Selective Cross-polarization Research Articles

Identification of beryllium fluoride complexes in mechanically distorted gels using quadrupolar split 9Be NMR spectra resolved with solution-state selective cross-polarization.

The uniformly anisotropic media afforded by hydrogels are being increasingly exploited in analytical (structure elucidation) nuclear magnetic resonance (NMR) spectroscopy, and in studies of mechanosensitive biophysical and biochemical properties of living cells. The 9Be NMR parameters of beryllium fluoride complexes formed in aqueous solutions are sensitive markers of the anisotropic molecular environments produced by gelatin gels. The electric quadrupole moment of the 9Be nucleus (spin I = 3/2) interacts with the electric field gradient tensor in a stretched (or compressed) gel, giving rise to the splitting of peaks in 9Be NMR spectra. These are in addition to those produced by scalar coupling to the 19F nuclei. Thus, an equilibrium mixture of beryllofluoride complexes (BeF2, BeF3-, and BeF42-) in mechanically distorted gels generates an envelope of overlapping 9Be NMR multiplets. In the present work, the multiplets were dissected apart by using selective excitation of 9Be-19F cross-polarization; and the spectral components were quantified with multi-parameter line-shape decomposition, coupled with SpinDynamica simulations. The effects of gel density and Bloom number (a measure of gelatin-gel rigidity under standard conditions of sample preparation) on residual quadrupolar splittings were examined. Cross-polarization experiments revealed a bimodal distribution of residual quadrupolar coupling constants (RQC) of the BeF3- complexes. The average RQC of the dominant BeF3- population was ∼3 times larger than that of BeF42-. The secondary BeF3- population existed in a tetrahedral configuration. It was attributed to BeF3- complexes associated with negatively charged -COO- groups of the denatured collagen matrix.

Selective spin inversion in solution by magic field cross polarization

A pulsed element is proposed allowing the selective inversion of a single 1H nucleus, without regard to the presence of other degenerate 1H nuclei, provided that it is coupled to a heteronuclear spin with adequate chemical shift resolution in a 2D heteronuclear correlation spectrum. The approach is based on selective cross polarization, in which matched weak RF fields of specific amplitude are applied on resonance to the targeted 1H-X spin pair. It is shown theoretically that when the RF field amplitudes are set to specific values (ie. at a magic field), transfer of coherence can be fruitfully achieved transverse to the applied RF fields in addition to the normal longitudinal transfers. This enables the construction of a pulsed element which has the characteristics of a BIRDr,X element (Uhrin et al., 1993), yet with 2D frequency selectivity. Demonstration of the pulsed element is made in the context of selective spin inversion, by which all 1H spins are inverted except the targeted 1H spin.

13C‐Detected Through‐Bond Correlation Experiments for Protein Resonance Assignment by Ultra‐Fast MAS Solid‐State NMR

We present two sequences which combine ((1)H,(15)N) and ((15)N,(13)C) selective cross-polarization steps with an efficient variant of the J-based homonuclear transfer scheme, in which a spin-state-selective (S(3)E) block is incorporated to improve both resolution and sensitivity in the direct (13)C dimension. We propose these two sequences as a part of a suite of four N-C correlation experiments allowing for the assignment of protein backbone resonances in the solid state. We illustrate these experiments under ultra-fast magic angle spinning conditions on two samples of microcrystalline dimeric human superoxide dismutase (SOD, 153×2 amino acids), in its diamagnetic ("empty", Zn(II)) and paramagnetic (Cu(II), Zn(II)) states.

Selective polarization transfer using a single rf field

NMR is a popular and mature technique used in fields as diverse as chemistry, biology, or material science. One reason for this versatility lies in its ability to correlate the nuclei that are present in one molecule to another. This provides the researcher with correlation maps allowing for studies of the molecules at an atomic level. Selective experiments allow isolation of one such correlation to focus on spins of interest. This leads to a savings in precious experimental time by reducing the dimension of the experiment, which in turn may enable one to record more elaborate experiments that would otherwise not be amenable within reasonable acquisition times. Here, we present an alternative method to selectively transfer magnetization using a single rf field. This technique, which we call single field polarization transfer, allows to obtain longitudinal two-spin order of two scalar-coupled spins when only one of them is irradiated. The method is easy to implement and does not depend on stringent conditions, such as Hartmann-Hahn matching for selective cross-polarization transfers or very long inversion pulses and identification of coupling satellites in selective population inversion experiments.

Quadrupolar coupling selective cross-polarization in solid state NMR

A new strategy is presented for achieving selective heteronuclear polarization transfers from half-integer quadrupolar spins in magic-angle spinning (MAS) NMR. By combining cross-polarization with a recently introduced RAPT pulse sequence that selectively excites the signal of a half-integer quadrupolar nucleus based on its quadrupolar coupling constant magnitude, we demonstrate that hetero-nuclei in its close proximity may be selectively excited. Selective 23Na --> 1H polarization transfers are demonstrated in Na2MoO4 x 2 H2O, Na2HPO4 x 2 H2O and a mixture of NaHCO3 and Na2HPO4 x 2 H2O.

A comparison of the association of spermine with duplex and quadruplex DNA by NMR

The association of [1′,1″- 13C 2]spermine ([1′,1″- 13C 2] N, N′-bis(3-aminopropyl)-1,4-butanediamine) with duplex and quadruplex DNA has been studied by nuclear magnetic resonance spectroscopy. 1D NOESY experiments using two-way selective cross-polarization (ISI-SCP-NOESY) showed spermine intramolecular NOEs are either weakly positive or weakly negative when spermine is complexed to duplex B-DNA and linear four-stranded quadruplex DNA. In contrast, large negative intramolecular NOEs are observed when spermine is complexed to two distinct forms of folded quadruplex DNA suggesting greater immobilization of spermine on these folded DNA quadruplexes. No changes in the quadruplex stem structure are observed but there are minor changes to the loop structure of a two-stranded folded quadruplex on binding spermine.

