Improved decoupling during symmetry-based C9-TOBSY sequences.

We show that heteronuclear decoupling during symmetry-based C9 TOBSY sequences can be improved by using phase-alternating pulse sequences (XiX) instead of cw irradiation. The use of XiX sequences makes the optimization of the decoupling rf-field amplitude simpler and lowers the decoupling rf-field requirements to attain a comparable performance. A Floquet analysis of the first-order resonance conditions was used to determine the correct timing of the XiX sequence to avoid interference between the C sequence and the decoupling. The decoupling performance is analyzed analytically using Floquet theory and verified using numerical simulations as well as experimental results.

Spatio-chemical characterization of a polymer blend by magnetic resonance force microscopy.

Magnetic resonance force microscopy (MRFM) is a promising method to obtain magnetic resonance images with nanometer resolution. One of the factors currently limiting its application to compounds with strong and possibly time-dependent dipolar couplings is the imperfect efficiency of the adiabatic inversion sweeps and the concomitant loss in the signal to noise ratio. We describe significant improvements to previous MRFM excitation schemes. Applying these schemes to a polymer blend of 80% PEEK and 20% PTFE allowed acquiring full-volume two-dimensional (2D) spatial images (with Hadamard multiplexing) of both the (1)H and (19)F-rich phases with a 6-fold enhancement in the SNR.

Second-order dipolar order in magic-angle spinning nuclear magnetic resonance.

Generating dipolar order under magic-angle spinning (MAS) conditions is explored for different pulse sequences and dipolar-coupling networks. It is shown that under MAS second-order dipolar order can be generated reliably with 10% to 30% efficiency using the Jeener-Broekaert sequence in systems where the second-order average Hamiltonian is a (near) constant of the motion. When using adiabatic demagnetization and remagnetization, second-order dipolar order can be generated and reverted back to Zeeman order with up to 60% efficiency. This requires a maximum field strength with a nutation frequency that is less than one-quarter of the rotor frequency, and that the spin system can be properly spinlocked under such conditions. A simple coherent description accounts for the principal features of the spin dynamics, even using the smallest possible system of three coupled spins. For the systems investigated, the lifetime of second-order dipolar order under MAS was found to be on the order of T(1).

Evidence for fully conjugated double-stranded cycles.

A chain of circumstantial evidence for the existence of the first fully conjugated, double-stranded cycles is presented. The products have the structure of the belt-region of fullerene C(84)(D(2)) and carry either four hexyl chains or four phenyl groups. The unsubstituted parent cycle is also presented. The chain of evidence is mainly based on mass spectrometric analysis and trapping reactions, the latter being supported by quantum mechanical calculations. It is also of importance that the phenyl-substituted and unsubstituted products cannot undergo a [1,5] hydrogen shift, the only reasonable side-reaction that recently could not be excluded for the alkyl-substituted analogue. It is concluded that the fully aromatic targets truly exist in the gas phase. Whether they can be generated in solution under the applied conditions cannot yet be firmly decided; theoretical evidence speaks against.

Characteristics of zero-quantum correlation spectroscopy in MAS NMR experiments.

Zero-quantum coherence generation and reconversion in magic-angle spinning solid-state NMR is analyzed. Two methods are discussed based on implementations using symmetry-based pulse sequences that utilize either isotropic J couplings or dipolar couplings. In either case, the decoupling of abundant proton spins plays a crucial role for the efficiency of the zero-quantum generation. We present optimized sequences for measuring zero-quantum single-quantum correlation spectra in solids, achieving an efficiency of 50% in ubiquitin. The advantages and disadvantages of zero-quantum single-quantum over single-quantum single-quantum correlation spectroscopy are explored, and similarities and differences with double-quantum single-quantum correlation spectroscopy are discussed. Finally, possible application of zero-quantum single-quantum experiments to polypeptides, where it can lead to better spectral resolution is investigated using ubiquitin, where we find high efficiency and high selectivity, but also increased line widths in the MQ dimension.

Operator-based Floquet theory in solid-state NMR.

This article reviews the application of operator-based Floquet theory in solid-state NMR. Basic expressions for calculating effective Hamiltonians based on van Vleck perturbation theory are reviewed for problems with a single frequency or multiple incommensurate frequencies. Such a treatment allows calculation of effective Hamiltonians for resonant and non-resonant problems. Examples from literature are given for single-mode to triple-mode Floquet problems, covering a wide range of applications in solid-state NMR under magic-angle spinning and radio-frequency irradiation of a single nucleus or multiple nuclei.

Tuning polymer thickness: synthesis and scaling theory of homologous series of dendronized polymers.

