Solid-state DNP

In this review, we focus on solution-state DNP studies utilizing the Overhauser effect. Solid-state DNP is not included in this review,9,10 nor are experiments where the hyperpolarization is performed in the solid state before melting11,12 or dissolving8,13,14 the sample into solution. Chemically induced DNP (CIDNP)15-17 and the Overhauser effect of conduction electrons18,19 are also excluded, and the interested reader is referred to the cited papers for more information. The principles and prospects of proton-electron double resonance imaging (PEDRI) are briefly included, as a thorough coverage of this research area is out of the scope of this document. 

Solid-state DNP (SS-DNP) at low temperatures (typically 90 K), driven by the cross effect (CE), using high-power microwave sources in conjunction with MAS for solid-state NMR. 

Table 4.24 summarises the main features of solid-state photolysis-GC for polymer/additive analysis. Photolytic degradation products obtained from PE/1 % DLTDP and PE/1 % DSTDP, PMMA/9 % DNP and PVC/19 % DOP were examined [189]. The method has found little follow-up. 

An important example of the solid state polymerization of disubstituted diacetylenes is 1,6-bis(2,4-dinitro-phenoxy)-2,4-hexadiyne (DNP) shown in Figure 9 - DNP is unique among known diacetylenes... 

Recently, Bunton et al. synthesized the Ci6H33-attached triamine copper(II) complex 8b [26], which promoted the hydrolysis of diphenyl 4-nitrophenyl phosphate (DNP) at alkaline pH. The catalytic activity was almost the same as that for Menger s previously reported comicellar system with 3b. The active species was proposed to be the hydroxide-bound copper(II) complex 8b. The pvalue of the copper(II)-bound water molecule was speculated to be about 8 from the fact that the nonalkylated and tetradecyl homologous copper(II) complexes have a pvalue of 8 (determined by DNP hydrolysis kinetics). Since the micellar metal complex 8b was not fully characterized either in the solid state or in a micellar solution, its hydrolysis mechanism remains to be elucidated. 

The only example of a twisted cuboctahedron was reported by Kimura and coworkers [159]. Cuboctahedron 63 was obtained by the 4 4 self-assembly of fris(Znn-cyclen) complex 61 with trianionic trithiocyanurate (TCA3-, 62) upon guest encapsulation and was characterized both in the solid state and in solution (Fig. 33). Suitable guests are lipophilic organic molecules of matching size, such as, ([D4]-2,2,3,3)-3-(trimethylsilyl)propionic acid (TSP), 1-adamantanecarboxylic acid, 2,4-dinitrophenol (2,4-DNP), adamantine (ADM), or the tetra-n-propylammonium (TPA) cation. 

Solid-state NMR signal enhancements of about two orders of magnitude (100-400) have been observed in dynamic nuclear polarization (DNP) experiments performed at high magnetic field (5 T) and low temperature (10 K) using the nitroxide radical 4-amino TEMPO as the source of electron polarization. ... 

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. 

Mak-Jurkauskas ML, Bajaj VS, Homstein MK et al (2008) Energy transformations early In the bacteriorhodopsin photocycle revealed by DNP-enhanced solid-state NMR. Proc Natl Acad Scl USA 105 883-888... 

Thus a high power microwave source is required for DNP experiments. Currently, the microwave sources can be classified into two different types solid-state and vacuum electronic devices. Several excellent reviews of the introduction of -and outlook for - microwave source technology are available [41 3]. 

Recent developments of DNP have shown great potential in solid-state and liquid-state NMR spectroscopy and in MRI. As promising as DNP is, the future development of hyperpolarization transition may require great improvement in the following aspects ... 

New DNP methods should be able to overcome the limitation of the very short lifetime of the hyperpolarization, and should be integrated into the currently available solid-state and liquid-state NMR experiments. 

A well-known and important phenomenon in the area of nuclear-spin resonance (NMR) in gases, liquids, or solid samples is dynamic nuclear-spin polarisation (DNP) (see e.g. [M6]). This term refers to deviations of the nuclear magnetisation from its thermal-equilibrium value, thus a deviation from the Boltzmann distribution of the populations of the nuclear Zeeman terms, which is produced by optical pumping (Kastler [31]), by the Overhauser effect [32], or by the effet solide or solid-state effect [33]. In all these cases, the primary effect is a disturbance of the Boltzmann distribution in the electronic-spin system. In the Overhauser effect and the effet solide, this disturbance is caused for example by saturation of an ESR transition. Owing to the hyperfine coupling, a nuclear polarisation then results from coupled nuclear-electronic spin relaxation processes, whereby the polarisation of the electronic spins is transferred to the nuclear spins. 

An alternative method, termed DNP (Dynamic Nuclear Polarisation), relies on the transfer of polarisation from unpaired electrons onto nuclear spins in the solid state but has been further developed to significantly enhance the sensitivity of solution-state NMR, as described briefly below. The technique has been applied to boost the polarisation levels for low-7 heteronuclear spins in particular and shows most promise to date for and N observation the initial report on the technique described a gain in signal-to-noise in excess of 10,000-fold for both these elements when compared to data collected at thermal equilibrium [115]. The hyperpolarisation of P and Si has also been demonstrated. 

Y.S. Ong, A. Lakatos, J. Becker-Baldus, K.M. Pos, C. Glaubitz, Detecting substrates bound to the secondary multidrug efflux pump EmrE by DNP-enhanced solid-state NMR, J. Am. Chem. Soc. 135 (2013) 15754-15762. 

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