Selective cross-polarization in solution state nuclear magnetic resonance of scalar coupled spin 12 and quadrupolar nuclei

Semiselective heteronuclear cross-polarization for achieving coherence transfer between a spin I=12 scalar coupled to a spin S⩾12 in isotropic solution is considered. The expansion of the density operator as products of irreducible tensor operators provides a compact formalism for describing cross-polarization involving scalar coupled quadrupolar nuclei. An analytical description of cross-polarization is presented in the limit of strong radio-frequency (rf) fields, with respect to the scalar-coupling constant. Numerical simulations show that reducing the rf field amplitudes does not have a detrimental effect on the efficiency of the transfer provided they are comparable to or greater than the scalar-coupling constant. The use of weak rf fields largely circumvents the reduced efficacy due to Hartmann–Hahn mismatch. Applications of the method for selective observation of scalar-coupled quadrupolar nuclei are considered and experimental results are presented for a mixture of beryllium fluoride complexes.

Selective 1D 1H → 13C → 13C polarization transfer experiments for small organic molecules

Selective polarization transfer from proton to adjacent carbon was accomplished by either selective cross-polarization or selective refocused-INEPT. Magnetization was transferred in the subsequent step to remote carbons separated by one, two or three bonds. The acquired data were processed by a new symmetrization protocol resulting in the signal-to-noise ratio enhancement by a factor of √2. Copyright © 1999 John Wiley & Sons, Ltd.

Selective 1D1H →13C →13C polarization transfer experiments for small organic molecules

Selective polarization transfer from proton to adjacent carbon was accomplished by either selective cross-polarization or selective refocused-INEPT. Magnetization was transferred in the subsequent step to remote carbons separated by one, two or three bonds. The acquired data were processed by a new symmetrization protocol resulting in the signal-to-noise ratio enhancement by a factor of √2. Copyright © 1999 John Wiley & Sons, Ltd.

Selective cross-polarization in solution state NMR

In two influential papers, Ernst and coworkers argued that cross-polarization, originallyproposed by Hartmann and Hahn, can be very useful in isotropic liquids to transfer coherencebetween scalar-coupled nuclei such as protons and carbon-13 (Maudsley, A. A., Müller, L.,and Ernst, R. R., 1977; J. magn. Reson., 28, 463; Müller, L., and Ernst, R. R., 1979, Molec.Phys., 38, 963). Unfortunately, the efficiency of cross-polarization in liquids tends to bestrongly attenuated if the radiofrequency fields are not perfectly homogeneous. In thispaper, it is demonstrated by experiments and simulations that imperfections in RF fieldshave little effect on the efficiency of magnetization transfer, provided that the RF amplitudesare comparable with the magnitudes of the heteronuclear scalar coupling constants. A comparisonbetween selective cross-polarization and selective INEPT shows clearly that crosspolarizationis more efficient. Selective cross-polarization does not require any careful calibrationand is insensitive to experimental instabilities.

Pharmacokinetics of the 13C labeled anticancer agent temozolomide detected in vivo by selective cross-polarization transfer.

The anticancer agent temozolomide labeled with 13C (8-Carbamoyl-3-13C-methylimidazo-[5,1-d]-1,2,3,5-tetrazin-4-(3H)-o ne), was noninvasively detected in subcutaneous RIF-1 tumors by a selective cross polarization 13C NMR method, at a field strength of 9.4T. Pharmacokinetics of the drug, at a dose of 150 mg/kg, were determined for intravenous and intraperitoneal models of administration (three animals per mode). The half-life of the drug in the tumors was approximately 60 min. The uptake and clearance of the drug, however, varied significantly between individual hosts, for both modes of administration. These results demonstrate the feasibility of obtaining pharmacokinetics of anticancer agents for individual tumors without the need for a label that might modify drug activity (e.g., fluorine). The variability of the in vivo measurements, even within the same tumor model, demonstrates the necessity of directly monitoring the tumor to evaluate drug pharmacokinetics.

Aluminum-27 cross polarization of aluminas. The NMR spectroscopy of surface aluminum atoms

The selective cross polarization of surface aluminum atoms is demonstrated and applied to {gamma}- and {alpha}-aluminas. The relatively high hydration levels of the {gamma}-alumina surface (out of the bottle) allow the observance of two surface species. Upon dehydration only one, somewhat broader resonance, with lower signal-to-noise ratio, can be resolved. Upon extensive dehydration this signal disappears. By comparison to Boehmite and {alpha}-alumina, these resonances are assigned to surface octahedral sites (3.0 ppm) and tetrahedral sites ({approximately} 62.0 ppm). All chemical shifts are reported relative to Al(NO{sub 3}){sub 3} {times} xH{sub 2}O. It is demonstrated that these resonances represent surface Broensted sites, and that they are not contaminated by Lewis acid sites or subsurface species. The Lewis acid sites can be selectively observed by replacing the surface hydroxyl protons with deuterons and adsorbing a Lewis base probe, pyridine, on the surface. In this case the {sup 1}H on the pyridine provide the polarization for the {sup 27}Al resonance. The {alpha}-alumina used in this study appears to have a low number of surface hydroxyls concomitant with its low surface area (5.5 m{sup 2}/g). This is reflected in the low (relative to {gamma}-alumina) {sup 27}Al signal-to-noise ratio obtained with this sample. Overall, thismore » approach appears to be an excellent way to nondestructively characterize these surfaces. More importantly, the success of surface selective cross polarization methodology illustrates that other nuclides are equally amenable to such experiments, and that this approach could be very useful as a general surface chemistry method for characterization of surfaces.« less