NMR spectra obtained under conditions of either simultaneous high power 31P and 1H decoupling, or under conditions of 31P rotary resonance recoupling are considered. Considerable anisotropy of the heteronuclear J coupling 1J(113Cd, 31P) is observed.

Long-range proton-carbon coupling constants in conformational analysis of oligosaccharides

Long-range heteronuclear coupling constants, 3J(C,H), were measured across glycosidic linkages in β-linked disaccharides and also for vicinally disubstituted trisaccharides. One-dimensional multiple-site 13C excitation experiments using Hadamard spectroscopy and band-selective 1H decoupling during the acquisition period were used in conjunction with pulsed field gradients for coherence selection in the 1H detected experiment. For one dihedral angle the 3J(C,H) value could only be unambiguously determined utilizing the band-selective 1H decoupling. The 2D EXSIDE was also used for the measurement of 3J(C,H) values and the results showed excellent agreement with those of the 1D measurements. The influence of solvent, water and water–dimethyl sulfoxide (7:3), on conformation was investigated, but showed only small changes in 3J(C,H) values. In general, the long-range heteronuclear coupling constants related to the ϕ dihedral angles (3.6–4.3 Hz) were smaller than those of the ψ dihedral angles (4.3–5.3 Hz). A comparison with 3J(C,H) values from Monte Carlo or Langevin dynamics simulations showed better agreement for the ψ dihedral angles than for the ϕ dihedral angles as calculated using a Karplus relationship. © 1998 John Wiley & Sons Ltd.

A NMR method for the analysis of mixtures of alkanes with different deuterium substitutions

13C NMR at 125.76 MHz with 1H and 2H decoupling, 2H NMR at 76.77 MHz with 1H decoupling, and 1H NMR at 500.14 MHz with 2H decoupling were employed as analytical tools to study the complex mixtures of deuterated ethanes resulting from the catalytic H–D exchange of normal ethane with gas-phase deuterium in the presence of a platinum foil. Reference samples consisting of 1:1 binary mixtures of pure normal ethane and ethane-d n ( n=1–6) were used to identify the peak positions in the 13C, 2H, and 1H NMR spectra due to each individual isotopomer, and the effect of isotopic substitution on the chemical shifts was determined in each case. While the NMR of all three nuclei worked well for the identification of the individual components of the 1:1 standard mixtures, both 1H and 2H NMR suffered from inadequate resolution when studying complex reaction mixtures because of the broadening of the lines due to 1H– 1H ( 1H NMR) and 2H– 2H ( 2H NMR) couplings. 13C NMR was therefore determined to be the method of choice for the quantitative analysis of the reaction mixtures. Using the 13C NMR results, a correlation that takes into account the primary and secondary isotope substitution effects on chemical shifts was deduced. This equation was used for the identification of the individual components of the mixtures, and integration of the individual observed resonances was then employed for quantification of their composition. This study shows that 13C NMR with 1H and 2H decoupling is a viable procedure for studying mixtures of deuterated ethanes. Furthermore, the additivity of the isotopic effects on chemical shifts and the transferability of the values obtained with ethane to other molecules makes this approach general for the analysis of other isotopomer mixtures.

NMR and polymorphism of a piperidinone-substituted benzopyran with a fluorinated sidechain

Two polymorphs of (3S-trans)-1-[3,4-dihydro-3-hydroxy-2,2-dimethyl-6-(pentafluoroethyl)-2H-1-benzopyran-4-yl]piperidin-2-one (A)‡ have been characterised by solid-state 13C and 19F cross-polarisation magic-angle spinning NMR, aided by IR spectroscopy, differential scanning calorimetry and X-ray powder diffraction methods. The two polymorphs show markedly different NMR spectral phenomena, including variations in residual dipolar coupling characteristics and crystallographic splitting of resonances, indicating that more than one molecule exists in the asymmetric unit of polymorphic form II. The 13C NMR studies involved triple-channel operation, with simultaneous decoupling of 1H and 19F, including cross polarisation from 19F as well as from 1H. Dual-channel 19F measurements have been carried out using 1H decoupling, with cross polarisation from 1H.

