Journal of Magnetic Resonance, Series A最新文献

The design of a solenoidal NMR coil with high-B₁homogeneity for high-frequency solid-state NMR is described. The coil is made of thin ribbon of continuously variable width. The width is varied to minimize inhomogeneity in the axial direction. The field distribution throughout the entire volume of the sample was taken into account. The gap between turns is constant and kept small to minimize field inhomogeneity in the radial direction. For large samples in particular, coils of this design tested at 300 MHz show significantly better homogeneity than helical coils with constant pitch or coils with variable pitch but fixed ribbon width.

A “Double-WURST” decoupling sequence is constructed for simultaneously decoupling the¹³CO and¹³C_αregions in¹⁵N/¹³C-double-labeled proteins. It uses frequency-shifted WURST decoupling combined with standard WURST decoupling. The power can be reduced by two-thirds compared to conventional WURST decoupling, which decouples the entire¹³C region including the unneeded region between¹³CO and¹³C_α. Well-decoupled spectra are acquired using the Double-WURST decoupling sequence with a test¹⁵N/¹³C-double-labeled peptide sample and 8 and 19.6 kDa¹⁵N/¹³C-double-labeled proteins. It has also been shown that adiabatic decoupling, WURST, for example, has already reached its highest decoupling indexn(=2) defined in a relation of decouple range Δfversus RF pulse strengthf_1rms, i.e., Δf= λ(f_1rms)ⁿ, where λ is a constant. Forngreater than 2, the energy conservation law would be violated, and therefore is not possible.

The spinning sidebands observed in the¹³C MAS NMR spectra ofcis,cis-mucononitrile oriented in liquid-crystalline media and of the neat sample in the solid state are studied. There are differences in the sideband intensity patterns in the two cases. These differences arise because the order parameters which characterize the orientation of the solute in the liquid-crystalline media differ for different axes. It is shown that, in general, the relative intensities of the sidebands contain information on the sign and magnitude of an effective chemical-shift parameter which is a function of the sum of the products of the principal components of the chemical-shift tensor and the corresponding order parameters with respect to the director. A method for obtaining the orientation of the carbon chemical-shift tensor is proposed. The carbon chemical-shift tensors obtained from gauge-including atomic orbital calculations are also presented for comparison.

The fabrication and analysis of¹⁹F-based and¹H-based thin films and the NMR techniques employed for studying them are discussed. The heteronuclear magnetization transfer results in disordered media that motivated the development of the ordered thin-film techniques are summarized. The experimental setup and sample geometry for the thin-film experiments are described, and the techniques for thin-film preparation and thickness measurement are discussed. For the NMR spectrometer, coil tuning via changing the applied magnetic field and the techniques employed for lowering the dead time are described. No signal averaging was required to obtain spin echoes from a sample composed of 0.5 μm thin films.T₁values on the order of one minute were measured in the thin films at a temperature of 0.5 K. These techniques should be of particular use in studying cross relaxation at surfaces and NMR of membrane proteins.