Jerry C. C. Chan

Recoupling of chemical shift anisotropies in solid-state NMR under high-speed magic-angle spinning and in uniformly 13C-labeled systems

We demonstrate the possibility of recoupling chemical shift anisotropy (CSA) interactions in solid-state nuclear magnetic resonance (NMR) under high-speed magic-angle spinning (MAS) while retaining a static CSA powder pattern line shape and simultaneously attenuating homonuclear dipole–dipole interactions. CSA recoupling is accomplished by a rotation-synchronized radio-frequency pulse sequence with symmetry properties that permit static CSA line shapes to be obtained. We suggest a specific recoupling sequence, which we call ROCSA, for which the scaling factors for CSA and homonuclear dipole–dipole interactions are 0.272 and approximately 0.05, respectively. This sequence is suitable for high-speed 13C MAS NMR experiments on uniformly 13C-labeled organic compounds, including biopolymers. We demonstrate the ROCSA sequence experimentally by measuring the 13C CSA patterns of the uniformly labeled, polycrystalline compounds L-alanine and N-acetyl-D,L-valine at MAS frequencies of 11 and 20 kHz. We also present ...

Synthesis and catalytic activity of amino-functionalized SBA-15 materials with controllable channel lengths and amino loadings

Aminopropyl-functionalized SBA-15 mesoporous materials (NH2-SBA-15) with different lengths of channeling pores and aminopropyl loadings up to 2.6 mmol g−1 were synthesized by one-pot co-condensation of tetraethyl orthosilicate (TEOS) and aminopropyltrimethoxysilane (APTMS) with the aid of appropriate amounts of Zr(IV) ions and NaCl. The morphologies and pore lengths of the resultant materials were tuned by varying the Zr/TEOS and NaCl/TEOS molar ratios in the synthesis gels. The assembly processes were examined by in situsmall-angle X-ray scattering (SAXS) and conventional characterization techniques. Accordingly, TEOS prehydrolysis for around 2 h in the synthesis solution before the addition of APTMS was essential for preparing platelet NH2-SBA-15 materials with well-ordered p6mmpore structure and narrowly distributed pore size distribution, due to the strong interference from APTMS. When applying these materials as solid base catalysts in solvent-free flavanone synthesis, the platelet material with short-channeling poresca. 200–300 nm was superior to the rod-like and fiber-like counterparts with lengthy channels in the micrometre scale due to better molecular diffusion and less possibility of pore blockage. Furthermore, the platelet materials with high surface areas (∼450 m2 g−1) and amino loadings around 1.4–1.8 mmol N g−1 were the most efficient catalysts for flavanone synthesis, and the high activity was retained by a simple regeneration method.

Steric Zipper of the Amyloid Fibrils Formed by Residues 109–122 of the Syrian Hamster Prion Protein

We report the results of atomic force microscopy, Fourier-transform infrared spectroscopy, solid-state nuclear magnetic resonance, and molecular dynamics (MD) calculations for amyloid fibrils formed by residues 109-122 of the Syrian hamster prion protein (H1). Our data reveal that H1 fibrils contain no more than two beta-sheet layers. The peptide strands of H1 fibrils are antiparallel with the A117 residues aligned to form a linear chain in the direction of the fibril axis. The molecular structure of the H1 fibrils, which adopts the motif of steric zipper, is highly uniform in the region of the palindrome sequence AGAAAAGA. The closest distance between the two adjacent beta-sheet layers is found to be about 5 A. The structural features of the molecular model of H1 fibrils obtained by MD simulations are consistent with the experimental results. Overall, our solid-state NMR and MD simulation data indicate that a steric zipper, which was first observed in the crystals of fibril-forming peptides, can be formed in H1 fibrils near the region of the palindrome sequence.

Adsorption of a Statherin Peptide Fragment on the Surface of Nanocrystallites of Hydroxyapatite

Statherin is an active inhibitor of calcium phosphate precipitation in the oral cavity. For many studies of the interaction between statherin and hydroxyapatite (HAp), the samples are prepared by a direct mixing of statherin or its fragment with well-crystalline HAp crystals. In this work, the HAp sample is precipitated in the presence of peptide fragment derived from the N-terminal 15 amino acids of statherin (SN-15). The in situ prepared HAp crystallites are nanosized, leading to a significant increase of the peptide amount adsorbed on the HAp surface. The enhancement in NMR sensitivity allows, for the first time, the measurement of a two-dimensional 13C-13C correlation spectrum for a 13C uniformly labeled peptide sample adsorbed on mineral surface. The measurement time is about 18.5 h at a field strength of 7.05 T. Preliminary results suggest that there may exist two different mechanisms for the interaction between SN-15 and HAp. In addition to the one which will cause a conformational change near the N-terminal, SN-15 may also be absorbed on the HAp surface by simple electrostatic interaction, without any significant conformational changes of the peptides.

C-rotational echo double resonance: Heteronuclear dipolar recoupling with homonuclear dipolar decoupling

We propose a series of heteronuclear dipolar recoupling schemes which are insensitive to the presence of homonuclear dipolar interaction. The schemes are based on the pulse symmetry CNnν. The optimum choices for N, n, and ν were determined with the help of Average Hamiltonian Theory (AHT). The lowest-order AHT shows that the pulse symmetries with n=N and ν=1 are suitable for the recoupling of heteronuclear dipolar interactions. Together with the parabolic approximation of the dephasing curve, we develop a powerful experimental strategy to characterize the van Vleck’s second moments for multiple-spin systems under conditions of very fast magic-angle spinning. These new pulse symmetries are superior to the rotational echo double resonance (REDOR) method and other recoupling schemes as far as the interference of the homonuclear dipolar interaction is concerned. Preliminary experimental results on crystalline model compounds are given to illustrate the utility of our approach.

