Excitation of magnetization using a modulated radiation damping field

In this work, pulsed-field gradients are used to modulate the radiation damping field generated by the detection coil in an NMR experiment in order that spins with significantly different chemical shifts can affect one another via the radiation damping field. Experiments performed on solutions of acetone/water and acetone/DMSO/water demonstrate that spins with chemical shift differences much greater than the effective radiation damping field strength can still be coupled by modulating the radiation damping field. Implications for applications in high-field NMR and for developing sensitive magnetization detectors are discussed.     

Spin amplification in solution magnetic resonance using radiation damping

The sensitive detection of dilute solute spins is critical to biomolecular NMR. In this work, a spin amplifier for detecting dilute solute magnetization is developed using the radiation damping interaction in solution magnetic resonance. The evolution of the solvent magnetization, initially placed along the unstable -z direction, is triggered by the radiation damping field generated by the dilute solute magnetization. As long as the radiation damping field generated by the solute is larger than the corresponding thermal noise field generated by the sample coil, the solute magnetization can effectively trigger the evolution of the water magnetization under radiation damping. The coupling between the solute and solvent magnetizations via the radiation damping field can be further improved through a novel bipolar gradient scheme, which allows solute spins with chemical shift differences much greater than the effective radiation damping field strength to affect the solvent magnetizations more efficiently. Experiments performed on an aqueous acetone solution indicate that solute concentrations on the order of 10(-5) that of the solvent concentration can be readily detected using this spin amplifier.     

On the Tuning of High‐Resolution NMR Probes

Three optimum conditions for the tuning of NMR probes are compared: the conventional tuning optimum, which is based on radio‐frequency pulse efficiency, the spin noise tuning optimum based on the line shape of the spin noise signal, and the newly introduced frequency shift tuning optimum, which minimizes the frequency pushing effect on strong signals. The latter results if the radiation damping feedback field is not in perfect quadrature to the precessing magnetization. According to the conventional RLC (resistor–inductor–capacitor) resonant circuit model, the optima should be identical, but significant deviations are found experimentally at low temperatures, in particular on cryogenically cooled probes. The existence of different optima with respect to frequency pushing and spin noise line shape has important consequences on the nonlinearity of spin dynamics at high polarization levels and the implementation of experiments on cold probes.     

Defeating Radiation Damping and Magnetic Field Inhomogeneity with Spatially Encoded Noise

A simple NMR experiment capable of providing well resolved spectra under conditions where either radiation damping or static magnetic field inhomogeneity would broaden otherwise high‐resolution NMR spectra is introduced. The approach involves using a strong pulsed magnetic field gradient and a selective radio‐frequency pulse to encode a predetermined noise pattern into the spatial distribution of magnetization. Following readout in a much smaller field gradient, the noise sequence may be deconvolved from the acquired data and a high‐resolution spectrum is obtained, eliminating the effects of either radiation damping or the static field inhomogeneity. In the presence of field inhomogeneity a field map is also obtained from the same single transient. A quasi‐two‐dimensional version of the experiment eliminates the need for deconvolution and produces improved results with simplified processing, but without requiring a full two‐dimensional experiment. Example spectra are shown for both radiation damping and one‐dimensional field inhomogeneity with improvement in linewidths of more than a factor of 40.     

NMR solvent shifts of adenine in aqueous solution from hybrid QM/MM molecular dynamics simulations

We present first principles calculations of the NMR solvent shift of adenine in aqueous solution. The calculations are based on snapshots sampled from a molecular dynamics simulation, which were obtained via a hybrid quantum-mechanical/mechanical modeling approach, using an all-atom force field (TIP3P). We find that the solvation via the strongly fluctuating hydrogen bond network of water leads to nontrivial changes in the NMR spectra of the solutes regarding the ordering of the resonance lines. Although there are still sizable deviations from experiment, the overall agreement is satisfactory for the 1H and 15N NMR shifts. Our work is another step toward a realistic first-principles prediction of NMR chemical shifts in complex chemical environments.     

Standardization of chemical shifts of TMS and solvent signals in NMR solvents

The standard for chemical shift is dilute tetramethylsilane (TMS) in CDCl3, but many measurements are made relative to TMS in other solvents, the proton resonance of the solvent peak or relative to the lock frequency. Here, the chemical shifts of TMS and the proton and deuterium chemical shifts of the solvent signals of several solvents are measured over a wide temperature range. This allows for the use of TMS or the solvent and lock signal as a secondary reference for other NMR signals, as compared with dilute TMS in CDCl3 at a chosen temperature; 25 degrees C is chosen here. An accuracy of 0.02 ppm is achievable for dilute solutions, provided that the interaction with the solvent is not very strong. The proton chemical shift of residual water is also reported where appropriate.     

