Overhauser Dynamic Nuclear Polarization at high magnetic fields (9.4 T) on Lipid Bilayers
Final Report Abstract
The ultimate aim of this project was to increase the nuclear magnetic resonance (NMR) signal from spins in fluid lipid bilayers through the phenomenon of dynamic nuclear polarization (DNP). Thanks to the comparatively high operating NMR frequency of 400 MHz, signals from polar and non-polar lipid protons could be clearly resolved. The polarization of the hydrophobic interior of the lipid bilayers could thus be measured. Experiments were performed on bilayers composed of DOPC (1,2-dioleoyl-sn-glycero- 3-phosphocholine) lipids and doped at a ratio of 20:1 with spin-labeled PSPC (1-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine) lipids. The selective labeling of the stearoyl chain of PSPC at the n-th carbon with the nitroxide spin label Doxyl yields the spin-labeled lipid n-Doxyl-PC. Separate experiments were performed for n = 5, 10 and 16 in order to explore the dependence of the polarization on the position of the electronic spin inside the hydrophobic core of the lipid bilayer. All these experiments demonstrated that, in addition to the anticipated Overhauser effect of polarization transfer, the so-called solid effect was also operative. It was thus necessary to properly model both of these DNP mechanisms in order to analyze the DNP measurements. The existing theoretical description of the solid effect, which had been successfully used for 65 years, turned out not to be easily generalizable to the liquid state, where the molecules undergo diffusive motions. In this project, we developed and published a new theoretical description of the solid effect, which explicitly accounted for the dynamics of the spin coherences. In a second publication, the developed time-domain description was extended to allow for the diffusive modulation of the dipolar interaction between the electronic and nuclear spins, thus modeling the situation in liquids. The ensuing formalism explained the presence of a strange feature in the DNP field profile of the free radical BDPA in DMPC lipid bilayers, which had been previously ascribed to the DNP mechanism of thermal mixing. Then, in a third publication, we further allowed for the slow rotational diffusion of the polarizing agent. After this generalization, it was finally possible to analyze quantitatively the DNP data of 10- Doxyl-PC and 16-Doxyl-PC in DOPC lipid bilayers. By fitting the DNP field profiles with simultaneous contributions from the Overhauser and solid effects, we could quantify dynamics on, respectively, fast (sub-picosecond) and slow (a few nanoseconds) time scales. The slow time scale, accessed here for the first time by modeling the solid-effect DNP in liquids, was in good agreement with the known translational diffusion of the lipids in the plane of the lipid bilayer.
Publications
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Non-perturbative treatment of the solid effect of dynamic nuclear polarization. Magnetic Resonance, 4(1), 129-152.
Sezer, Deniz
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The solid effect of dynamic nuclear polarization in liquids – accounting for g -tensor anisotropy at high magnetic fields. Magnetic Resonance, 4(2), 243-269.
Sezer, Deniz; Dai, Danhua & Prisner, Thomas F.
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The solid effect of dynamic nuclear polarization in liquids. Magnetic Resonance, 4(1), 153-174.
Sezer, Deniz
