Construction of universal rotations from point-to-point transformations

For a desired range of offsets, universal rotations of arbitrary flip angle can be constructed based on point-to-point rotations of I(y) with half the flip angle. This approach allows, for example, creation of broadband or bandselective refocusing pulses from broadband or bandselective excitation pulses. Furthermore, universal rotations about any axis can be obtained from point-to-point transformations that can easily be optimized using optimal control algorithms. The construction procedure is demonstrated on the examples of a broadband refocusing pulse, a broadband 120(x) degrees rotation and a z-rotation with offset pattern.     

Entropy-stabilized smectic C phase in a system of zigzag-shaped molecules

We report Monte Carlo simulations of a system of rigid zigzag-shaped molecules that demonstrate that simple excluded-volume interactions are sufficient to produce a fluid tilted lamellar [smectic C (SmC)] liquid crystal phase. The molecules are composed of three rigidly linked hard spherocylinders arranged in a zigzag fashion. By varying the zigzag angle we have mapped out the whole phase diagram as a function of pressure and zigzag angle Psi. For Psi between 35 degrees and 80 degrees our model simulation exhibits the SmC phase. This is the first conclusive evidence where steric interactions arising out of molecular shape alone induce the occurrence of the SmC phase for a wide range of zigzag angles. For smaller Psi, a transition from tilted crystal to crystal is observed.     

Susceptibility corrections in solid-state NMR experiments with oriented membrane samples. Part I: applications

Chemical shift referencing of solid-state NMR experiments on oriented membranes has to compensate for bulk magnetic susceptibility effects that are associated with the non-spherical sample shape, as described in the accompanying paper [J. Magn. Reson. 164 (2003) 115-127]. The resulting frequency deviations can be on the order of 10 ppm, which is serious for nuclei with a narrow chemical shift anisotropy such as 1H or 13C, and in some cases even 19F. Two referencing schemes are proposed here to compensate for these effects: A flat (0.4 mm) glass container with an isotropic reference molecule dissolved in a thin film of liquid is stacked on top of the oriented membrane sample. Alternatively, the intrinsic proton signal of the hydrated lipid can be used for chemical shift referencing. Further aspects related to magnetic susceptibility are discussed, such as air gaps in susceptibility-matched probeheads, the benefits of shimming, and limitations in the accuracy of orientational constraints. A biological application is illustrated by a series of experiments on the antimicrobial peptide PGLa, aimed at understanding its concentration-dependent membranolytic effect. To address a wide range of molar peptide/lipid ratios between 1:3000 and 1:8, multilayers of hydrated DMPC containing a 19F-labeled peptide were oriented between stacked glass plates. Maintaining an approximately constant amount of peptide gives rise to thick samples (18 plates) at low, and thin samples (3 plates) at high peptide/lipid ratio. Accurate referencing was critical to reveal a small but significant change over 5 ppm in the anisotropic chemical shift of the 19F label on the peptide, indicative of a change in the orientation and/or dynamics of PGLa in the membrane.     

Transverse magnetization transfer under planar mixing conditions in spin systems consisting of three coupled spins 1/2

Polarization transfer under planar mixing conditions is a widely used tool in modern NMR-experiments. In the case of two coupled spins 1/2 or a chain of three or more spins 1/2 with only nearest neighbor couplings, it is only possible to transfer a single magnetization component (longitudinal magnetization in the principle axis system of the planar coupling tensors). However, if all couplings in a three-spin system are non-zero, it turns out that all magnetization components can be efficiently transferred even under strictly planar mixing conditions. In this article a detailed theoretical analysis is presented based on analytical transverse coherence transfer functions and on the underlying commutator algebra. In addition, transverse magnetization transfer is demonstrated experimentally. The results show that in highly coupled spin systems, as for example in the case of partially aligned samples with many residual dipolar couplings, special care has to be taken to avoid phase distortions if planar mixing steps are used.     

Template assisted formation of micro- and nanotubular carbon nitride materials

Micro- and nanotubes of an amorphous carbon nitride material were synthesized by metathesis reactions between cyanuric chloride (C₃N₃Cl₃) and different nitrogen sources, such as Li₂(CN₂) or Li₃(BN₂). The intermediate formation of needle-shaped crystals of N(C₃N₃Cl₂)₃ was always observed in our reactions, and investigated with respect to their role as a template in the formation of tubes. Chemical analyses of the micro- and nanotubes reveal carbon to nitrogen ratios near 3:4, consistent with the suspected material C₃N₄. Synthesized carbon nitride materials were thermally stable up to 600 ^°C in inert atmosphere. They were inspected by a number of physical measurements, mainly using TEM, EDX and IR investigations.     

Ueber die eudiometrische Bestimmung der Salpeters?ure

Ohne Zusammenfassung(Mittheilung aus dem Laboratorium der landwirthschaftlichen Versuchsstation für Elsass-Lothringen zu Rufach.)