Calculating nuclear magnetic resonance chemical shifts in solvated systems

The nuclear magnetic resonance (NMR) chemical shift is extremely sensitive to molecular geometry, hydrogen bonding, solvent, temperature, pH, and concentration. Calculated magnetic shielding constants, converted to chemical shifts, can be valuable aids in NMR peak assignment and can also give detailed information about molecular geometry and intermolecular effects. Calculating chemical shifts in solution is complicated by the need to include solvent effects and conformational averaging. Here, we review the current state of NMR chemical shift calculations in solution, beginning with an introduction to the theory of calculating magnetic shielding in general, then covering methods for inclusion of solvent effects and conformational averaging, and finally discussing examples of applications using calculated chemical shifts to gain detailed structural information.     

Intrinsic carbon-13 NMR solvent shifts in hydrocarbons. Relevance of long range substituent parameters

Carbon-13 NMR chemical shifts have been measured in a number of binary mixtures of normal alkanes. Intrinsic solvent shifts are deduced from the shifts and the relevance of some small substituent effects in alkanes is discussed. A comparison is made between solvent effects and thermally induced chemical shift differences in alkanes.     

Effect of chemical exchange on radiation damping in aqueous solutions of the osmolyte glycine

Radiation damping is of central relevance in NMR spectroscopy especially with the advent of ultrahigh-field magnets and of supersensitive probes. Furthermore, the recent realization that the combined effect of the distant dipole field and of radiation damping causes the resurrection of undesired crushed water magnetization emphasizes the need for a thorough understanding of all the factors affecting radiation damping. While the effects of pulsed-field gradients and of active feedback have been extensively investigated, the consequences on radiation damping of chemical exchange between water and co-solutes is not as well understood. Here it is demonstrated that the rate of water radiation damping is significantly affected by free glycine (Gly), a representative of an important class of biocompatible osmolytes often used at molar concentrations as protein stabilizers. The pH and temperature dependencies of this effect were investigated and rationalized in terms of radiation damping attenuation caused by incoherent dephasing occurring in the intermediate exchange regime. For instance, at pH 6.0 and at a temperature of 313 K the Gly NH3+/water exchange has the same dramatic effect on radiation damping as a series of repeated weak PFGs, increasing the water inversion-recovery zero-crossing delay from approximately 30 ms to approximately 2.3 s. In addition, under these conditions, the Gly NH3+/water exchange suppresses the resurrection of unwanted crushed water magnetization. When used in combination with PFGs and water flip-back schemes, glycine is therefore expected to tame chaotic dynamics and improve the reproducibility of the NMR experiments affected by it.     

Optical absorption and NMR spectroscopic studies on paramagnetic trivalent lanthanide complexes with 2,2'-bipyridine.; The solvent effect on 4f-4f hypersensitive transitions

The optical absorption and NMR studies of trivalent lanthanide chloride complexes with 2,2'-bipyridine (bpy) are presented and discussed. The NMR spectra of paramagnetic complexes exhibit lower as well as higher field shifts of bpy resonances that reflect change in geometry and reveals importance of the factor (3 cos2 theta - 1 ) in changing sign of the shift. The paramagnetic shifts recorded have been analyzed and the intramolecular shift ratios suggest that the paramagnetic shift is predominantly dipolar in origin. Electronic spectral studies of the Pr, Nd, Ho, Er and Dy complexes in different solvents (viz. methanol, pyridine, DMSO and DMF), which differ with respect to donor atoms, reveal that the chemical environment around the lanthanide ion has great impact on f-f transitions and any change in the environment results in modifications of the spectra. The oscillator strength for the hypersensitive and non-hypersensitive transitions have been determined and changes in the oscillator strength and band shape with respect to solvent type is rationalized in terms of ligand (solvent) structure and coordination properties.     

NMR solvent shifts of acetonitrile from frozen density embedding calculations

We present a density functional theory (DFT) study of solvent effects on nuclear magnetic shielding parameters. As a test example we have focused on the sensitive nitrogen shift of acetonitrile immersed in a selected set of solvents, namely water, chloroform, and cyclohexane. To include the effect of the solvent environment in an accurate and efficient manner, we employed the frozen-density embedding (FDE) scheme. We have included up to 500 solvent molecules in the NMR computations and obtained the cluster geometries from a large set of conformations generated with molecular dynamics. For small solute-solvent clusters comparison of the FDE results with conventional supermolecular DFT calculations shows close agreement. For the large solute-solvent clusters the solvent shift values are compared with experimental data and with values obtained using continuum solvent models. For the water --> cyclohexane shift the obtained value is in very good agreement with experiments. For the water --> chloroform NMR solvent shift the classical force field used in the molecular dynamics simulations is found to introduce an error. This error can be largely avoided by using geometries taken from Car-Parrinello molecular dynamics simulations.     

