Overhauser dynamic nuclear polarization

Recendy, Song et al. measured the local water diffusion coefficient and proton diffusion coefficient Dp within 5—10 A of spin probes that are partitioned into selectively different local environments of the swollen NAFION using Overhauser dynamic nuclear polarization relaxometry (ODNP) for nuclei of water at 9.8 GHz. This experiment concluded that... 

One technique, Overhauser Dynamic Nuclear Polarization (ODNP), is based on the well-known chemical shift of water in NMR spectra. Ordinarily, the liquid water signal intensity is low however, intensity can be magnified 1000-fold by addition of a nitroxide spin label such as TEMPO. Precession of the unpaired electron in TEMPO at the Larmor frequency results in Nuclear Overhauser-mediated polarization of the protons in water. These get polarized within 15 A of the spin labels and then relax with a relaxation time determined by the local diffusivity, i.e. in bulk water, the diffusivity is high and so relaxation is rapid by contrast, in hydration layers, relaxation takes 10-fold longer than in bulk water. Next, the trick is to covalently tether spin labels to surfaces of interest and measure how relaxation rates in hydration layers change as adhesive proteins approach and locally dehydrate the surfaces. 

Methods of disturbing the Boltzmann distribution of nuclear spin states were known long before the phenomenon of CIDNP was recognized. All of these involve multiple resonance techniques (e.g. INDOR, the Nuclear Overhauser Effect) and all depend on spin-lattice relaxation processes for the development of polarization. The effect is referred to as dynamic nuclear polarization (DNP) (for a review, see Hausser and Stehlik, 1968). The observed changes in the intensity of lines in the n.m.r. spectrum are small, however, reflecting the small changes induced in the Boltzmann distribution. 

Hence, provided that I g is known and that R has been determined by means of an independent experiment, provides the cross-relaxation rate ct. This enhancement is called nuclear Overhauser effect (nOe) (17,19) from Overhauser (20) who was the first to recognize that, by a related method, electron spin polarization could be transferred to nuclear spins (such a method can be worked out whenever EPR lines are relatively sharp it is presently known as DNP for Dynamic Nuclear Polarization). This effect is usually quantified by the so-called nOe factor p... 

The second approach is the use of the dynamic nuclear polarization (DNP) detection principle. Dorn and co-workers have pioneered the application of this technique [9,10], Whereas the NOE enhancement of 13C nuclei in the conventional 13C H recording is dependent upon the 7h/7c ratio (NOE = Th/ Tc = 2 1), the DNP enhancement relates to the ye/yuc ratio (2640 1). In an electron-nucleus spinsystem, the electron-electron transitions are saturated by microwave irradiation and magnetization transfer from electron to nucleus (Overhauser effect) occurs via a scalar and/or dipolar mechanism. The DNP enhancement, A, is described by the following equation ... 

Key Words Dynamic nuclear polarization, DNP, Overhauser effect, Hyperpolarization, Sensitivity enhancement,... 

First attempts to explain the new NMR phenomena invoked electron-nuclear cross-relaxations in intermediate radicals and were based on a formalism similar to that of dynamic nuclear polarization or Overhauser effects ).Accord-... 

Such anomalous NMR spectra as observed in the above reactions have been called Chemically Induced Dynamic Nuclear Polarization (CIDNP) . CINDP should be due to nonequilibrium nuclear spin state population in reaction products. At first, the mechanism of CIDNP was tried to be explained by the electron-nuclear cross relaxation in free radicals in a similar way to the Overhauser effect [4b, 5b]. In 1969, however, the group of Closs and Trifunac [6] and that of Kaptain and Oosterhoff [7] showed independently that all published CIDNP spectra were successfully explained by the radical pair mechanism. CIDEP could also be explained by the radical pair mechanism as CIDNP. In this and next chapters, we will see how CIDNP and CIDEP can be explained by the radical pair mechanism, respectively. 

