Carbon Chemical Shift Anisotropy Research Articles

Estimation of carbon-13 chemical shift anisotropy from aligned phase parameters in liquid crystals

ABSTRACT An experimental approach for estimating carbon chemical shift anisotropy (CSA) of liquid crystals is demonstrated. The alignment-induced chemical shift (AIS) along with local order parameters calculated from dipolar couplings are utilized to determine CSA parameters of a particular carbon of interest. The experiment is carried out as a function of temperature so that reliable values are obtained and the best fit to data at all temperatures will increase the accuracy of the approach. The molecular orientational order information at different temperatures can be obtained from CSA values from the single 1D spectrum in a straightforward way from the AIS values. The advantage of this approach is that due to variations in the local order and the orientation of the CSA tensor with respect to the magnetic field, the spectra show wide dispersion of resonance frequencies, thereby avoiding the problem of overlap of spectral lines which is encountered generally in the solid state.

Quantifying conformational dynamics using solid-state R1ρ experiments

We demonstrate the determination of quantitative rates of molecular reorientation in the solid state with rotating frame (R1ρ) relaxation measurements. Reorientation of the carbon chemical shift anisotropy (CSA) tensor was used to probe site-specific conformational exchange in a model system, d6-dimethyl sulfone (d6-DMS). The CSA as a probe of exchange has the advantage that it can still be utilized when there is no dipolar mechanism (i.e. no protons attached to the site of interest). Other works have presented R1ρ measurements as a general indicator of dynamics, but this study extracts quantitative rates of molecular reorientation from the R1ρ values. Some challenges of this technique include precise knowledge of sample temperature and determining the R20 contribution to the observed relaxation rate from interactions other than molecular reorientation, such as residual dipolar couplings or fast timescale dynamics; determination of this term is necessary in order to quantify the exchange rate due to covariance between the 2 terms. Low-temperature experiments measured an R20 value of 1.8±0.2s−1 Allowing for an additional relaxation term (R20), which was modeled as both temperature-dependent and temperature-independent, rates of molecular reorientation were extracted from field strength-dependent R1ρ measurements at four different temperatures and the activation energy was determined from these exchange rates. The activation energies determined were 74.7±4.3kJ/mol and 71.7±2.9kJ/mol for the temperature-independent and temperature-dependent R20 models respectively, in excellent agreement with literature values. The results of this study suggest important methodological considerations for the application of the method to more complicated systems such as proteins, such as the importance of deuterating samples and the need to make assumptions regarding the R20 contribution to relaxation.

NMR Cross-Correlated Relaxation Rates Reveal Ion Coordination Sites in DNA

In this work, a novel NMR method for the identification of preferential coordination sites between physiologically relevant counterions and nucleic acid bases is demonstrated. In this approach, the NMR cross-correlated relaxation rates between the aromatic carbon chemical shift anisotropy and the proton-carbon dipolar interaction are monitored as a function of increasing Na(+), K(+), and Mg(2+) concentrations. Increasing the counterion concentration modulates the residence times of the counterions at specific sites around the nucleic acid bases. It is demonstrated that the modulation of the counterion concentration leads to sizable variations of the cross-correlated relaxation rates, which can be used to probe the site-specific counterion coordination. In parallel, the very same measurements report on the rotational tumbling of DNA, which, as shown here, depends on the nature of the ion and its concentration. This methodology is highly sensitive and easily implemented. The method can be used to cross-validate and/or complement direct but artifact-prone experimental techniques such as X-ray diffraction, NMR analysis with substitutionary ions, and molecular dynamics simulations. The feasibility of this technique is demonstrated on the extraordinarily stable DNA mini-hairpin d(GCGAAGC).

Carbon‐13 chemical shift anisotropy in DNA bases from field dependence of solution NMR relaxation rates

Knowledge of (13)C chemical shift anisotropy (CSA) in nucleotide bases is important for the interpretation of solution-state NMR relaxation data in terms of local dynamic properties of DNA and RNA. Accurate knowledge of the CSA becomes particularly important at high magnetic fields, prerequisite for adequate spectral resolution in larger oligonucleotides. Measurement of (13)C relaxation rates of protonated carbons in the bases of the so-called Dickerson dodecamer, d(CGCGAATTCGCG)(2), at 500 and 800 MHz (1)H frequency, together with the previously characterized structure and diffusion tensor yields CSA values for C5 in C, C6 in C and T, C8 in A and G, and C2 in A that are closest to values previously reported on the basis of solid-state FIREMAT NMR measurements, and mostly larger than values obtained by in vacuo DFT calculations. Owing to the noncollinearity of dipolar and CSA interactions, interpretation of the NMR relaxation rates is particularly sensitive to anisotropy of rotational diffusion, and use of isotropic diffusion models can result in considerable errors.