Structure and conformation of DAB dendrimers in solution via multidimensional NMR techniques.

NOESY-HSQC 3D-NMR and NOESY 2D-NMR techniques have been used on a 750 MHz spectrometer to study the chain conformations of different generation DAB dendrimers (poly[propylene imine] dendrimers) in chloroform and benzene solutions. The high-field multidimensional NMR techniques provided the chemical shift dispersion needed to resolve all of the unique resonances in the dendrimers. By studying the NOE interactions among dendritic chain protons, information about through space interactions between protons on different parts of the dendrimer chain is obtained, which is directly related to the conformation of the dendrimer. These experiments also give further proof of the chemical shift assignments obtained from the HMQC-TOCSY 2D and 3D NMR experiments. The concentration effects on chemical shifts have also been observed, revealing information about the interactions between solvent and different parts of dendrimer molecules. These studies clearly show for DAB dendrimers, that folded chain conformations can occur in nonpolar solvents such as benzene and extended chain conformations are predominant in polar solvents such as chloroform.     

Computational studies of 13C NMR chemical shifts of saccharides

The (13)C NMR chemical shifts for alpha-D-lyxofuranose, alpha-D-lyxopyranose (1)C(4), alpha-D-lyxopyranose (4)C(1), alpha-D-glucopyranose (4)C(1), and alpha-D-glucofuranose have been studied at ab initio and density-functional theory levels using TZVP quality basis set. The methods were tested by calculating the nuclear magnetic shieldings for tetramethylsilane (TMS) at different levels of theory using large basis sets. Test calculations on the monosaccharides showed B3LYP(TZVP) and BP86(TZVP) to be cost-efficient levels of theory for calculation of NMR chemical shifts of carbohydrates. The accuracy of the molecular structures and chemical shifts calculated at the B3LYP(TZVP) level is comparable to those obtained at the MP2(TZVP) level. Solvent effects were considered by surrounding the saccharides by water molecules and also by employing a continuum solvent model. None of the applied methods to consider solvent effects was successful. The B3LYP(TZVP) and MP2(TZVP)(13)C NMR chemical shift calculations yielded without solvent and rovibrational corrections an average deviation of 5.4 ppm and 5.0 ppm between calculated and measured shifts. A closer agreement between calculated and measured chemical shifts can be obtained by using a reference compound that is structurally reminiscent of saccharides such as neat methanol. An accurate shielding reference for carbohydrates can be constructed by adding an empirical constant shift to the calculated chemical shifts, deduced from comparisons of B3LYP(TZVP) or BP86(TZVP) and measured chemical shifts of monosaccharides. The systematic deviation of about 3 ppm for O(1)H chemical shifts can be designed to hydrogen bonding, whereas solvent effects on the (1)H NMR chemical shifts of C(1)H were found to be small. At the B3LYP(TZVP) level, the barrier for the torsional motion of the hydroxyl group at C(6) in alpha-D-glucofuranose was calculated to 7.5 kcal mol(-1). The torsional displacement was found to introduce large changes of up to 10 ppm to the (13)C NMR chemical shifts yielding uncertainties of about +/-2 ppm in the chemical shifts.     

Sensitive high resolution inverse detection NMR spectroscopy of proteins in the solid state

A new indirect detection scheme for obtaining (15)N/(1)H shift correlation spectra in crystalline proteins is described. Excellent water suppression is achieved without the need for pulsed field gradients, and using only a 2-step phase cycle. Careful attention to overall NMR instrument stability was found critical for obtaining the best resolution and sensitivity. Magnetic dilution by deuteration of the protein in combination with high-speed magic angle spinning produces (1)H resonances averaging only 0.22 ppm in width, and in some cases lines as narrow as 0.17 ppm are obtained. In application to two different polymorphs of ubiquitin, structure dependent differences in both (15)N and (1)H amide chemical shifts are observed. In one case, distinct shifts for different molecules in the asymmetric unit are seen, and all differ substantially from solution NMR shifts. A gain of 7 in sensitivity makes the method competitive with solution NMR as long as nanocrystalline samples are available.     

