Dynamic nuclear polarization modelling

The first discovery of chemically induced dynamic electron polarization (CIDEP) was made by Fessenden and Schuler in 1963 (58). These authors observed the abnormal spectra of the H atoms produced during the irradiation of liquid methane. The low-field line in the esr spectrum was inverted compared to the corresponding high-field line. The related chemically induced dynamic nuclear polarization effect (CIDNP) was reported independently four years later by Bargon et al. (22) and by Ward and Lawler (134). Because of the wider application of nmr in chemistry, the CIDNP effect immediately attracted considerable theoretical and experimental attention, and an elegant theory based on a radical-pair model (RPM) was advanced to explain the effect. The remarkable development of the radical-pair theory has obviously brought cross-fertilization to the then-lesser-known CIDEP phenomenon. 

Membranes and model membranes exhibit liquid crystalline behavior and this has been exploited in a number of studies to obtain valuable information on the structure and dynamics of membrane associated peptides and proteins as well as on the interaction of the peptides with the membranes themselves. NMR spectroscopy of nuclei such as proton, carbon, deuterium, nitrogen and phosphorus has been utilized for such purposes. Structure elucidation of membrane-associated peptides and proteins in oriented bilayers by solid-state NMR spectroscopy has been reviewed. A survey on the use of static uniaxially oriented samples for structural and topological analysis of membrane-associated polypeptides is available. The theoretical background has been dealt with and a number of examples of applications provided. In addition, ongoing developments combining this method with information from solution NMR spectroscopy and molecular modelling as well as exploratory studies using dynamic nuclear polarization solid-state NMR have been presented. The use of N chemical shift anisotropy, dipolar interactions and the deuterium quadrupolar split-... 

A novel analytieal tool for the selective detection of local water inside soft mol. assemblies (hydrophobic cores, vesicular bilayers, and micellar structures) suspended in bulk water has been presented. Through the use of dynamic nuclear polarization (DNP), the NMR signal of water is ampUfied, as it interacts with stable radicals that possess about 658 times higher spin polarization. Stable nitroxide radicals covalently attached along the hydrophobic tail of stearic acid molecules that incorporate themselves into surfactant-based micelle or vesicle structures have been used, allowing to study the local water content and fluid viscosity inside oleate micelles and vesicles and Triton X-100 micelles to serve as model systems for soft molecular assembhes. ... 

The simulations to investigate electro-osmosis were carried out using the molecular dynamics method of Murad and Powles [22] described earher. For nonionic polar fluids the solvent molecule was modeled as a rigid homo-nuclear diatomic with charges q and —q on the two active LJ sites. The solute molecules were modeled as spherical LJ particles [26], as were the molecules that constituted the single molecular layer membrane. The effect of uniform external fields with directions either perpendicular to the membrane or along the diagonal direction (i.e. Ex = Ey = E ) was monitored. The simulation system is shown in Fig. 2. The density profiles, mean squared displacement, and movement of the solvent molecules across the membrane were examined, with and without an external held, to establish whether electro-osmosis can take place in polar systems. The results clearly estab-hshed that electro-osmosis can indeed take place in such solutions. 

The fast component is clearly related to electronic polarization, Pfast = Pd, while the slow component, connected to nuclear motions of the solvent molecules, is often called the orientational polarization (Pslow = Pot), or inertial component (PsioW = Pin)- This simplified model has been developed and applied by many authors we shall recall here Marcus (see the papers already quoted), who first had the idea of using Psiow as a dynamical coordinate. For description of solvent dynamical coordinates in discrete solvent models see Warshel (1982) and other papers quoted in Section 9. 

The analysis presented so far on the difrerent specificities of LR and SS descriptions of excitation processes within QM/continuum approaches also ap-phes to the polarizable QM/MM approaches. In those cases, however, the picture is simpler because there is no need to partition the polarization into dynamic and inertial terms as in continuum models, since the inertial (nuclear) degrees of freedom are considered expUcidy through the fixed multipolar expansion while the dynamic response is represented by the polarizable term, such as the induced dipoles in the ID formulation described earlier. 

Walls et al investigated the dynamics for dipole-coupled nuclear spin systems under conditions of high spin polarization both theoretically and numerically. Quantum spin simulations were performed which demonstrate that the transverse magnetization decays more rapidly as 9 > ti/2, where theta is the initial tip angle applied to the spins. Simple analytical models are used to demonstrate the increased decay of the transverse magnetization as 9 > ti/2. 

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