Rotational sidebands

In these spectra, the protein has been regenerated with retinal specifically 13 C labeled at positions 11,12 and 13, and in each case the retinal resonance exhibits a sharp centerband at the isotropic chemical shift and is flanked by rotational sidebands. Other lines in the spectrum are the natural-abundance 13C resonances of the protein carbonyls ca 175 ppm) and aliphatic carbons (0-100 ppm). Contributions from the Ammonyx-LO detergent in these spectra are seen in the different intensities in the 0-100 ppm region. Ammonyx-LO does not exhibit NMR resonances above 100 ppm. Spectra of the 9-cis pigment isorhodopsin are similar. Table 38 summarizes the isotropic chemical shifts from the solid-state NMR spectra of rhodopsin regenerated with retinal13 C labeled at each position along... 

FIGURE 39. MAS 13C NMR spectra of (a) 13C-11, (b) 13C-12 and (c) 13C-13 rhodopsin. Center-bands and rotational sidebands of the retinal resonances are marked with asterisks. Reprinted with permission from Reference 55. Copyright (1990) American Chemical Society... 

Table 5 Calculated dipolar rotational sideband intensities for an aromatic CH pair undergoing various molecular motions, under MAS at 1.894 kHz (from [40])...

When the sample is placed in a gas-driven rotor and spun at the rate of a few kilohertz under 6 equal 54.7° (Figure IB and C), the anisotropic part is zero. In such an experiment, the so-called MAS, narrowing of the resonance lines is observed. However, it is only true when the spin rotation is larger than anisotropy A8 given in hertz. In cases when the rotor rotation (cor) < AS, the isotropic signal is flanked on both sides by rotational sidebands. In a strong applied field B0, the only measurable quantity of the shift tensor is the component oriented along z-axis in the laboratory frame ... 

Various ID and 2D experiments related to first-order quadrupole broadening also benefit from the fast rotation frequency. For heavier nuclei, quadrupole interaction and chemical shift anisotropy averaging at higher speeds results in better focusing of signal intensity to the remaining rotation sidebands (Fig. 5). 

Following the first procedure, mineral standards were synthesized and characterized by P MAS NMR [31,42]. BD, CP and DD spectra of mineral standards and bone samples were compared, giving particular attention to chemical shifts (Table 2),linewidths and rotational sideband patterns [35]. A typical set of spectra, acquired for mineral standards, is given in Fig. 7. Each BD spectrum comes from all P-sites, while its CP counterpart exposes a fraction of the P-sites capable of obtaining polarization from surrounding protons. The DD experiment is performed with a time interval without proton decoupling inserted just after CP. This is called a dipolar suppression period (DS), because... 

We have so far discussed centrebands of the P CP/MAS signal from bone mineral, but rotational sidebands are also of considerable interest. The P MAS sidebands disclose phosphorus sites with substantial chemical shift anisotropy, while CP reveals those located close to protons. In consequence, the CP rota-... 

Table 4 Fitted parameters of the H—PCP kinetics for the first, high-frequency rotational sidebands (MAS at 1 kHz, 81 MHz for P) of phosphate and human cortical hone samples (B1-B3) [20]. HAh and HAc denote hydrated (as-prepared) and calcined HA, respectively. The I-H-S CP model has heen used. The sideband amphtudes Ajj are in % of the absolute amplitudes A j of the corresponding centrebands. AH the time constants are reported in ms...

The resolution of conventional proton MAS spectra presented by Kaflak-Hachulska et al. [20] is much better than those discussed earlier (Figs. 19 and 20). For MAS at 40 kHz rotational sidebands are missing. Ultra-high speed MAS... 

Because the Fourier transform of a delta comb is a delta comb again, 5((w) corresponds to a stick spectrum similar to a MAS spectrum with rotational sidebands (cf. Fig. 3.3.7(c)). 

Fig. 5. (a) Two-dimensional 1SN- H dipolar/chemical shift spectrum obtained from [lSN]acetylvaline showing the dipolar and chemical shift projections. Linewidths are typically 50-150 Hz for the dipolar and 0.5-1.0 ppm for the chemical shift dimension. vR = 1.07 kHz. (b) Dipolar cross-sections taken from the 2D spectrum. Each trace runs parallel to ivh through a particular rotational sideband in u>2. (i) Experimental i5N-H spectra from [15N]acetylvaline, vR = 1.07 kHz. The two simulations (ii and iii) assume two different orientations of the dipolar and shielding tensors, (ftD = 22°, D = 0°) and (Ai = 17°, aD = 0°), respectively, and illustrate the subtle differences in orientation which can be detected in the spectra. 

Schaefer and coworkers, in another chapter in this text, used 1h - 13(j dipole-dipole "line shapes" obtained in a very clever way to investigate rotational motion of the aromatic rings in polystyrene. The method used a WAHUHA pulse sequence to decouple proton-proton dipolar interactions, cross polarization to enhance signal acquisition and an overall sampling technique synchronous with the sample rotation. The dipole-dipole interaction was mapped in rotational sideband spectra obtained from 16 "normal" CP/MAS spectra. The method, though somewhat involved, provided a measure of dipole-dipole line-shapes which can be interpreted in terms of side-chain rotation in the polymer. 

Dipolar Rotational Sideband Intensities for Poly(p-isopropylstyrene) with Magic Angle Spinning at 1894 Hz... 

Table III Protonated Aromatic s of some Polystyrenes Scaled by the Amplitude of Root-Mean-Square Angular Fluctuations Deduced from Experimental Dipolar Rotational Sideband Intensities...

Figure 22 Br MAS NMR spectra of powdered alkali metal bromides. All experiments were carried outat Bq = 11.7T and used v,ot = 10 kHz. (A) a mixture of LiBr and KBr (B) a mixture of CsBr, RbBr, KBr and NaBr (C) 0.03 M NaBr in D2O. For (B), the single, double and triple primes correspond to the rotational sidebands of NaBr, KBr and CsBr, respectively. The sidebands for RbBr were not fully resolved due to overlap with the KBr sidebands, which are shown for comparison in (A). Inset measured bromine chemical shift values (errors).

Rotation of powdered samples involves another difficulty [27,41,42] magic-angle spinning makes the orientation of each crystallite time-dependent. In the presence of substantial chemical shift anisotropy, this modulation leads, after Tr/2 pulses, to unwanted rotor lines [41], which appear in both dimensions and form cross-peaks of their own. The situation becomes even more complicated when or pulses or a train of different pulses is involved, especially if the magic angle is set incorrectly [42]. In addition, the rotational sidebands can be enhanced and the observed peaks correspondingly broadened for certain MAS frequencies [27]. 

The sensitivity of the C-NMR technique is limited by diverse factors such as rotational sidebands, couplings, and so on, and at least by the accumulation time. For practical use, the... 

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