Study of the tautomeric equilibrium of pyridoxine in 1,4-dioxane/water mixtures by 13C nuclear magnetic resonance. Thermodynamic characterization and solvent effects

A significant temperature dependence has been found for the (13)C NMR chemical shifts of pyridoxine in 10%, 20%, 30%, 40%, 50%, and 60% v/v 1,4-dioxane/water mixtures (pH = 7.0). The nuclei most sensitive to the temperature effect were C-3 and C-6 in all of the mixtures. This dependence has been explained on the basis of a thermally induced tautomeric equilibrium shift between the neutral and the dipolar forms of the pyridoxine molecule. The thermodynamic characterization of this tautomeric equilibrium, which interconverts quickly on the NMR time scale, has been achieved by considering the observed average (13)C NMR chemical shifts at different temperatures through fitting the experimental data to a theoretical curve. The fitting accuracy is greatly improved on using linear correlations between the average chemical shifts obtained from different nuclei at the same temperature. The methodology outlined above allows the DeltaH degrees value to be calculated for the tautomeric process and the chemical shifts of the pure extreme forms, i.e., neutral and dipolar, to be deduced. These values have been used to calculate the thermodynamic parameters of the tautomerization equilibrium in each dioxane/water mixture. The effect of solvent on the tautomeric equilibrium and the averaged chemical shift has been explained in terms of a multiparameter equation developed by Kamlet and Taft. The overall solvent effect is the sum of two different effects: the dipolarity and polarizability of the solvent and the ability of the solvent to act as a hydrogen-bond donor toward a solute.     

Modeling amino acid side chains in proteins: 15N NMR spectra of guanidino groups in nonpolar environments

Natural-abundance 15N NMR spectroscopy on dodecylguanidine reveals solvent and protonation effects that model those that could occur for the arginine side chain in proteins. Our results demonstrate that the 15N chemical shifts of the terminal guanine nitrogens strongly depend on the solvent chosen for measurements. A polar H-bond-donating solvent like water has strongly deshielding effects on the neutral guanidine group (with the latter acting predominantly as an H-bond acceptor). As a result, a substantial upfield shift occurs when neutral guanidine is dissolved instead in a non-H-bonding solvent (chloroform). These solvent effects can be as large as those induced by protonation changes. This limits the ability of 15N chemical shifts to distinguish the protonation state of the arginine side chain, at least without specific knowledge of its environment. These results help to reconcile previous interpretations about the protonation state arg-82 in the M state of bacteriorhodopsin based on FTIR and 15N NMR spectroscopy. That is, contrary to earlier conclusions from solid-state NMR, the side chain of arg-82 could undergo a deprotonation between the bR and M states, but only if it also experienced a significant decrease in the H-bonding character and polarity of its environment. In fact, the average 15N chemical shift of the two Neta of arg-82 in bacteriorhodopsin's M intermediate (from the previous NMR measurements) is 17 ppm upfield from the corresponding value for the deprotonated arginine side chain in aqueous solution at pH >14, but only 3 ppm upfield from the value for deprotonated dodecylguanidine in chloroform.     

13C and 15N NMR studies of iron-bound cyanides of heme proteins and related model complexes: sensitive probe for detecting hydrogen-bonding interactions at the proximal and distal sides

Studies of the 13C and 15N NMR paramagnetic shifts of the iron-bound cyanides in the ferric cyanide forms of various heme proteins containing the proximal histidine and related model complexes are reported. The paramagnetic shifts of the 13C and 15N NMR signals of the iron-bound cyanide are not significantly affected by the substitution of the porphyrin side chains. On the other hand, the paramagnetic shifts of both the 13C and 15N NMR signals decrease with an increase in the donor effect of the proximal ligand, and the 13C NMR signal is more sensitive to a modification of the donor effect of the proximal ligand than the 15N NMR signal. With the tilt of the iron-imidazole bond, the paramagnetic shift of the 13C NMR signal increases, whereas that of the 15N NMR signal decreases. The hydrogen-bonding interaction of the iron-bound cyanide with a solvent decreases the paramagnetic shift of both 13C and 15N NMR signals, and the effect is more pronounced for the 15N NMR signal. Data on the 13C and 15N NMR signals of iron-bound cyanide for various heme proteins are also reported and analyzed in detail. Substantial differences in the 13C and 15N NMR shifts for the heme proteins can be explained on the basis of the results for the model complexes and structures around the heme in the heme proteins. The findings herein show that the paramagnetic shift of the 13C NMR signal of the iron-bound cyanide is a good probe to estimate the donor effect of the proximal imidazole and that the ratio of 15N/13C NMR shifts allows the hydrogen-bonding interaction on the distal side to be estimated.     