Insight into the CSA tensors of nucleobase carbons in RNA polynucleotides from solution measurements of residual CSA: Towards new long-range orientational constraints

Using residual chemical shift anisotropies (RCSAs) measured in a weakly aligned stem–loop RNA, we examined the carbon chemical shift anisotropy (CSA) tensors of nucleobase adenine C2, pyrimidine C5 and C6, and purine C8. The differences between the measured RCSAs and the values back-calculated using three nucleobase carbon CSA sets [D. Stueber, D.M. Grant, 13C and (15)N chemical shift tensors in adenosine, guanosine dihydrate, 2′-deoxythymidine, and cytidine, J. Am. Chem. Soc. 124 (2002) 10539–10551; D. Sitkoff, D.A. Case, Theories of chemical shift anisotropies in proteins and nucleic acids, Prog. NMR Spectrosc. 32 (1998) 165–190; R. Fiala, J. Czernek, V. Sklenar, Transverse relaxation optimized triple-resonance NMR experiments for nucleic acids, J. Biomol. NMR 16 (2000) 291–302] reported previously for mononucleotides (1.4 Hz) is significantly smaller than the predicted RCSA range (−10–10 Hz) but remains larger than the RCSA measurement uncertainty (0.8 Hz). Fitting of the traceless principal CSA values to the measured RCSAs using a grid search procedure yields a cytosine C5 CSA magnitude ( CSA a = ( 3 / 2 · ( δ 11 2 + δ 22 2 + δ 33 2 ) ) 1 / 2 = 173 ± 21 ppm ) , which is significantly higher than the reported mononucleotide values (131–138 ppm) and a guanine C8 CSA a (148 ± 13 ppm) that is in very good agreement with the mononucleotide value reported by solid-state NMR [134 ppm, D. Stueber, D.M. Grant, 13C and (15)N chemical shift tensors in adenosine, guanosine dihydrate, 2′-deoxythymidine, and cytidine, J. Am. Chem. Soc. 124 (2002) 10539–10551]. Owing to a unique sensitivity to directions normal to the base plane, the RCSAs can be translated into useful long-range orientational constraints for RNA structure determination even after allowing for substantial uncertainty in the nucleobase carbon CSA tensors.

Ordering of a lateral crown ether and terminal short POE chains in some symmetrical nematogens by 13 C NMR

A lateral crown ether fragment can be introduced on symmetrical mesogens containing only three aromatic rings. The replacement of the terminal alkoxy chains by chains containing oxyethylene units decreases the melting and clearing temperatures allowing one to obtain nematic compounds near room temperature. These compounds dissolve LiBF4 salt only to a small extent, but the nematic arrangement, is thereby destroyed. The carbon chemical shift anisotropy and the local C-H bond order parameters were obtained for the nematic phase for the crown ether fragment and the terminal chains. This study indicates that the crown ether average conformation changes insignificantly on decreasing the temperature. The lateral crown ether protrudes markedly from the core with the consequence that the molecular shape is far from rod-like in geometry.

Solid-state (13)C NMR chemical shift anisotropy tensors of polypeptides.

Carbon-13 chemical shift anisotropy (CSA) tensors for various carbon sites of polypeptides, and for carbon sites in alpha-helical and beta-sheet conformations of poly-L-alanine, and polyglycine, are presented. The carbonyl (13)C CSA tensors were determined from one-dimensional CPMAS spectra obtained at a slow spinning speed, whereas the CSA tensors of C(alpha) and other carbons in side chains of peptides were determined using 2D PASS experiments on powder samples. The results suggest that the spans of (13)Carbonyl CSA tensors of alanine and glycine residues in various peptides are similar, even though the magnitude of individual components of the CSA tensor and the isotropic chemical shift are different. In addition, the delta(22) element is the only component of the (13)Carbonyl CSA tensor that significantly depends on the CO.HN hydrogen-bond length. Solid-state NMR experimental results also suggest that (13)Carbonyl and (13)C(alpha) CSA tensors are similar for alpha-helical and beta-sheet conformations of poly-L-alanine, which is in agreement with the reported quantum chemical calculation studies and previous solid-state NMR experimental studies on other systems. On the other hand, the (13)C(alpha) CSA tensor of the first alanine residue is entirely different from that of the second or later alanine residues of the peptide. While no clear trends in terms of the span and the anisotropic parameter were predicted for (13)C(beta) CSA tensors of alanine, they mainly depend on the conformation and dynamics of the side chain as well as on the packing interactions in the solid state of peptides.

