Friday, June 27, 2008

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja8017253

Calcium-43 Chemical Shift Tensors as Probes of Calcium Binding Environments. Insight into the Structure of the Vaterite CaCO3 Polymorph by 43Ca Solid-State NMR Spectroscopy

David L. Bryce, Elijah B. Bultz, and Dominic Aebi

Natural-abundance 43Ca solid-state NMR spectroscopy at 21.1 T and gauge-including projector-augmented-wave (GIPAW) DFT calculations are developed as tools to provide insight into calcium binding environments, with special emphasis on the calcium chemical shift (CS) tensor. The first complete analysis of a 43Ca solid-state NMR spectrum, including the relative orientation of the CS and electric field gradient (EFG) tensors, is reported for calcite. GIPAW calculations of the 43Ca CS and EFG tensors for a series of small molecules are shown to reproduce experimental trends; for example, the trend in available solid-state chemical shifts is reproduced with a correlation coefficient of 0.983. The results strongly suggest the utility of the calcium CS tensor as a novel probe of calcium binding environments in a range of calcium-containing materials. For example, for three polymorphs of CaCO3 the CS tensor span ranges from 8 to 70 ppm and the symmetry around calcium is manifested differently in the CS tensor as compared with the EFG tensor. The advantages of characterizing the CS tensor are particularly evident in very high magnetic fields where the effect of calcium CS anisotropy is augmented in hertz while the effect of second-order quadrupolar broadening is often obscured for 43Ca because of its small quadrupole moment. Finally, as an application of the combined experimental−theoretical approach, the solid-state structure of the vaterite polymorph of calcium carbonate is probed and we conclude that the hexagonal P63/mmc space group provides a better representation of the structure than does the orthorhombic Pbnm space group, thereby demonstrating the utility of 43Ca solid-state NMR as a complementary tool to X-ray crystallographic methods.

ASAP J. Am. Chem. Soc., ASAP Article, 10.1021/ja0776881

Characterization of Mg2+ Binding to the DNA Repair Protein Apurinic/Apyrimidic Endonuclease 1 via Solid-State 25Mg NMR Spectroscopy

A. S. Lipton, R. W. Heck, S. Primak, D. R. McNeill, D. M. Wilson, III, and P. D. Ellis

Apurinic/apyrimidinic endonuclease 1 (APE1), a member of the divalent cation-dependent phosphoesterase superfamily of proteins that retain the conserved four-layered α/β-sandwich structural core, is an essential protein that functions as part of base excision repair to remove mutagenic and cytotoxic abasic sites from DNA. Using low-temperature solid-state 25Mg NMR spectroscopy and various mutants of APE1, we demonstrate that Mg2+ binds to APE1 and a functional APE1−substrate DNA complex with an overall stoichiometry of one Mg2+ per mole of APE1 as predicted by the X-ray work of Tainer and co-workers (Mol, C. D.; Kuo, C. F.; Thayer, M. M.; Cunningham, R. P.; Tainer, J. A. Nature 1995, 374, 381−386). However, the NMR spectra show that the single Mg2+ site is disordered. We discuss the probable reasons for the disorder at the Mg2+ binding site. The most likely source of this disorder is arrangement of the protein−ligands about the Mg2+ (cis and trans isomers). The existence of these isomers reinforces the notion of the plasticity of the metal binding site within APE1.

Wednesday, June 25, 2008

J Phys Chem B, Phys Chem C, Organometallics and Macromolecules Update

J. Phys. Chem. B, 112 (22), 6747–6752, 2008. 10.1021/jp711417t
Local Structure of Magnesium in Silicate Glasses: A 25Mg 3QMAS NMR Study
Keiji Shimoda,
* Takahiro Nemoto,§ and Koji Saito
Abstract: We have reported the 25Mg triple-quantum magic-angle spinning (3QMAS) NMR spectra of silicate glasses. The two-dimensional spectra suggest that the magnesium ions in MgSiO3, CaMgSi2O6, Ca2MgSi2O7, Mg3Al2Si3O12, and Li2MgSi2O6 glasses are mainly in octahedral environments, although in Na2MgSi2O6, K2MgSi2O6, and K2MgSi5O12 glasses they form tetrahedral species. We discussed the coordination environments of Mg based on the field strength of competing Mg2+, Ca2+, Na+, K+, and Li+ cations, and convincingly demonstrated that there is a correlation between them. These results indicate that the two-dimensional NMR spectroscopy such as MQMAS technique is a very useful method to analyze the local environments of nonframework cations in noncrystalline solids.

J. Phys. Chem. B, 112 (25), 7363–7369, 2008. 10.1021/jp800769u
Solvation and Aggregation of N,N′-Dialkylimidazolium Ionic Liquids: A Multinuclear NMR Spectroscopy and Molecular Dynamics Simulation Study
Richard C. Remsing, Zhiwei Liu, Ivan Sergeyev, and Guillermo Moyna*
Abstract: The solvation and aggregation of the ionic liquid (IL) 1-n-butyl-3-methylimidazolium chloride ([C4mim]Cl) in water and dimethylsulfoxide (DMSO) were examined by analysis of 1H and 35/37Cl chemical shift perturbations and molecular dynamics (MD) simulations. Evidence of aggregation of the IL n-butyl chains in aqueous environments at IL concentrations of 75−80 wt% was observed both in the NMR experiments and in the MD simulations. The studies also show that [C4mim]Cl behaves as a typical electrolyte in water, with both ions completely solvated at low concentrations. On the other hand, the data reveal that the interactions between the [C4mim]+ and Cl− ions strengthen as the DMSO content of the solutions increases, and IL-rich clusters persist in this solvent even at concentrations below 10 wt%. These results provide an experimentally supported atomistic explanation of the effects that these two solvents have on some of the macroscopic properties of [C4mim]Cl. The implications that these findings could have on the design of IL-based solvent systems are briefly discussed.