On the dynamics of the neutral soliton, in 1980 Nechtschein and co-workers have demonstrated the evidence for the rapid diffusion of the neutral soliton along a one-dimensional chain from the observation of the pure Overhauser effect (OE) in trans-PA using a dynamic nuclear polarization (DNP) experiment and from the l/v frequency dependence of the H NMR spin-lattice relaxation rate 7Yil [143]. These observations give quantitative estimations that the pure OE implies the condition T (iJc 10" rad/s for the... 

Good evidence for the rapid motion and trapping of the soliton is demonstrated by a DNP experiment as shown in Figure 6.30 [146,173]. The dynamic nuclear polarization (DNP) experiments are carried out at 9 GHz between 1.5 and 300 K in cis [143,174] and irons PA [143,145,146,173,174]. At room temperature the pure Overhauser effect (OE) was observed in a -irans-PA without air or oxygen but a mixed solid state effect (SSE) together with OE was found in c/s-rich PA without air [143,174] and al -lrans-PA with air [146]. In particular, below 150 K, the mixed effect was observed, even in a -trans-PA without air [145,173], The OE is characteristic of dynamic interaction between nuclear spins and electron spins, with rapid motion, On the other hand, the SSE is of the static interaction between them. In other words, observation of the pure OE is clear evidence for the electron spin for motion with an inverse of the correlation time comparable with or larger than Wj,. At 300 K, the pure OE observed in all-/ra s-PA, is consistent with the conclusions for the neutral soliton to diffuse rapidly compared with 1/We as concluded from the ESR linewidth narrowed by motion [53] and the proton NMR 7Y ] [143] as a function of frequency over a broad range. 

Figure 6.30. Enhancement of the proton NMR signal amplitude (P/P -l) is shown as a ftinction of microwave frequency as taken by the dynamic nuclear polarization experiments, where P and / are the signal amplitude with and without microwave pumping, respectively [143,146,173,174], The Overhauser effect (OE) typical to the mobile spins makes a shaip peak at the Lannor frequency cve of the electron spin but the Solid State Effect (SSE) shows two extrema separated by o from (O charactcrictic with the fixed paramagnetic spins [8], The solid curves are a guide for the eye. (a). t a is-(CH), (A//pp 0.8 G) and , trs-rich (CH)> (Affpp 7.8 G) 1174]. (b) The temperature dependence of the peak-ratio in //ww-(CH), 173], y/x = 0 means pure Overhauser enhancement and y/x— 1, the solid slate effect [173]. Note the change from the mobile above 150 K to the trapped below 150 K, Most of the solitons are trapped below 10 K, (after (a) [174] and (b) [173])...

A developing approach measures by NMR the local hydration dynamics around nitroxides at room temperature via Overhauser d3mamic nuclear polarization (DNP). 

Hyperpolarization is a term which collects a set of dilferent signal-enhancement techniques in NMR spectroscopy. They have in common that they create population differences (polarizations) of the nuclear spin, which are far larger than the thermal equilibrium value (Boltzmann polarization). Following a proposal by Overhauser in 1953, which was experimentally verified by Carver and Slichter Dynamic Nuclear Polarization (DNP) schemes were developed. These techniques employ an auxiliary reservoir of fast and effectively polarizable spins to generate spin order which is subsequently transferred to the spin system of interest. In case of DNP experiments the strong polarization of electron spins is transferred to the nuclear spins by means of microwave irradiation. In the early days DNP experiments were restricted to relatively low fields ( H-frequency below 60 MHz) due to the technical limitations of klystrons used as microwave sources. 

Another method of polarizing nuclear spins is dynamic nuclear polarization (DNP), whereby the comparably large electron spin polarization (see Fig. 1) is transferred to nuclear spins by saturating the electron resonance. DNP is almost as old as NMR spectroscopy, building on the aforementioned theoretical work by Overhauser [21] who predicted what is today known as the Overhauser effect (OE). DNP was soon after demonstrated experimentally by Carver and Slichter [22, 23]. The enhancement, s, that can be obtained by DNP is determined by the gamma ratio 7e/7n> which is 660 for protons and 2,625 for... 

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