33S nuclear quadrupole resonance study of dibenzyl disulfide toward understanding of cross-linked structures in rubber.

Experimental and theoretical investigations of a sulfur-33 electric-field-gradient (EFG) tensor of disulfide bonds in 33S-labled dibenzyl disulfide have been presented. Temperature dependence of quadrupolar frequencies, νQ, is observed in the temperature range between 80 and 280 K, in which single peaks appear in all the 33S nuclear quadrupole resonance (NQR) spectra. Analysis of nutation echo 33S NQR spectra at 200 K yields the quadrupolar coupling constant, CQ value, of 46.8(6) MHz and the asymmetry parameter, ηQ, of 0.48(7). The orientation of the 33S EFG tensor of the disulfide is obtained by quantum chemical calculations: the largest EFG tensor component, VZZ, is approximately perpendicular to the molecular plane (C-S-S), and the smallest component, VXX, is approximately 41° off the C-S bond. Extensive quantum chemical calculations are systematically performed to investigate dependences of 33S EFG tensors on changes of torsion angles in disulfide and trisulfide bonds, indicating that analysis of νQ and CQ values potentially makes it possible to assign the secondary structures of cross-linking in a rubber.

Homo- and heteronuclear two-dimensional covariance solid-state NMR spectroscopy with a dual-receiver system.

Two-dimensional (2D) covariance NMR spectroscopy, which has originally been established to extract homonuclear correlations (HOMCOR), is extended to include heteronuclear correlations (HETCOR). In a (13)C/(15)N 2D chemical shift correlation experiment, (13)C and (15)N signals of a polycrystalline sample of (13)C, (15)N-labeled amino acid are acquired simultaneously using a dual-receiver NMR system. The data sets are rearranged for the covariance data processing, and the (13)C-(15)N heteronuclear correlations are obtained together with the (13)C-(13)C and (15)N-(15)N homonuclear correlations. The present approach retains the favorable feature of the original covariance HOMCOR that the spectral resolution along the indirect dimension is given by that of the detection dimension. As a result, much fewer amounts of data are required to obtain a well-resolved 2D spectrum compared to the case of the conventional 2D Fourier-Transformation (FT) scheme. Hence, one can significantly save the experimental time, or enhance the sensitivity by increasing the number of signal averaging within a given measurement time.

Solid-state NMR analysis of interaction sites of curcumin and 42-residue amyloid ß-protein fibrils.

Aggregation of 42-residue amyloid ß-protein (Aß42) plays a pivotal role in the etiology of Alzheimer's disease (AD). Curcumin, the yellow pigment in the rhizome of turmeric, attracts considerable attention as a food component potentially preventing the pathogenesis of AD. This is because curcumin not only inhibits the aggregation of Aß42 but also binds to its aggregates (fibrils), resulting in disaggregation. However, the mechanism of interaction between curcumin and the Aß42 fibrils remains unclear. In this study, we analyzed the binding mode of curcumin to the Aß42 fibrils by solid-state NMR using dipolar-assisted rotational resonance (DARR). To improve the quality of 2D spectra, 2D DARR data were processed with the covariance NMR method, which enabled us to detect weak cross peaks between carbons of curcumin and those of the Aß42 fibrils. The observed (13)C-(13)C cross peaks indicated that curcumin interacts with the 12th and 17-21st residues included in the ß-sheet structure in the Aß42 fibrils. Interestingly, aromatic carbons adjacent to the methoxy and/or hydroxy groups of curcumin showed clear cross peaks with the Aß42 fibrils. This suggested that these functional groups of curcumin play an important role in its interaction with the Aß42 fibrils.

Efficient cross-polarization using a composite 0 degrees pulse for NMR studies on static solids.

In most solid-state NMR experiments, cross-polarization is an essential step to detect low-gamma nuclei such as (13)C and (15)N. In this study, we present a new cross-polarization scheme using spin-locks composed of composite 0 degrees pulses in the RF channels of high-gamma and low-gamma nuclei to establish the Hartmann-Hahn match. The composite 0 degrees pulses with no net nutation-angle{(2pi)(X)-(2pi)(-X)-(2pi)(Y)-(2pi)(-Y) -}(n) applied simultaneously to both high-gamma (I) and low-gamma (S) nuclei create an effective heteronuclear dipolar Hamiltonian H(d)((0))=d/2(2I(Z)S(Z)+I(X)S(X)+I(Y)S(Y)), which is capable of transferring the Z-component of the I spin magnetization to the Z-component of the S spin magnetization. It also retains a homonuclear dipolar coupling Hamiltonian that enables the flip-flop transfer among abundant spins. While our experimental results indicate that the new pulse sequence, called composite zero cross-polarization (COMPOZER-CP) performs well on adamantane, it is expected to be more valuable to study semi-solids like liquid crystalline materials and model lipid membranes. Theoretical analysis of COMPOZER-CP is presented along with experimental results. Our experimental results demonstrate that COMPOZER-CP overcomes the RF field inhomogeneity and Hartmann-Hahn mismatch for static solids. Experimental results comparing the performance of COMPOZER-CP with that of the traditional constant-amplitude CP and rampCP sequences are also presented in this paper.

Double-acquisition: utilization of discarded coherences in a 2D separation experiment using the States method.

We propose a new data-acquisition scheme for 2D separation experiments to save the spectrometer time by 1/2. This scheme, referred to as a double-acquisition scheme, is applicable to most of separation experiments with the hypercomplex time-domain data-acquisition scheme (the States method) for data collection.

Combination of (13)C-(13)C COSY and DARR (COCODARR) in solid-state NMR.

In this work, we describe a new 2D (13)C-(13)C correlation experiment in solids, in which (13)C-(13)C J-correlation (COSY) and dipolar correlation (DARR) are recorded at the same time. The sequence is similar to COCONOSY in the liquid-state NMR, in which (1)H-(1)H COSY and NOESY spectra are obtained in a single experiment. The combined COSY and DARR experiment facilitates assignment of (13)C signals in solids.

15N solid-NMR and X-ray diffraction studies of N-confused porphyrins.

Using (15)N high-resolution solid-state NMR and X-ray diffraction, the structure of N-confused porphyrin (NCP) in the solid state was studied. A 1D (15)N magic angle spinning (MAS) experiment and a 2D dipolar assisted rotational resonance (DARR) (15)N-(15)N spin exchange experiment of N-confused tetratolylporphyrin (Tol) crystallized from CH(2)Cl(2)/hexane indicate that Tol is the inner 3H-type tautomer and has two magnetically different molecules in the unit cell. Further, a FSLG-2 & 4macr; 2 (1)H-(15)N dipolar recoupling NMR measurement indicates no fast ring flipping motion which is consistent with the planar structure in the X-ray analysis. The planarity of Tol is ascribed to crystal packing enforced by pi-pi stacking and CH-pi interactions.