J. Phys. Chem. C, 112 (21), 7818–7828, 2008. 10.1021/jp710981g Web Release Date: May 8, 2008
Aggregation of Borate Salts in Hydrocarbon Solvents†
B. Endeward,‡§ P. Brant,∥ R. D. Nielsen,‡ M. Bernardo,‡⊥ K. Zick,# and H. Thomann*‡
Abstract: The aggregation of ammonium borate salts in hydrocarbon was investigated by 1H, 19F, and 11B pulsed field gradient (PFG) NMR. The molecular self-diffusion coefficients of the borate salts, [C(C6H5)3][B(C6F5)4], [N(CH3)2(n-C18H37)2][B(C6F5)4], [N(n-C18H37)4][B(C6F5)4], a neutral borane compound, and a siloxane model compound were measured in toluene, cyclohexane, hexane, and a solvent of mixed alkanes. Diffusion coefficients were determined from the echo attenuation of the stimulated echo pulsed field gradient NMR signal as described by Stejskal and Tanner. In all of the samples studied, the echo decay was observed to be a pure exponential decay, corresponding to a single diffusion coefficient within the resolution of the NMR experiment. The hydrodynamic radius of an equivalent diffusing sphere was calculated from the experimental diffusion coefficients using the Stokes−Einstein relation. We found that the neutral borane and siloxane model compounds are monomeric (nonaggregated) in both the aliphatic and aromatic solvents. In contrast, the borate salts exist as simple ion pairs in aromatic solvents, and as larger aggregates in aliphatic solvents for concentrations above approximately 1 mM. In the aliphatic solvents ion pair aggregate numbers are found which range from 5 1 to 11 2 ion pairs. Energy minimized structures of ion-pair multiplets were obtained using molecular mechanics simulations, and were used to establish the dependence of molecular volume on aggregate size. The aggregation of ions in nonpolar solvents with low dielectric constant is consistent with the known chemistry of electrolyte solutions. Ethylene−propylene copolymerizations were carried out in hexanes diluent where 0−2 molar equivalents of [N(n-C18H37)4][B(C6F5)4] was added to a metallocene catalyst and the concentration of the metallocene catalyst was held constant at 0.2 µM. Catalyst productivity decreases and ethylene incorporation increases with increasing ratio of [N(n-C18H37)4][B(C6F5)4] to metallocene catalyst. From these observations, it is inferred that, even under typical catalytic conditions, the ammonium borate salt is in close contact with the metallocene catalyst during polymerization.

J. Phys. Chem. C, 112 (21), 7895–7899, 2008. 10.1021/jp711727c
13C and 15N NMR and ab Initio/GIAO-CCSD(T) Study of the Structure of Mono-, Di-, and Triprotonated Guanidine, Urea, and Thiourea

George A. Olah,* G. K. Surya Prakash, and Golam Rasul
Abstract: Structures and 13C and 15N NMR chemical shifts of mono- and diprotonated guanidine, urea, and thiourea were studied by the ab initio/GIAO-SCF, GIAO-MP2, and GIAO-CCSD(T) methods. Calculated 15N NMR chemical shifts by these methods correlate very well with the experimental data. Also the calculated 13C NMR chemical shifts of the ions agree well with the available experimental values. Possible solvent effects on the 15N NMR chemical shifts in superacid media are also discussed. Structures and 15N and 13C NMR chemical shifts of yet experimentally unknown triprotonated guanidine, urea, and thiourea trications were also calculated.

J. Phys. Chem. C, 112 (22), 8481–8485, 2008. 10.1021/jp800486n
Hydrogen Sorption Behavior of the ScH2−LiBH4 System: Experimental Assesment of Chemical Destabilization Effects
Justin Purewal,
* Son-Jong Hwang, Robert C. Bowman, Jr.,§ Ewa Rönnebro, Brent Fultz, and Channing Ahn
Abstract: The hydrogen storage reaction ScH2 + 2LiBH4 → ScB2 + 2LiH + 4H2 (8.91 wt %), based on the thermodynamic destabilization of LiBH4, is predicted to have a reaction enthalpy of ΔH300K = 34.1 kJ/mol H2. The isothermal kinetic desorption behavior in this system was measured. At temperatures up to 450 °C, less than 5 wt % H2 is released, which is only half of the theoretical capacity. Powder X-ray diffraction data indicate that LiBH4 has decomposed into LiH in the final desorption product, but the data provide no evidence that ScH2 has participated in the reaction. Magic angle spinning NMR (MAS NMR) results do not show that the expected ScB2 equilibrium product phase has formed during desorption. While the addition of 2 mol % TiCl3 catalyst does improve desorption kinetics at 280 °C, it does not otherwise assist the destabilization reaction. The calculated reaction enthalpy suggests that this system should be of interest at moderate temperatures, but the large heats of formation of the reactant phases in this system appear to play a critical role in determining overall kinetics. Furthermore, the formation of a Li2B12H12 intermediate phase was determined by MAS NMR, which is an undesirable stable product if reaction reversibility is to be accomplished.

J. Phys. Chem. C, 112 (23), 8575–8586, 2008. 10.1021/jp711137c
Directly Probing the Metal Center Environment in Layered Zirconium Phosphates by Solid-State 91Zr NMR
Zhimin Yan, Christopher W. Kirby, and Yining Huang
Abstract: Layered zirconium phosphates (ZrPs) and their derivatives potentially have many important applications. In the past, these materials have been mainly characterized by X-ray diffraction and 31P MAS NMR. Their metal centers have not been directly probed by solid-state NMR spectroscopy. In this work, we present solid-state 91Zr NMR spectra acquired at several magnetic fields for several representative layered zirconium phosphates including α-Zr(HPO4)2·H2O, γ-Zr(PO4)(H2PO4)·2H2O, Zr(NH4PO4)2·H2O, and Zr2(NaPO4)4·6H2O. The NMR interaction tensors were extracted from the spectra. The results indicate that the 91Zr spectra are sensitive not only to the relatively small distortion in ZrO6 polyhedron but also to the difference in the geometry of Zr(OP)6 units in these materials. We show that 91Zr quadrupolar coupling constants (CQ) correlate well with several angular distortion parameters reflecting the deviation from a perfect ZrO6 octahedron such as distortion index, shear strain, and mean O−Zr−O angle. The relationships between CQ and structural parameters related to the Zr(OP)6 unit including the mean Zr−P distance and Zr−O−P angle also appear to exist. The theoretical calculations at both restricted Hartree−Fock and density functional levels were performed on model clusters to establish the relationships of various structural parameters with 91Zr EFG tensors, and the calculation results are consistent with the empirical correlations. For the related layered zirconium phosphates whose structures are unknown or poorly described, we have shown that 91Zr NMR can be used to directly obtain structural information on the local environment around the metal centers, which is complementary to that obtained from powder XRD and 31P MAS NMR, as demonstrated by a novel meso-lamellar ZrP as an example.

J. Phys. Chem. C, 112 (25), 9145–9154, 2008. 10.1021/jp711398h
Dynamics and Disorder in Surfactant-Templated Silicate Layers Studied by Solid-State NMR Dephasing Times and Correlated Line Shapes
Sylvian Cadars,
Nicolas Mifsud, Anne Lesage, Jan Dirk Epping,§ Niklas Hedin, Bradley F. Chmelka, and Lyndon Emsley*
Abstract: Surfactant-templated layered silicates are shown to possess complex compositional, structural, and dynamic features that manifest rich and interrelated order and disorder at molecular length scales. Temperature-dependent 1D and 2D solid-state 29Si NMR measurements reveal a chemical-exchange process involving the surfactant headgroups that is concomitant with reversible broadening of 29Si NMR line shapes under magic-angle-spinning (MAS) conditions at temperatures in the range 205−330 K. Specifically, the temperature-dependent changes in the 29Si transverse dephasing times T2′ can be quantitatively accounted for by 2-fold reorientational dynamics of the surfactant headgroups. Variable-temperature analyses demonstrate that the temperature-dependent 29Si shifts, peak broadening, and 2D 29Si{29Si} correlation NMR line shapes are directly related to the freezing of the surfactant headgroup dynamics, which results in local structural disorder within the silicate framework.

