Motionally-driven spin diffusion

S-spin zero-quantum line shape is sufficiently broadened to allow S-spin polarization transfer. The broadening of the zero-quantum line can also be caused by chemical exchange ( motionally-driven spin diffusion ) [15, 18]. In a different approach one decouples the protons and removes the chemical-... 

The dynamics of the rotor speed can be conveniently analysed and adjusted by feeding a signal from the rotor motion monitor (via an optical fibre) to a console ADC (Fig. 7). The corresponding spectra are shown in Fig. 8. The faster sweep clearly reduces K12, and to a lesser extent K13. Both direct complementary peaks (1-Kmn) and relayed peaks depend on the product of complementary transfer coefficients, and correlate well with the expected influence of the sweep rate variation. The observed relay process can be shorted by proton-driven spin diffusion. Consequently, efficient rotation-speed independent (or carefully synchronized) decoupling is required during the entire mixing period. 

As an alternative means, it is a natural consequence to expect that high-resolution sohd-state NMR could be conveniently utilized to reveal the 3D structure and dynamics of a variety of membrane proteins, because the expected NMR line widths available from sohd-state NMR are not any more influenced by motional fluctuation of proteins under consideration as a whole as encountered in solution NMR. For instance, an attempt was made to determine 3D structure of uniformly C-labeled a-spectrin SH3 domain as a globular protein, based on distance constraints estimated from proton-driven spin-diffusion (PDSD) measure-... 

Finally, structural investigations of a human calcitonin-derived carrier peptide in a membrane enviromnent by solid-state NMR have been reported. The typical axially symmetric powder patterns of NMR spectra were used to confirm the presence of lamellar bilayers in the samples studied. The chemical shift anisotropy of the NMR spectra was monitored in order to reveal weak interaction of the peptide with the lipid headgroups. In addition, paramagnetic enhancement of relaxation rates and NMR order parameters of the phospholipid fatty acid chains in the absence and presence of the carrier peptide were measured. All peptide signals were resolved and fully assigned in 2D proton-driven spin diffusion experiments. The isotropic chemical shifts of CO, C and provided information about the secondary structure of the carrier peptide. In addition, dipolar eoupling measurements indicated rather high amplitudes of motion of the peptide. 

The temperature dependence of the spectral spin diffusion and crossrelaxation was examined by Mueller et a/.287,288 with spin- and spin-1 systems. They showed that the diffusion rate can be strongly temperature dependent if it is motionally driven. It is therefore, unreliable to discriminate spin diffusion and chemical exchange by variable-temperature measurement of 2D exchange spectra. Mueller et al. suggested that the dependence of the polarization transfer rate on the spectral difference of the relevant resonances should be measured in a single crystal to safely distinguish the two different polarization transfer processes (see also ref. 289). They also explained satisfactorily why the relaxation of the quadrupolar order is much faster than the Zeeman order. This... 

It should be realized that unlike the study of equilibrium thermodynamics for which a model is often mapped onto Ising system, elementary mechanism of atomic motion plays a deterministic role in the kinetic study. In an actual alloy system, diffusion of an atomic species is mainly driven by vacancy mechanism. The incorporation of the vacancy mechanism into PPM formalism, however, is not readily achieved, since the abundant freedom of microscopic path of atomic movement demands intractable number of variational parameters. The present study is, therefore, limited to a simple spin kinetics, known as Glauber dynamics [14] for which flipping events at fixed lattice points drive the phase transition. Hence, the present study for a spin system is regarded as a precursor to an alloy kinetics. The limitation of the model is critically examined and pointed out in the subsequent sections. 

This is the simplest motional model for relaxation, in which all of the spins are rigidly attached to the rotating macromolecule, and the relaxation is driven entirely by rotational diffusion. While it is clear that this model is not rigorously correct for any macromolecule, it has proved to be a useful and powerful framework for the elucidation of solution structures from NMR data. Its limitations can be explored only through an explicit consideration of the effects of molecular motion, which is the subject of the next section. 

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