R sequences for the scalar-coupling mediated homonuclear correlation spectroscopy under fast magic-angle spinning

Abstract High efficiency of homonuclear polarization transfer via scalar spin–spin coupling is achieved using a pulse sequence designed based on R pulse symmetry. Numerical simulations show that this new pulse sequence is robust with respect to rf inhomogeneity and resonance offsets. The required rf nutation frequency is five times the spinning frequency. Experimental data obtained for Ag 4 P 2 S 6 -II indicate that the new pulse sequence has a maximum efficiency about 75% and therefore is very promising for the measurement of homonuclear correlation spectra.

(CuI)3P4S4: Preparation, Structural, and NMR Spectroscopic Characterization of a Copper(I) Halide Adduct with β-P4S4

(CuI)(3)P(4)S(4) is obtained by reaction of stoichiometric amounts of CuI, P, and S in evacuated silica ampoules. The yellow compound consists of monomeric beta-P(4)S(4) cage molecules that are separated by hexagonal columns of CuI. (CuI)(3)P(4)S(4) crystallizes isotypic to (CuI)(3)P(4)Se(4) in the hexagonal system, space group P6(3)cm (no. 185) with a=19.082(3), c=6.691(1) A, V=2109.9(6) A(3), and Z=6. Three of the four phosphorus atoms are bonded to copper, whereas no bonds between copper and sulfur are observed. The two crystallographically distinct copper sites are clearly differentiated by (65)Cu magic-angle spinning (MAS) NMR spectroscopy. Furthermore, an unequivocal assignment of the (31)P MAS-NMR spectra is possible on the basis of homo- and heteronuclear dipole-dipole and scalar interactions. Dipolar coupling to the adjacent quadrupolar spins (63, 65)Cu generates a clear multiplet structure of the peaks attributable to P1 and P2, respectively. Furthermore, the utility of a newly developed two-dimensional NMR technique is illustrated to reveal direct connectivity between P atoms based on ((31)P-(31)P) scalar interactions.

C-REDOR: rotational echo double resonance under very fast magic-angle spinning

Abstract We propose a heteronuclear dipolar interaction recoupling scheme based on the pulse symmetry POST-CNnν. Numerical simulations showed that POST-C331 and POST-C771 are suitable for the characterization of heteronuclear dipolar couplings for multiple-spin systems under very fast magic-angle spinning condition. These new pulse symmetries are superior to the rotational echo double resonance method and other recoupling schemes as far as the interference of homonuclear dipolar interaction is concerned. The experiment was carried out for fluorapatite at a spinning frequency of 25 kHz and the results were in good agreement with X-ray data.

Double-quantum cross-polarization between half-integer quadrupolar spin systems: 11B↔23Na and 11B↔27Al

Abstract Cross-polarization/magic angle spinning (CPMAS) experiments are reported in which both the source and recipient spin systems are based on half-integer nuclei with strong second-order quadrupolar interactions. Double-quantum (DQ) CP is particularly well suited for this purpose, as demonstrated with 11 B { 23 Na } CPMAS spectra of sodium diborate. Furthermore, a two-dimensional 11 B { 27 Al } heteronuclear correlation spectrum is obtained on a magnesium aluminoborate glass, demonstrating preferential connectivities among various distinct boron and aluminum sites. Novel spectral editing opportunities arise from semi-selective coherence transfer rates measured by exploiting off-resonance effects on spin-locking efficiencies.

NMR studies of phosphorus chalcogenide–copper iodide coordination compounds

The local structures of the new phosphorus chalcogenide – copper iodide coordination compounds (CuI)P4Se4, (CuI)2P8Se3, (CuI)3P4Se4, and (CuI)3P4S4 are investigated using comprehensive 63Cu, 65Cu, and 31P magic angle spinning NMR techniques. Peak assignments are proposed on the basis of homo- and heteronuclear indirect spin–spin interactions, available from lineshape analysis and/or two-dimensional correlation spectroscopy. In particular, the 31P-63,65Cu scalar coupling constants have been extracted from detailed lineshape simulations of the 31P resonances associated with the Cu-bonded P atoms. In addition, the RNνn pulse symmetry concept of Levitt and coworkers has been utilized for total through-bond correlation spectroscopy (TOBSY) of directly-bonded phosphorus species. The resonance assignments obtained facilitate a discussion of the 31P and 63,65Cu NMR Hamiltonian parameters in terms of the detailed local atomic environments. Analysis of the limited data set available for this group of closely related compounds offers the following conclusions: (1) bonding of a special phosphorus site in a given P4Xn (X = S, Se) molecule to Cu+ ions shifts the corresponding 31P NMR signal upfield by about 50 ppm relative to the uncomplexed molecule, (2) the magnitude of the corresponding scalar 31P-63,65Cu spin–spin coupling constant tends to decrease with increasing Cu–P distance, and (3) the 63,65Cu nuclear electric quadrupolar coupling constants appear to be weakly correlated with the shear strain parameter specifying the degree of local distortion present in the four-coordinated [CuI2P2] and [CuI3P] environments. Overall, the results illustrate the power and potential of advanced solid state NMR methodology to provide useful structural information in this class of materials.