J. Phys. Chem. C, 112 (25), 9267–9271, 2008. 10.1021/jp800873c
Determining the Diameter of Functionalized Single-Walled Carbon Nanotubes with 13C NMR: A Theoretical Study
Eva Zurek,
* Chris J. Pickard, and Jochen Autschbach*§
Abstract: NMR chemical shifts of N–R-functionalized (n, 0) single-walled carbon nanotubes (SWNTs) with n = 11, 13−17 were computed from first principles. The chemical shifts of functionalized carbons at a bond diagonal to the SWNT axis are strongly dependent upon the C−C distance in the C−NR−C moiety. This distance, in turn, is sensitive to the SWNT radius. Monitoring these shifts could therefore help to determine the diameter distribution within a sample. Proton shifts are also reported.

J. Phys. Chem. C, 112 (25), 9486–9492, 2008. 10.1021/jp802631u
Role of Pentacoordinated Al3+ Ions in the High Temperature Phase Transformation of γ-Al2O3
Ja Hun Kwak, Jianzhi Hu, Adrienne Lukaski,
Do Heui Kim, János Szanyi, and Charles H. F. Peden*
Abstract: In this work, the structural stability of γ-alumina (γ-Al2O3) was investigated by a combination of XRD and high-resolution solid-state 27Al MAS NMR at an ultrahigh magnetic field of 21.1 T. XRD measurements show that γ-Al2O3 undergoes a phase transition to θ-Al2O3 during calcination at 1000 °C for 10 h. The formation of the θ-Al2O3 phase is further confirmed by 27Al MAS NMR; additional 27Al peaks centered at 10.5 and ~78 ppm were observed in samples calcined at this high temperature. Both the XRD and NMR results indicate that, after calcination at 1000 °C for 10 h, the ratio of the θ-Al2O3 phase to the total alumina in samples modified by either BaO or La2O3 is significantly reduced in comparison with γ-Al2O3. 27Al MAS NMR spectra revealed that the reduction in the extent of θ-Al2O3 formation was highly correlated with the reduction in the amount of pentacoordinated aluminum ions, measured after 500 °C calcination, in both BaO- and La2O3-modified γ-Al2O3 samples. These results strongly suggest that the pentacoordinated aluminum ions, present exclusively on the surface of γ-Al2O3, play a critical role in the phase transformation of γ-Al2O3 to θ-Al2O3. The role of the modifiers, in our case BaO or La2O3, is to convert the pentacoordinated aluminum ions into octahedral ones, thereby improving the thermal stabilities of the samples. Oxide additives, however, seem to have little, if any beneficial effect on preventing reductions in specific surface areas that occurred during high-temperature (≤1000 °C) calcination

ASAP Organometallics, ASAP Article, 10.1021/om800260c
6Li NMR Study of the Equilibrium between Methyllithium and n-Butyllithium in THF. Influence of a Third Partner, a Chiral Lithium Amide
Franck Paté,
Hassan Oulyadi,* Anne Harrison-Marchand, and Jacques Maddaluno*
Abstract: A 6Li NMR structural study of mixtures of labeled methyllithium, n-butyllithium, and 3-aminopyrrolidine lithium amide has been achieved in THF at −78 °C. In the first part, the study was focused on the mixed aggregates formed between methyllithium and n-butyllithium. The spectral data suggest that the two reactants form a series of tetramers (MeLi)4(n-BuLi)4−n, of which abundance depends on the proportion of the two alkyllithiums following a purely statistical distribution. The labeled chiral lithium amide was next introduced in the sample, and the competitive formation of the amide−methyllithium and amide−butyllithium mixed dimers was monitored in situ. The NMR spectra show a juxtaposition of signals typical of the entities characterized separately, suggesting that there is no preferential formation of one aggregate over the others.

Organometallics, 27 (12), 2756–2760, 2008. 10.1021/om800096f
Homoleptic Complexes of Bisstannylenes with Nickel(0): Synthesis, X-Ray Diffraction Studies, and 119Sn NMR Investigations
Alexander V. Zabula, Tania Pape, Alexander Hepp, and F. Ekkehardt Hahn
Abstract: The benzannulated bisstannylenes linked by a CH2C(CH3)2CH2 (3) or 1,3-(CH2)2C6H4 bridging unit (4) react with [Ni(cod)2] to give the intensely colored homoleptic Ni0 complexes 5 and 6. X-ray diffraction and 119Sn NMR studies for the complex of the bisstannylene with the CH2C(CH3)2CH2 linker 5 reveal the presence of four identical tin atoms. For the nickel complex 6 of the bisstannylene ligand, which contains a 1,3-(CH2)2C6H4 bridging unit, two types of tin atoms with different coordination environments were found both in solution and in the solid state.

Monday, June 23, 2008

Physical Review Letters - Vol 100 No 24

Two-Dimensional NMR of Diffusion Systems
Yi-Qiao Song, Lukasz Zielinski, and Seungoh Ryu
We consider diffusion in porous media with well-connected pore space for which isolated-pore models are insufficient. Explicit pore-to-pore exchange parameters were introduced in recent 2D NMR experiments. However, such parameters capture only certain aspects of the interpore spin dynamic which, for single-fluid saturated media, are wholly determined by diffusion. Here, we develop a theoretical approach suitable for a quantitative description of such 2D NMR taking a full account of the underlying diffusion modes. We use simple models of one pore and two coupled pores to demonstrate the rich behavior of 2D NMR.

Friday, June 20, 2008

Chemical Physics, Volume 351, Issues 1-3, 3 July 2008, Pages 83-90

Theoretical study of 14N quadrupole coupling constants in some NO-containing complexes: N2O3 and FNO

R. Polák, , and J. Fišer

The nuclear quadrupole coupling constants at nitrogen centers have been computed for N2O3 and FNO by employing the complete-active-space self-consistent field, internally contracted multireference configuration interaction and single-configuration coupled-cluster methods with correlation-consistent basis sets at the levels of attainable accuracy. To examine the overall quality of the wave functions used in our calculations, also electric dipole moments and potential energy characteristics were calculated and compared with available experimental and recent theoretical data. The effects of the choice of the basis set and reference configuration space were investigated. The robust changes in the electric field gradients occurring in the course of complex formation from isolated subunits were interpreted in terms of wave function composition. Our calculations confirm the assignment of the 14N nuclear quadrupole coupling constants to nuclear centers in N2O3 provided by the microwave measurements of Cox et al. [A.P. Cox, J. Randell, A.C. Legon, Chem. Phys. Lett. 126 (1986) 481.].

Chemical Physics Volume 351, Issues 1-3, 3 July 2008, Pages 33-36

Double quantum CRAZED NMR signal in inhomogeneous fields
Bin Jiang, Huili Liu, Maili Liu, Chaohui Ye and Xian Mao


It has been well accepted that the double quantum (DQ) correlated-spectroscopy revamped by asymmetric z-gradient echo detection (CRAZED) signal is enveloped in the profile function t2 exp [−(t2 + 2t1)/T2], but this function is too simple to describe the spin echo characteristics of the CRAZED free induction decay signal. In this paper the DQ CRAZED experiment is analyzed by including the homogeneous and inhomogeneous broadening effects, and a formula for the time domain DQ CRAZED signal is obtained. This formula includes the chemical shift echo and the inhomogeneous echo, both appearing at t2 = 2t1. Experiments have confirmed the theory.