Probing the intermediates of halogen addition to alkynes: bridged halonium versus open halovinyl cation; a theoretical study

[Reaction: see text]. Intermediates formed in halogen addition (X = Br, Cl, F) to alkynes (ethyne, propyne, 2-butyne, trifluoromethylethyne, trimethylsilylethyne, and 1-trimethylsilylpropyne) were studied computationally by MP2 at the MP2/6-311++G(3df,3pd) level and/or by DFT at the B3LYP/6-31+G(d) level. Structure optimization and frequency calculations were performed to identify the minima and to obtain their relative energies. PCM calculations (with H2O, CH2Cl2, and CCl4 as model solvents) were employed to examine solvation effects on the relative stabilities in the resulting bridged halonium, -halovinyl, or -halovinyl cations. GIAO-MP2 and GIAO-DFT calculations were employed to compute NMR chemical shifts (13C, 19F, and 29Si as appropriate). In selected cases, PCM-GIAO calculations were also performed to investigate the extent of solvent effects on the computed NMR shifts. The NPA-derived charges and the GIAO shifts were examined in comparative cases to shed light on structural features. In several cases, structure optimization starting with the -halovinyl cations resulted in -halovinyl cations (via formal hydride shift or trimethylsilyl shift). With the CF3 derivative (when X = F), a formal F shift results in polyfluoroallyl cation generation from fluorovinyl cation as starting geometry.     

Application of a 1H δ‐resolved 2D NMR experiment to the visualization of enantiomers in chiral environment,using sample spatial encoding and selective echoes

We present the application of a 2D broadband homodecoupled proton NMR experiment to the visualization of enantiomers. In a chiral environment, the existence of diastereoisomeric intermolecular interactions can yield—generally slight—variations of proton chemical shifts from one enantiomer to another. We show that this approach, which relies on a spatial encoding of the NMR sample, is particularly well suited to the analysis of enantiomeric mixtures, since it allows, within one single 2D experiment, to detect subtle chemical shift differences between enantiomers, even in the presence of several couplings. This sequence, which uses semiselective radio‐frequency (rf) pulses combined to a z‐field gradient pulse, produces different selective echoes in various parts of the sample. The resulting homonuclear decoupling provides an original δ‐resolved spectrum along the diagonal of the 2D map where it becomes possible to probe the chiral differentiation process through every proton site where the resulting variation in the chemical shift is detectable. We discuss the advantages and drawbacks of this approach, regarding other experiments which provide homodecoupled proton spectra. This methodology is applied to the observation of enantiomers of (1) ( ± )2‐methyl‐isoborneol coordinated to europium (III) tris[3‐(trifluoromethyl‐hydroxymethylene)‐(+)‐camphorate] in isotropic solution, and (2) ( ± )3‐butyn‐2‐ol dissolved in a chiral liquid‐crystal solvent, in order to show the robustness of this pulse sequence for a wide range of chiral samples. Copyright © 2009 John Wiley & Sons, Ltd.     

Advanced solvent signal suppression for the acquisition of 1D and 2D NMR spectra of Scotch Whisky

A simple and robust solvent suppression technique that enables acquisition of high‐quality 1D ¹H nuclear magnetic resonance (NMR) spectra of alcoholic beverages on cryoprobe instruments was developed and applied to acquire NMR spectra of Scotch Whisky. The method uses 3 channels to suppress signals of water and ethanol, including those of ¹³C satellites of ethanol. It is executed in automation allowing high throughput investigations of alcoholic beverages. On the basis of the well‐established 1D nuclear Overhauser spectroscopy (NOESY) solvent suppression technique, this method suppresses the solvent at the beginning of the pulse sequence, producing pure phase signals minimally affected by the relaxation. The developed solvent suppression procedure was integrated into several homocorrelated and heterocorrelated 2D NMR experiments, including 2D correlation spectroscopy (COSY), 2D total correlation spectroscopy (TOCSY), 2D band‐selective TOCSY, 2D J‐resolved spectroscopy, 2D ¹H, ¹³C heteronuclear single‐quantum correlation spectroscopy (HSQC), 2D ¹H, ¹³C HSQC‐TOCSY, and 2D ¹H, ¹³C heteronuclear multiple‐bond correlation spectroscopy (HMBC). A 1D chemical‐shift‐selective TOCSY experiments was also modified. The wealth of information obtained by these experiments will assist in NMR structure elucidation of Scotch Whisky congeners and generally the composition of alcoholic beverages at the molecular level.