The thickness of dendronized polymers can be tuned by varying their generation g and the dendron functionality X. Systematic studies of this effect require (i) synthetic ability to produce large samples of high quality polymers with systematic variation of g, X and of the backbone polymerization degree N, (ii) a theoretical model relating the solvent swollen polymer diameter, r, and persistence length, lambda, to g and X. This article presents an optimized synthetic method and a simple theoretical model. Our theory approach, based on the Boris-Rubinstein model of dendrimers predicts r approximately n(1/4)g(1/2) and lambda approximately n(2) where n = [(X - 1)(g) - 1]/(X - 2) is the number of monomers in a dendron. The average monomer concentration in the branched side chains of a dendronized polymer increases with g in qualitative contrast to bottle brushes whose side chains are linear. The stepwise, attach-to, synthesis of X = 3 dendronized polymers yielded gram amounts of g = 1-4 polymers with N approximately = 1000 and N approximately = 7000 as compared to earlier maxima of 0.1 g amounts and of N approximately = 1000. The method can be modified to dendrons of different X. The conversion fraction at each attach-to step, as quantified by converting unreacted groups with UV labels, was 99.3% to 99.8%. Atomic force microscopy on mixed polymer samples allows to distinguish between chains of different g and suggests an apparent height difference of 0.85 nm per generation as well as an increase of persistence length with g. We suggest synthetic directions to allow confrontation with theory.

matNMR: a flexible toolbox for processing, analyzing and visualizing magnetic resonance data in Matlab.

matNMR (matnmr.sourceforge.net) is a toolbox for processing, analyzing and visualizing magnetic-resonance data within the Matlab environment (www.mathworks.com) that aims for control, flexibility and extendability. Processing can be done using either a graphical user interface or with command-line scripts, both of which allow user-defined processing or analysis functions to be incorporated at any time. The direct access to data points during processing, and the extensive library of mathematical and visualization routines provided by Matlab, afford the high degree of control and flexibility needed in modern magnetic-resonance research.

Theory and applications of supercycled symmetry-based recoupling sequences in solid-state nuclear magnetic resonance.

We present the theoretical principles of supercycled symmetry-based recoupling sequences in solid-state magic-angle-spinning NMR. We discuss the construction procedure of the SR26 pulse sequence, which is a particularly robust sequence for double-quantum homonuclear dipole-dipole recoupling. The supercycle removes destructive higher-order average Hamiltonian terms and renders the sequence robust over long time intervals. We demonstrate applications of the SR26 sequence to double-quantum spectroscopy, homonuclear spin counting, and determination of the relative orientations of chemical shift anisotropy tensors.

Symmetry-based recoupling of 17O-1H spin pairs in magic-angle spinning NMR.

We have performed magic-angle-spinning solid-state NMR experiments in which protons are recoupled to oxygen-17 nuclei by applying a symmetry-based recoupling sequence at the proton Larmor frequency. Two-dimensional quadrupole-dipole correlation spectra are produced, in which the second-order quadrupolar shift of the oxygen-17 central transition is correlated with the recoupled heteronuclear dipole-dipole interaction. These spectra are sensitive to the relative orientation of the electric field gradient at the site of the oxygen-17 nucleus and the O-H internuclear vector. We also demonstrate experiments in which polarization is transferred from protons to oxygen-17, and show that oxygen-17 signals may be selected according to the protonation state of the oxygen site. We discuss the small observed value of the heteronuclear dipolar splitting in the central-transition oxygen-17 spectra.

A DOQSY approach for the elucidation of torsion angle distributions in biopolymers: application to silk.

Silk from the wild silkworm Samia cynthia ricini with a molecular mass of about 300kDa consists of alternating repeats of nano-crystalline poly-(Ala) and non-crystalline glycine-rich domains. The backbone torsion angles between pairs of these two amino acids is determined by DOQSY solid-state NMR spectroscopy: the alanine-rich domains are predominantly in a beta-sheet conformation, whereas the glycine-rich domains are found to be partially in an extended beta-sheet conformation and partially in an approximately 3(1)-helical conformation. In the cast film from liquid silk significantly different secondary structures were found: the alanine-rich domains are alpha-helical conformation, whereas the results for glycine-labelled sample are explained by a random-coil state. A detailed error analysis of the technique is presented.

Spherical tensor analysis of nuclear magnetic resonance signals.

In a nuclear magnetic-resonance (NMR) experiment, the spin density operator may be regarded as a superposition of irreducible spherical tensor operators. Each of these spin operators evolves during the NMR experiment and may give rise to an NMR signal at a later time. The NMR signal at the end of a pulse sequence may, therefore, be regarded as a superposition of spherical components, each derived from a different spherical tensor operator. We describe an experimental method, called spherical tensor analysis (STA), which allows the complete resolution of the NMR signal into its individual spherical components. The method is demonstrated on a powder of a (13)C-labeled amino acid, exposed to a pulse sequence generating a double-quantum effective Hamiltonian. The propagation of spin order through the space of spherical tensor operators is revealed by the STA procedure, both in static and rotating solids. Possible applications of STA to the NMR of liquids, liquid crystals, and solids are discussed.

Inverse methods in two-dimensional NMR spectral analysis.

Solid-state NMR is a valuable technique for the study of disordered materials. Analysis of such spectra usually involves solution of so-called ill-posed inverse problems. Here we present a strategy for the analysis of two-parameter two-dimensional NMR problems and test it on 2D DECODER and DOQSY experiments. Using Monte Carlo tests, constraints are determined for the resolution and accuracy of the analysis for both experiments. The methods are finally applied to spectra of spider dragline silk, a heterogeneous solid fibrous protein.