Bryan's Mini-Update

As per popular request, I'm always here to give the people what they want.

Chemical Physics Letters
Vol: 459, Issue: 1-6, June 27, 2008
pp. 188-193

Title: Solid-state single-scan 2D NMR under magic-angle-spinning

Authors: Gal, M.; Melian, C.; Demco, D.E.; Blumich, B.; Frydman, L.

Abstract: 2D NMR rubber spectrum collected within a single-scan (27ms total time) using a 2ms mixing time and magic-angle-spinning at 17kHz

Tuesday, June 17, 2008

Chem. Mater., 20 (12), 3944–3953, 2008.

Characterization of Layered γ-Titanium Phosphate (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O Intercalate: A Combined NMR, Synchrotron XRD, and DFT Calculations Study
Luís Mafra* and João Rocha,* Christian Fernandez, Germán R. Castro, Sergei A. Khainakov, and José R. García

Organically templated titanium phosphate, (C2H5NH3)[Ti(H1.5PO4)(PO4)]2·H2O, has been prepared by hydrothermal synthesis from titanium(IV) chloride, phosphoric acid, and ethylamine. The structure of this material has been characterized by synchrotron X-ray powder diffraction, 1H, 13C, 15N, and 31P (including 1H{FS-LG}-1H HOMCOR, 1H{FS-LG}-31P HETCOR, 1H-1H DQ-SQ, 13C{31P} REDOR) MAS NMR, and FTIR spectroscopies and thermal analyses (TG and DSC). Its triclinic structure was solved in the space group P1 (No. 1) with the following final unit cell parameters: a = 5.1126(1), b = 6.3189(2), c = 12.0396(5) Å, α = 100.931(2), β = 97.211(2), γ = 90.683(3)°, and V = 378.62(2) Å3 (Z = 1). This pseudo-three-dimensional compound is built up of anionic titanium phosphate layers, similar to those present in the γ-type titanium phosphate, and ethylammonium cations residing in the interlayer one-dimensional channels. On the basis of empirical (particularly NMR and powder XRD) and theoretical (DFT calculations using plane waves basis set) data, the 1H NMR spectrum has been assigned, and evidence was found for a very strong interlayer P−O···H···O−P interaction.

Monday, June 16, 2008

Journal of Magnetic Resonance - Vol 193 Issue 1

Probing the validity of average Hamiltonian theory for spin I = 1, 3/2 and 5/2 nuclei by analyzing a simple two-pulse sequence
E.S. Manangaa, b, C.D. Hsuc, S. Ishmaela, T. Islama and G.S. Boutis
In this work, we investigate the accuracy of controlling spin I = 1, 3/2 and 5/2 spin systems by average Hamiltonian theory. By way of example, we consider a simple two-pulse echo sequence and compare this perturbation scheme to a numerical solution of the Von Neumann equation. For the different values of I, we examine this precision as a function of the quadrupolar coupling as well as various experimental parameters such as the pulse spacing and pulse width. Experiments and simulations on I = 3/2 and I = 5/2 spin systems are presented that highlight a spectral artifact introduced due to finite pulse widths as predicted by average Hamiltonian theory. The control of these spin systems by this perturbation scheme is considered by investigating a phase cycling scheme that suppresses these artifacts to zeroth-order of the Magnus expansion.

Direct measurement of dipole–dipole/CSA cross-correlated relaxation by a constant-time experiment
Yizhou Liua and James H. Prestegard
Relaxation rates in NMR are usually measured by intensity modulation as a function of a relaxation delay during which the relaxation mechanism of interest is effective. Other mechanisms are often suppressed during the relaxation delay by pulse sequences which eliminate their effects, or cancel their effects when two data sets with appropriate combinations of relaxation rate effects are added. Cross-correlated relaxation (CCR) involving dipole–dipole and CSA interactions differ from auto-correlated relaxation (ACR) in that the signs of contributions can be changed by inverting the state of one spin involved in the dipole–dipole interaction. This property has been exploited previously using CPMG sequences to refocus CCR while ACR evolves. Here we report a new pulse scheme that instead eliminates intensity modulation by ACR and thus allows direct measurement of CCR. The sequence uses a constant time relaxation period for which the contribution of ACR does not change. An inversion pulse is applied at various points in the sequence to effect a decay that depends on CCR only. A 2-D experiment is also described in which chemical shift evolution in the indirect dimension can share the same constant period. This improves sensitivity by avoiding the addition of a separate indirect dimension acquisition time. We illustrate the measurement of residue specific CCR rates on the non-myristoylated yeast ARF1 protein and compare the results to those obtained following the conventional method of measuring the decay rates of the slow and fast-relaxing 15N doublets. The performances of the two methods are also quantitatively evaluated by simulation. The analysis shows that the shared constant-time CCR (SCT–CCR) method significantly improves sensitivity.

An analysis of phase-modulated heteronuclear dipolar decoupling sequences in solid-state nuclear magnetic resonance
Rajendra Singh Thakura, Narayanan D. Kururb and P.K. Madhu
The design of variants of the swept-frequency two-pulse phase modulation sequence for heteronuclear dipolar decoupling in solid-state NMR is reported, their performance evaluated, and compared with other established sequences like TPPM and SPINAL. Simulations performed to probe the role of the homonuclear 1H–1H bath show that the robustness of the decoupling schemes improves with the size of the bath. In addition, these simulations reveal that the homonuclear 1H–1H bath also leads to broad baselines at high MAS rates. Results from a study of the SPINAL decoupling scheme indicate that optimisation of the starting phase and phase increment improves its performance and efficiency at high MAS rates. Additionally, experiments performed on a liquid crystal display the role of the initial phase in SPINAL-64 and sequences in the SWf-TPPM family.

Fast amplitude-modulated pulse trains with frequency sweep (SW-FAM) in solid-state NMR of spin-7/2 nuclei
Thomas Bräunigera and P.K. Madhu
We here investigate the sensitivity enhancement of central-transition NMR spectra of quadrupolar nuclei with spin-7/2 in the solid state, generated by fast amplitude-modulated RF pulse trains with constant (FAM-I) and incremented pulse durations (SW-FAM). Considerable intensity is gained for the central-transition resonance of single-quantum spectra by means of spin population transfer from the satellite transitions, both under static and magic-angle-spinning (MAS) conditions. It is also shown that incorporation of a SW-FAM train into the excitation part of a 7QMAS sequence improves the efficiency of 7Q coherence generation, resulting in improved signal-to-noise ratio. The application of FAM-type pulse trains may thus facilitate faster spectra acquisition of spin-7/2 systems.