Use of deuterium labeling in NMR: overcoming a sizeable problem

In summary, the use of deuteration in combination with uniformly 13C- and 15N-labeling dramatically improves the quality of NMR spectra of larger proteins. These improvements allow the backbone and side-chain signals of larger proteins to be assigned and aids in the acquisition and analysis of NOE data. Using these methods, three-dimensional structures of proteins up to 30kDa have been determined [[[20]xStructure and ligand recognition of the phosphotyrosine binding domain of Shc. Zhou, M.-M...., and Fesik, S.W. Nature. 1995; 378: 584–592Crossref | PubMedSee all References, [31]xX-ray and NMR structure of human Bcl-xL, an inhibitor of programmed cell death. Muchmore, S.W...., and Fesik, S.W. Nature. 1996; 381: 335–341Crossref | PubMedSee all References, [32]xStructure of Bcl-xL–Bak peptide complex reveals how regulators of apoptosis interact. Sattler, M...., and Fesik, S.W. Science. 1996; : in pressSee all References] SWF, unpublished data]. Further developments of NMR methods and variation of labeling techniques promise to push the molecular weight limit even further.

Field-dependent isotropic shifts and limitations to linewidths in solid state nuclear magnetic resonance: A Floquet treatment

Previously we have provided a quantitative theory that explains the apparent field dependence observed for chemical shifts in high resolution solids state 13C nuclear magnetic resonance spectroscopy (where magic angle spinning is combined with high-power 1H decoupling). We have expanded that theoretical treatment so as to include the effect of homonuclear dipole–dipole couplings among the abundant nuclear spin species, and to the impact of off-resonance rf fields. These differing contributions to the residual linewidths are assessed in a unified theoretical treatment using the Floquet theory for time-dependent Hamiltonians. Some comments are provided as to the circumstances in which the various contributing terms dominate.

A new goniometer design for single-crystal nuclear magnetic resonance spectroscopy

A new goniometer for single-crystal nuclear magnetic resonance (NMR) spectroscopy has been constructed and built into a multinuclear X–1H double-tuned probe for cross-polarization and/or 1H decoupling. Improvements in the ease of mounting/remounting the crystal on the three rotational axes and in the accuracy of the stepwise rotation about these axes have been achieved compared to the usual cube goniometer. A similar improvement is achieved for the determination of the orientation of the crystal relative to the goniometer frame. The NMR goniometer has three dovetail mortises that enable rotation of the crystal about three orthogonal axes, all perpendicular to the magnetic field. The crystal is glued onto a tenon that fits precisely into the dovetails of the NMR goniometer and also into a slightly modified x-ray diffraction goniometer, thereby allowing a high-accuracy determination of the orientation for the tensorial NMR interactions with respect to the crystal frame. The performance of the goniometer is illustrated by single-crystal 23Na NMR spectra of NaNO3.

A Solid-State NMR Study of Tungsten Methyl Group Dynamics in [W(η5-C5Me5)Me4][PF6

We report the results of Magic Angle Spinning (MAS) 13C and static 2H NMR studies of the dynamics of the methyl groups coordinated to tungsten in [WCp*Me4][PF6] (Cp* = η5-C5Me5). The temperature-dependent broadening of the axial methyl 13C line can be ascribed to interference between 1H decoupling and methyl motion when the motional rate and decoupling nutation frequency are comparable. This proposal is consistent with the absence of broadening in the 2H labeled compound and the motional rate constants inferred from 2H NMR lineshape and T1 studies, which range from 107 s-1 at 25 °C to 103 s-1 at −125 °C. The measured barrier to axial methyl hopping (26 kJ/mol) is among the highest reported to date. An X-ray crystal study of [W(η5-C5Me4Et)Me4][PF6] reveals no evidence (in terms of the core geometry) for an agostic interaction in either the axial methyl group or the equatorial methyl groups.