Proton-Decoupled Carbon-13 NMR Spectroscopy in a Lyotropic Chiral Nematic Solvent as an Analytical Tool for the Measurement of the Enantiomeric Excess

Organic solutions of poly-γ-(benzyl-l-glutamate) (PBLG) generate a sufficient differential ordering effect (DOE) to discriminate enantiomers using proton decoupled carbon-13 NMR in natural abundance. Discrimination between enantiomers is observed through the carbon-13 chemical shift anisotropy (CSA) differences. This method is successfully applied to a large number of chiral molecules including a case of axial chirality and offers the advantage that no labeling or chemical modification of molecules is needed. In most cases, the chemical shift differences are large enough to measure the enantiomeric excess with accuracy. We show that this new tool is an attractive and powerful alternative to the existing enantiomeric analytical techniques.

Carbon-13 NMR study of the C60 cluster in the solid state: molecular motion and carbon chemical shift anisotropy

{sup 13}C NMR spectra of solid buckminsterfullerene, the soccerball-like cluster of 60 carbon atoms, have been obtained at temperatures down to 77 K. The ambient spectrum shows rapid isotropic rotational motion. The motion is sufficiently slow at 77 K that a measurement of the chemical shift tensor of the carbon nucleus can be made. The tensor components (220, 186, 40 ppm) have values that are typical for an aromatic carbon. Spectra at intermediate temperatures suggest the possibility of either growth of a low-temperature phase in which C60 rotation is inhibited or a distribution of rotational correlation times.

Differential line broadening in the NMR spectrum of methanol adsorbed on sol-gel silica

The nuclear magnetic relaxation characteristics of ({sup 13}C)methanol adsorbed to sol-gel silica were studied over a wide range of temperatures. The four-line spectra displayed different T{sub 2} values for each line. Inner and outer lines of the quartet displayed different T{sub 1} values. This behavior indicates that temporal correlations between different carbon-proton dipolar interactions are significant. Cross correlations between carbon-proton dipolar couplings and the carbon chemical shift anisotropy are discernible. Adsorbed methanol presents an interesting situation where both even and odd ranks of multispin order are spawned from athermal magnetizations via orientationally dependent spin interactions. These interactions are modulated by highly anisotropic molecular motions.

Resolution in 13C NMR of organic solids using high-power proton decoupling and magic-angle sample spinning

The 13C NMR linewidths observed in organic solids by means of high-power proton decoupling and magic-angle sample spinning are roughly 10 to 100 times broader than resonances in the liquid phase. This paper investigates important 13C line-broadening mechanisms in organic solids and their dependences on experimental parameters, notably static and rf magnetic field strength. The discussion is limited to glassy (disordered, partially mobile) and crystalline (ordered, rigid) organics at natural isotopic abundance. Excluded are elastomers and systems with a third dipolar coupled nuclear species. Experimental data, primarily at 1.4 T, for glassy and semi-crystalline polymers as well as crystalline materials, illustrate and confirm the linebroadening mechanisms identified. For some specimens, 13C linewidths are compared at 1.4 and 4.7 T. It is found that a substantial linebroadening (0.5 to 6 ppm) corresponding to a dispersion of isotropic chemical shifts can arise from distributions of anisotropic sources of magnetic susceptibility, bond angles, or frozen molecular conformations; in other cases, the resonance lines may be split into many distinct lines by magnetic inequivalences present in the solid but not the liquid phase. For crystalline materials, methods for reducing the broadening from anisotropic bulk susceptibility are discussed. Other broadening mechanisms considered are: insufficient proton-decoupling fields, off-resonance decoupling, imperfections in magic-angle sample spinning, and motional modulation of both the carbon-proton dipolar coupling and the carbon chemical shift anisotropy. On consideration of these mechanisms, it is anticipated (and shown experimentally in limited cases) that no significant gain in resolution will be enjoyed at high magnetic fields, especially when variable-temperature operation is available. In some instances, degradation of resolution may occur at high field if large rf field strengths or high spinning rates cannot be achieved.