The generation of intense radiofrequency fields in μcoils
Edward W. Hagamana, Jian Jiaoa and Tony Moore
Large amplitude radiofrequency (rf) fields hold great promise in wide line NMR where it becomes possible to excite the full breadth of the line in a quantitative fashion. Applications in quadrupole NMR and in NMR of paramagnetic systems benefit greatly from intense fields. Spin manipulations in multiple quantum NMR experiments, specifically, the generation of multiple quantum coherence in MQ NMR, are more efficiently produced using intense rf fields. In this work we describe a μcoil probe that produces an rf field of 25 MHz, more than five times larger than the greatest rf field reported in the literature. We accomplish this in a robust, 127 μm diameter solenoid coil using 1 kW of rf power.

An alternative tuning approach to enhance NMR signals
Denis J.-Y. Mariona and Hervé Desvaux
By using spin-noise type measurement we show that the resonance frequency of the reception circuit of classical NMR spectrometers does not match the Larmor frequency even if, in emission, the electronic circuit is perfectly tuned at the Larmor frequency and matches the amplifier impedance. We also show that this spin-noise method can be used to ensure a match between the Larmor frequency and the reception circuit resonance frequency. In these conditions, (i) the radiation damping field is in perfect quadrature to the magnetization and (ii) the NMR signal level and potentially the signal-to-noise ratio, are enhanced. This choice induces a change of the probe resonance frequency by several hundreds of kHz for 500 or 700 MHz spectrometer. We show that the resulting mismatch condition for emission can be removed by adding other tuning and matching degrees of freedom located on the excitation line (or by symmetry on the reception line) decoupled to the probe resonance circuit by the crossed diodes.

Friday, June 13, 2008

ASAP Cryst. Growth Des., ASAP Article, 10.1021/cg800196s

Monotropic Polymorphism in Copper(II) Decanoate

M. Ramos Riesco, F. J. Martínez Casado, S. López-Andrés,‡ M. V. García Pérez, M. I. Redondo Yélamos, M. R. Torres, L. Garrido, and J. A. Rodríguez Cheda*

Copper(II) decanoate was synthesized, purified and studied by DSC, optical microscopy, XRD, and FTIR and C-13 NMR spectroscopies. Depending on the solvent used for crystallization two polymorphs were obtained. A combination of single-crystal and powder XRD was used to study their structures. One of the polymorphs (recrystallized in n-heptane) has similar structure to that previously reported at room temperature: triclinic (a = 7.94(1) Å, b = 5.28(3) Å, c = 28.13(5) Å; α = 94.6(10)°, β = 97.1(5)°, γ = 98.6(10)°). A single crystal of the second polymorph was obtained from an ethanol solution and has a triclinic structure (a = 5.2035(1) Å, b = 14.331(3) Å, c = 16.440(4) Å; α = 65.390(4)°, β = 86.889(4)°, γ = 82.886(4)°). This polymorph is monotropic and was identified by calorimetry as the metastable form. Both room temperature crystalline polymorphs belong to the same space group, P1̅. However, they differ in (1) unit cell parameters; (2) one is bilayered whereas the other presents a columnar distribution of the binuclear complexes inside a net of two-by-two interdigitated chains. Thermal coefficients for both polymorphs have been estimated by indexation of their powder X-ray patterns versus temperature. The FTIR and C-13 NMR techniques confirm the structural differences. Both polymorphs melt to the same columnar discotic liquid crystal (identified by optical microscopy), decomposing before reaching the clearing point.

Friday, June 06, 2008

ASAP Mol. Pharmaceutics, ASAP Article, 10.1021/mp7001522

Web Release Date: June 5, 2008
Copyright © 2008 American Chemical Society
NMR Spectroscopic Evaluation of the Internal Environment of PLGA Microspheres
Kirby C. Wong-Moon, Xin Sun, Xichdao C. Nguyen, Bin P. Quan, Kai Shen, and Paul A. Burke*

Department of Pharmaceutics, Amgen Inc., Thousand Oaks, California 91320

The internal environment of poly(lactide-co-glycolide) (PLGA) microspheres was characterized using 31P and 13C solid-state and solution NMR spectroscopy. Physical and chemical states of encapsulated phosphate- and histidine-containing porogen excipients were evaluated using polymers with blocked (i.e., esterified) or unblocked (free acid) end groups. Spectroscopic and gravimetric results demonstrated that the encapsulated porogen deliquesced upon hydration at 84% relative humidity to form a solution environment inside the microspheres. Dibasic phosphate porogen encapsulated in unblocked PLGA was partially titrated to the monobasic form, while in the same formulation 13C NMR showed partial protonation of the histidine imidazole. Similarly, encapsulated monobasic phosphate was partially converted to phosphoric acid. Coencapsulation of monobasic and dibasic phosphate porogens resulted in a single peak on hydration, indicating chemical exchange between discrete excipient microphases. Exogenous buffer addition differentiated external from internal, nontitratable, excipient populations. Microspheres containing dibasic phosphate porogen were hydrated with fetal calf serum, incubated at 37 °C, and characterized by 31P NMR through the polymer erosion phase. Within 48 h the 31P chemical shift moved over 2 ppm upfield and the line width narrowed to <60 Hz; there was little additional change through day 14. This indicated complete conversion to the monobasic phosphate form throughout the polydisperse sample and that pH remained below 4 but above the phosphoric acid pKa during matrix erosion.

Wednesday, June 04, 2008

Bryan's Literature Update up to June 4th, 2008

What follows is all the news fit to print on NMR since the last time I had the pleasure of communicating to the masses on March 31, 2008.

Fun game: Spot the paper by the current post-doctoral fellow in Rob Schurko's group! (It's really fun!)


Chemical Physics
Vol: 345, Issue: 1, April 3, 2008
pp. 116-118

Title: The relationship between crystal structure and NMR relaxation in molecular solids with tert-butyl groups

Authors: Rheingold, A.L.; DiPasquale, A.G.; Beckmann, P.A.

Keywords: Solid state NMR; NMR relaxation; Molecular solids; Intramolecular motion; X-ray diffraction

Abstract (English): We correlate an X-ray determination of the molecular and crystal structures of 2-tert-butylanthracene and 2-tert-butylanthraquinone reported here with the previously reported dynamical nuclear magnetic resonance determination of the motions of the tert-butyl groups and their resident methyl groups in the solid state [P.A. Beckmann, K.S. Burbank, M.M.W. Lau, J.N. Ree, T.L. Weber, Chem. Phys. 290 (2003) 241].


Chemical Physics
Vol: 348, Issue: 1-3, June 2, 2008
pp. 175-180

Title: 14N and 17O electric field gradient tensors in benzamide clusters: Theoretical evidence for cooperative and electronic delocalization effects in N-H...O hydrogen bonding

Authors: Esrafili, M.D.; Behzadi, H.; Hadipour, N.L.

