Quadrupolar alignment

Instead of measuring only the time-dependent dipolar interaction via NOE, it is also possible to determine dipolar couplings directly if the solute molecule is partially aligned in so-called alignment media. The most important resulting anisotropic parameters are RDCs, but residual quadrupolar couplings (RQCs), residual chemical shift anisotropy (RCSA) and pseudo-contact shifts (PCSs) can also be used for structure determination if applicable. 

A. M. Apparatus for rapid adjustment of the degree of alignment of NM R samples in aqueous media verification with residual quadrupolar splimngs in Na and "Cs spectta. /. Magn. Reson. 2006, 180, 256-265. 

In the presence of B0, nonzero spin nuclei adopt a specific number of orientations. The number of allowed orientations is dependent on the spin value and is equal to (21 + 1). For the simplest case of 1= 1/2, two orientations are allowed one aligned parallel to B0 and one antiparallel. Nuclei with I > 1/2 have a quadrupole moment (called quadrupolar nuclei) that allows them to interact with electric fields produced by neighboring nuclei and electrons. 

The anisotropic nature of the dipolar, quadrupolar and chemical shift anisotropy interactions requires that the isotropy of molecular orientation relative to the applied magnetic field be broken in order to allow their direct observation in terms of shifts in the frequencies of resonances.20,32,38 40 For high resolution NMR studies this has meant, thus far, that some degree of alignment of the molecule needs to be established. Almost all molecules will align to a small extent due to the anisotropy of their magnetic... 

Figure 5 Importance of the right alignment strength. Amide region of the H-1D spectrum of the peptide efrapeptin C in two PDMS/CDCl3 gels with deuterium quadrupolar splittings of the solvent of 9 Hz (A) and 35 Hz (B). The increased alignment strength leads to lines of a 60 Hz of the amide signals which usually prohibits accurate extraction of RDCs.

The basic concept of utilizing anisotropic chemical shifts in macroscopically aligned membranes, pioneered by Opella and Cross, was initially developed using gramicidin A. The 15 amino acid peptide is readily synthesized with selective N or C-labels in the backbone, " or with N or F-labels in the side chains (as well as H for quadrupolar couplings). Numerous individual orientational constraints were thus used to determine the detailed p-helical structure of the membrane-bound ion channel, which was refined... 

H- and P- NMR studies on the methyl-ds amide derivative of the antibiotic amphotericin B incorporated into mechanically aligned bilayers of DPPC have been reported. Variations of the quadrupolar and dipolar splittings as a function of macroscopic sample orientation and temperature indicate that the amide derivative undergoes fast rotation about an axis parallel to the bilayer normal. 

To bypass receiver deadtime effects, wideline spectra are derived by Fourier transformation of the decay of an echo. By use of the Hahn echo and the stimulated echo (Section 2.2.1), wideline spectra of and other spin-5 nuclei can be measured, for example, but not the spectra of dipolar coupled spins and of quadrupolar nuclei like H. The magnetization of nuclei with spin / = 1 can be refocused by the quadrupole echo or the solid echo, and by the Jeener-Broekaert echo or the alignment echo [Slil] (Fig. 3.2.6). 

Fig. 1. Schematic diagrams of the director field distortions black lines) around particles in an aligned nematic liquid crystal. For a normal anchoring of the liquid crystal molecules at the surface of the particles, there are two possible configurations, a Dipole configuration with a companion point defect (indicated by an arrow) located in the immediate vicinity of the particle, b Quadrupolar Saturn-ring configuration with a disclination ring surrounding the particle at the equator...

---