Keywords: Electron delocalization effects; Cooperative effects; Electric field gradient; Benzamide; Hydrogen bond; Natural bonding orbital (NBO) analysis

Abstract (English): Nuclear quadrupole coupling tensors of 14N and 17O nuclei in benzamide clusters (up to n=6) were calculated via density functional theory. Results revealed that N-H...O hydrogen bonds around the benzamide molecule in crystalline lattice have significant influence on 14N and 17O nuclear quadrupole coupling tensors. n-dependent trend in 14N quadrupole coupling tensors significantly correlates with cooperative effects of R(N-H...O) hydrogen bond distance. Natural bonding orbital analysis, NBO, was used to rationalize the quadrupole coupling results in terms of substantial nO->sN-H^* charge transfer in (benzamide)n=1-6 clusters. It was found that intermolecular nO->sN-H^* interactions play a key role in cooperative change of 14N and 17O quadrupole coupling tensors. There is an acceptable linear correlation between 14N or 17O quadrupole coupling tensors with strength of Fock matrix elements (Fij). This suggests that both 14N and 17O quadrupole coupling measurements can provide a useful probe for electron delocalization effects in both gaseous and condensed media.


Chemical Physics Letters
Vol: 455, Issue: 4-6, April 10, 2008
pp. 178-183

Title: Quantification of crystalline phases and measurement of phosphate chain lengths in a mixed phase sample by 31P refocused INADEQUATE MAS NMR

Authors: O'Dell, L.A.; Guerry, P.; Wong, A.; Abou Neel, E.A.; Pham, T.N.; Knowles, J.C.; Brown, S.P.; Smith, M.E.

Abstract (English): A 31P refocused INADEQUATE NMR pulse sequence has been used to identify the crystalline phases present in a sodium calcium phosphate ceramic. An unidentified calcium phosphate phase was observed, featuring a chain of six crystallographically distinct PO4 tetrahedra, the 31P NMR chemical shifts of which have been identified in the correct sequence along the chain.


Chemical Physics Letters
Vol: 456, Issue: 1-3, April 21, 2008
pp. 116-121

Title: Estimation of internuclear couplings in the solid-state NMR of multiple-spin systems. Selective spin echoes and off-magic-angle sample spinning

Authors: Pileio, G.; Mamone, S.; Mollica, G.; Montesinos, I.M.; Gansmuller, A.; Carravetta, M.; Brown, S.P.; Levitt, M.H.

Abstract (English): A combination of modulated pulses and off-magic-angle spinning NMR allows the estimation of distances between selected pairs of nuclei in the solid-state.


Chemical Physics Letters
Vol: 456, Issue: 4-6, May 5, 2008
pp. 227-230

Title: Proton exchange in hybrid sulfonated poly(ether ether ketone)-silica membranes by 1H solid-state NMR

Authors: Baias, M.; Demco, D.E.; Colicchio, I.; Blumich, B.; Moller, M.

Abstract (English): The Eyring function is used for obtaining enthalpy of activation for proton exchange in sulfonated poly(ether ether ketone)-silica membranes by NMR.


Chemical Physics Letters
Vol: 456, Issue: 4-6, May 5, 2008
pp. 253-256

Title: 1H homonuclear dipolar decoupling at high magic-angle spinning frequencies with rotor-synchronised symmetry sequences

Authors: Paul, S.; Thakur, R.S.; Madhu, P.K.

Abstract (English): Symmetry based rotor synchronised RNnn sequences were applied to obtain high resolution 1H spectra in solid-state NMR. The experimental and simulated data shows the potential of such RNnn sequences in obtaining high resolution spectra at high spinning speeds.


Chemical Physics Letters
Vol: 456, Issue: 4-6, May 5, 2008
pp. 262-267

Title: Decomposition of a NMR signal comprising a single resonance in exchange with a Pake doublet: Application to the analysis of ^1H NMR spectra of water molecules embedded in synthetic clays

Authors: Trausch, G.; Canet, D.; Turq, P.

Abstract (English): NMR of water in clays include several overlapping resonances, possibly with a Pake doublet. Proportions and dipolar coupling were determined.


Chemical Physics Letters
Vol: 458, Issue: 1-3, June 6, 2008
pp. 219-222

Title: Enantiomeric analysis using natural abundance deuterium 3D NMR spectroscopy in polypeptide chiral oriented media

Authors: Lesot, P.; Lafon, O.

Abstract (English): The use of natural abundance deuterium 3D NMR experiments for analyzing chiral molecules dissolved in chiral liquid crystals is reported.


Dalton Trans., 2008, 2241 - 2247, DOI: 10.1039/b718438g

Title: NMR Transversal relaxivity of aqueous suspensions of particles of Ln3+-based zeolite type materials

Authors: Giovannia A. Pereira, Magorzata Norek, Joop A. Peters, Duarte Ananias, João Rocha and Carlos F. G. C. Geraldes

Abstract: A series of zeolite-type silicates containing stoichiometric amounts of Ln3+ ions in the framework (Ln-AV-9 materials), with composition (Na4K2)(Ln2Si16O38)10H2O (Ln = Nd, Sm, Eu, Tb, Gd, Dy) has recently been synthesized and characterized. They form paramagnetic microparticles, which as aqueous suspensions have negligible water 1H longitudinal relaxivities (t1) for all Ln3+ ions studied and quite large transverse relaxivities (t2). In this work we further analysed the size distribution of the Ln-AV-9 particles and their t2* and t2 relaxivities. The t2* relaxivity effects are explained by the static dephasing regime (SDR) theory. The t2 relaxivities appear to be strongly dependent on the interval between two consecutive refocusing pulses (CP) in the train of 180̊ pulses applied. For long CP values, the t2 of the systems saturates at a value, which is always an order of magnitude smaller than t2*. These features are explained by a crude model, which takes into account the residual diffusion effect in the static dephasing regime. The large microparticles, although not efficient in T1 relaxation, are quite effective in enhancing T2 relaxation, particularly at high magnetic fields. The t2* values and the saturation values for t2 were found to increase linearly with B0 and 02. The largest transversal relaxation rate enhancements were observed for Dy-AV-9 with a saturation value of t2 of 60 s-1 mM-1 and a t2* value of 566 s-1 mM-1 at 9.4 T and 298 K.


Chem. Commun., 2008, 2361 - 2363, DOI: 10.1039/b801154k

Title: Proton chemical shift anisotropy measurements of hydrogen-bonded functional groups by fast magic-angle spinning solid-state NMR spectroscopy

Authors: Luminita Duma, Daniel Abergel, Piotr Tekely and Geoffrey Bodenhausen

Abstract: The suitability of fast MAS solid-state NMR spectroscopy for probing 1H chemical shift anisotropy of hydrogen-bonded species has been demonstrated.

J. Am. Chem. Soc., 130 (23), 7427–7435, 2008.

Solid-State NMR Spectroscopy Applied to a Chimeric Potassium Channel in Lipid Bilayers†

Robert Schneider, Christian Ader, Adam Lange, Karin Giller,Sönke Hornig, Olaf Pongs, Stefan Becker, and Marc Baldus*

We show that solid-state NMR can be used to investigate the structure and dynamics of a chimeric potassium channel, KcsA-Kv1.3, in lipid bilayers. Sequential resonance assignments were obtained using a combination of 15N−13C and 13C−13C correlation experiments conducted on fully labeled and reverse-labeled as well as C-terminally truncated samples. Comparison of our results with those from X-ray crystallography and solution-state NMR in micelles on the closely related KcsA K+ channel provides insight into the mechanism of ion channel selectivity and underlines the important role of the lipid environment for membrane protein structure and function.

J. Am. Chem. Soc., 130 (23), 7305–7314, 2008.

Solid-State Nuclear Magnetic Resonance Spectroscopy Studies of Furanose Ring Dynamics in the DNA HhaI Binding Site

Gary A. Meints,† Paul A. Miller,‡ Kari Pederson,‡ Zahra Shajani,‡ and Gary Drobny*‡

The dynamics of the furanose rings in the GCGC moiety of the DNA oligomer [d(G1A2T3A4G5C6G7C8T9A10T11C12)]2 are studied by using deuterium solid-state NMR (SSNMR). SSNMR spectra obtained from DNAs selectively deuterated on the furanose rings of nucleotides within the 5′-GCGC-3′ moiety indicated that all of these positions are structurally flexible. The furanose ring within the deoxycytidine that is the methylation target displays the largest-amplitude structural changes according to the observed deuterium NMR line shapes, whereas the furanose rings of nucleotides more remote from the methylation site have less-mobile furanose rings (i.e., with puckering amplitudes < 0.3 Å). Previous work has shown that methylation reduces the amplitude of motion in the phosphodiester backbone of the same DNA, and our observations indicate that methylation perturbs backbone dynamics through the furanose ring. These NMR data indicate that the 5′-GCGC-3′ is dynamic, with the largest-amplitude motions occurring nearest the methylation site. The inherent flexibility of this moiety in DNA makes the molecule more amenable to the large-amplitude structural rearrangements that must occur when the DNA binds to the HhaI methyltransferase.

J. Am. Chem. Soc., 130 (23), 7206–7207, 2008.

An EPR and NMR Study of Supramolecular Effects on Paramagnetic Interaction between a Nitroxide Incarcerated within a Nanocapsule with a Nitroxide in Bulk Aqueous Media

Judy Y.-C. Chen,† Nithyanandhan Jayaraj,‡ Steffen Jockusch,† M. Francesca Ottaviani,§ V. Ramamurthy,*‡ and Nicholas J. Turro*†

A 15N-labeled nitroxide was incarcerated into an octa acid nanocapsule, which was confirmed by 1H NMR and EPR spectroscopy. Electron paramagnetic interaction between the 15N-labeled incarcerated nitroxide and a 14N-labeled free nitroxide in the external aqueous solution was observed by EPR spectroscopy. The observation of spin−spin interaction, through the walls of the carcerand is reflected in the simultaneous line-broadening of both the 15N-labeled and 14N-labeled nitroxides. The computer-assisted analysis of the EPR data further provides direct information on the motion and the polarity of both the incarcerated paramagnetic nitroxide and the nitroxides in the external bulk aqueous phase. We also show how communication between an incarcerated guest and molecules in the bulk solvent can be enhanced or inhibited by supramolecular factors such as Coulombic attraction or repulsion between a charged guest@host complex (incarcerated 15N nitroxide) and charged molecules in the aqueous phase.

J. Am. Chem. Soc., 130 (23), 7172–7173, 2008.

Structural and Dynamical Characterization of Fibrils from a Disease-Associated Alanine Expansion Domain Using Proteolysis and Solid-State NMR Spectroscopy

Mirko Sackewitz,† Holger A. Scheidt,† Grit Lodderstedt,† Angelika Schierhorn,‡ Elisabeth Schwarz,† and Daniel Huster*†§

The nuclear poly(A) binding protein PABPN1 possesses a natural 10 alanine stretch that can be extended to 17 Ala by codon expansion. The expansions are associated with the disease oculopharyngeal muscular dystrophy (OPMD), which is characterized histopathologically by intranuclear fibrillar deposits. Here, we have studied the Ala extended fibrillar N-terminal fragment of PABPN1, (N-(+7)Ala), comprising 152 amino acids. At natural abundance, cross-polarized 13C MAS NMR spectra are dominated by the three Ala signals with characteristic β-sheet chemical shifts. In contrast, directly polarized 13C MAS spectra show a multitude of narrow lines, suggesting a large portion of highly mobile sites. Proteolytic cleavage of the protein combined with MALDI-TOF mass spectrometry revealed a protease-resistant peptide encompassing residues 13/14 to 50−52 with the poly-Ala stretch in the center. Measurements of the 1H−13Cα dipolar couplings of 13C/15N-labeled N-(+7)Ala revealed high order parameters of 0.77 for the poly-Ala stretch of the fibril, while the majority of the residues of N-(+7)Ala exhibited very low order parameters between 0.06 and 0.15. Only some Gly residues that are flanking the Ala-rich region had significant order parameters of 0.47. Thus, site-specific dynamic mapping represents a useful tool to identify the topology of fibrillar proteins.

Cryst. Growth Des., 8 (6), 1878–1885, 2008.

Guest Loading and Multiple Phases in Single Crystals of the van der Waals Host p-tert-Butylcalix[4]arene

Darren H. Brouwer, Igor L. Moudrakovski, Kostantin A. Udachin, Gary D. Enright, and John A. Ripmeester*

We have followed the loading of xenon into the low density form of a van der Waals solid host, p-tert-butylcalix[4]arene (tBC), with solid-state NMR and X-ray diffraction (XRD), techniques sensitive to local and long-range order, respectively. Even though there was little change in the unit cell parameters, 13C and 129Xe solid-state NMR spectra indicate that significant structural changes occur in local order even at low levels of loading. In particular, 129Xe double quantum experiments, which probe distance-dependent 129Xe−129Xe dipolar interactions, reveal that the closest Xe−Xe distances increase with Xe loading into the tBC host, suggesting that the tBC undergoes structural rearrangements as it absorbs Xe. Indeed, in light of the solid-state NMR results, a re-examination of partially loaded single crystals by XRD showed that up to a loading level (Xe/tBC ratio) of 0.25, the structure was closely related to that of the empty form with the typical calixarene bilayer structure; however, at higher loading (0.5), the structure is substantially different with a constricted zigzag channel for Xe. In the latter structure, alternate bilayers are distinct because of different molecular orientations and much enhanced thermal parameters. What is remarkable is that the changes described take place with the different structural motifs apparently coexisting in the same single crystal. These different structures have almost identical unit cell parameters; however, the structures are quite different, and the phase transitions are more easily followed with NMR spectroscopy than with diffraction. The fact that ordered domains are always present suggests that cooperative dynamics play an important role, with the experimental results giving snapshots of the loading process at different stages.

Tuesday, June 03, 2008

Chem. Mater., 20 (11), 3638–3648, 2008.

Variable-Temperature 17O NMR Study of Oxygen Motion in the Anionic Conductor Bi26Mo10O69
Lesley Holmes, Luming Peng, Ivo Heinmaa, Luke A. O’Dell, Mark E. Smith, Rose-Noelle Vannier, and Clare P. Grey*

Variable-temperature 17O NMR spectroscopy, spanning a temperature range from −238 to 1000 °C, has been used to investigate mechanisms for ionic conduction in Bi26Mo10O69, a material that contains both MoO42− tetrahedra and [Bi12O14]8−∞ columns. Two 17O NMR resonances are observed that are assigned to oxygen atoms in the MoO42− tetrahedra and in the [Bi12O14]8−∞ columns. On the basis of the nutation curves for the two groups of resonances, extremely rapid, but local, reorientational motion of the MoO42− units occurs at −70 °C and above (with a frequency of >50 kHz), whereas the Bi−O oxygen ions are rigid in this temperature regime. This is confirmed by both an analysis of the line broadening of the 17O MoO42− satellite transitions (under MAS) and the spin−lattice relaxation (T1) times of these sites, the T1 times indicating that the MoO42− reorientation rates rapidly increase, reaching >100 MHz at 400 °C. Line narrowing of the MoO42− central-transition resonance indicates that exchange between the tetrahedral units, a motion required for long-range anionic conduction, is much slower, involving only jump rates of approximately 1 kHz at 200 °C. Both the changes in line width of the MoO42− resonance, and the jump in the T1 times of the oxygen atoms in the [Bi12O14]8−∞ columns at around the triclinic-monoclinic phase transition temperature (310 °C) are consistent with a mechanism for motion involving all the oxygen atoms. The predicted conductivity based on the [Bi−O] T1 times is now of the order of that extracted from ac impedance measurements reported by Vannier et al. (J. Solid State Chem. 1996, 122, 394). On the basis of this detailed NMR analysis, we propose that motion at ambient temperatures primarily involves the MoO42− tetrahedral rotation: exchange between these sites is very slow. At higher temperatures (above 310 °C), the conduction process now appears to involve the oxygen atoms coordinated to Bi3+, in the [Bi12O14]8−∞ columns, and most likely in the partially vacant O[19] site. The involvement of these sites allows for long-range conduction processes that do not involve concerted, multiple Mo−O bond breakages.

Chem. Mater., 20 (11), 3549–3550, 2008.

The Organic−Mineral Interface in Teeth Is Like That in Bone and Dominated by Polysaccharides: Universal Mediators of Normal Calcium Phosphate Biomineralization in Vertebrates?

David G. Reid, Melinda J. Duer,* Rachel C. Murray, and Erica R. Wise

Teeth of mammals and many other vertebrates are made up of the three distinct substances, enamel, dentin, and cement.1 Cement is in many respects modified bone; it occurs as a thin film attached to the outside of dentin and holds the collagen fibers of the periodontal ligament. Enamel provides the hard surface of teeth and is about 97% apatitic mineral. Dentin, the major constituent, is a less mineralized material, extremely tough and impact resistant. The mineral is apatite-like and is bound into an organic matrix in which collagen is prominent. As in bone, the relationship between these two phases must be fundamental for toughness and hardness. We have shown that the solid-state NMR (SSNMR) technique called rotational echo double resonance (REDOR)2 is a powerful tool for studying such interfaces in calcium phosphate biominerals.3 This is because practically all the phosphorus is confined to the mineral and all the carbon to the organic phases, so the 13C{31P} REDOR experiment, which restores the through-space dipole−dipole coupling between nuclei of the two elements, is highly selective for the biomolecules at the boundary layer. Using this approach, we have shown that the molecules in bone most strongly associated with the interface are sugars, probably acidic glycosaminoglycans (GAGs), and not collagenous or other proteins as is widely assumed.4 Their prominence at the boundary argues an important role in controlling the formation and properties of the composite material, perhaps by directing calcium phosphate solidification and preventing inappropriate overcrystallization. Here, we report that the organic−mineral interface in teeth is very similar to that in bone and also dominated by polysaccharides.

Monday, June 02, 2008

Joel's Literature Update

Sensitive NMR Sensors detect Antibodies to Influenza
Isaac Koh, Rui Hong, Ralph Weissleder, Lee Josephson
Angwandte (2008)47, 4119.
Catching the flu: Magnetic particle aggregation has been used to provide a highly sensitive, NMR-based sensor for detecting antibodies to an influenza virus. To achieve high sensitivity, a homogeneous magnetic field (H) was employed to enhance antibody-based cross-linking between particles suspended in solution (see scheme; T2: change in the transverse relaxation time).

Temperature-Controlled Molecular Depolarization Gates in Nuclear Magnetic Resonance
Leif Schröder, Lana Chavez, Tyler Meldrum, Monica Smith, Thomas J. Lowery, David E. Wemmer, Alexander Pines
Angwandte (2008)47, 4316.
Down the drain: Cryptophane cages in combination with selective radiofrequency spin labeling can be used as molecular transpletor units for transferring depletion of spin polarization from a hyperpolarized source spin ensemble to a drain ensemble. The flow of nuclei through the gate is adjustable by the ambient temperature, thereby enabling controlled consumption of hyperpolarization (see scheme).

Broadband Dipolar Recoupling for Magnetization Transfer in Solid-State NMR Correlation Spectroscopy

Luminita Duma, Daniel Abergel, Fabien Ferrage, Philippe Pelupessy, Piotr Tekely, Geoffrey Bodenhausen
ChemPhysChem(2008)9, 1104.
Efficient recoupling: A new experimental procedure using a broadband variant of rotary resonance recoupling (B2R3, see figure) improves 13C-13C transfer of magnetization in biological molecules. The robustness of the method, combined with its simplicity, results in improved structural analysis of crystalline and non-crystalline systems.

Regioselective H/D Exchange at the Side-Chain of Ethylbenzene on Dealuminated Zeolite H-Y Studied by In Situ MAS NMR-UV/Vis Spectroscopy
Jun Huang, Yijiao Jiang, V. R. Reddy Marthala, Yean Sang Ooi, Michael Hunger
ChemPhysChem(2008)9, 1107.
Regioselective H/D exchange at the side-chain methyl group of ethylbenzene on dealuminated zeolite H-Y and formation of carbenium ions are observed for the first time by the novel in situ pulsed-flow 1H MAS NMR-UV/Vis technique. Hydride abstraction on Lewis acid sites lead to the formation carbenium ions which are protonated following Markovnikov's rule (see scheme).

High temperature NMR study of the local structure of molten LaF3–AF (A = Li, Na, K and Rb) mixtures
Anne-Laure Rollet, Stéphanie Godier and Catherine Bessada
The local structures of molten lanthanum alkali fluoride binaries have been studied using HT NMR technique. The chemical shifts of 19F, 23Na and 139La in solid and in liquid have been compared for AF (A = alkali) and LaF3. In pure molten alkali fluorides, the polarisability of anion–cation pairs appears to be a key parameter to depict the observed evolution of 19F chemical shifts. The influence of the composition has also been studied by measuring the chemical shifts in molten LaF3–AF as a function of LaF3 concentration. A strong influence of the alkali influence is observed. The coordination number of lanthanum is decreased versus AF amount all the more since the alkali atomic number is high. Moreover, the more polarisable the alkali, the less bridging fluorines between the LaFx units.