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Magnetic resonance techniques are successfully utilized in a broad range of scientific disciplines and in a number of practical applications, with medical MRI being the most widely-known example. Currently, both fundamental and applied magnetic resonance are enjoying a major boost owing to the dramatic signal enhancement provided by the rapidly-developing field of spin hyperpolarization. Such techniques are able to enhance signal intensities in magnetic resonance by several orders of magnitude, and thus to largely overcome its major disadvantage of relatively low sensitivity compared to other analytical techniques. This provides new impetus for existing applications, and, even more importantly, this opens the gates to numerous novel and exciting possibilities in the broad fields of fundamental and applied magnetic resonance. There are many different techniques that fall under the umbrella term “hyperpolarization”. Existing reviews cover the various subfields, but they are mostly addressed separately, and are seldom perceived as integral parts of the same field. In this review we attempt to unify the many methods that are used to hyperpolarize nuclear spins into one picture. Specifically, before delving into the individual techniques, we provide a detailed analysis of the underlying principles of spin hyperpolarization. We attempt to uncover and classify the origins of hyperpolarization; to establish its sources and the specific mechanisms that enable the flow of polarization from a source to the target nuclear spins. After outlining the inner workings of hyperpolarization, we give a more detailed analysis of individual hyperpolarization techniques: the mechanisms by which they work, fundamental and technical requirements, characteristic applications, unresolved issues and possible future directions. While a substantial progress has been achieved in the field of spin hyperpolarization in recent years, the continuing growth of activity indicates that this is just the beginning. It is unlikely that fundamentally new sources of hyperpolarization will be uncovered in the near future, but we expect the field to flourish as new ways to improve and utilize current hyperpolarization techniques are identified and implemented. There is great scope for cross-fertilization between known methods, and developments in one area (e.g., prolonging polarization lifetimes, or creating more efficient excitation-detection schemes) can have a very broad impact across the entire field of hyperpolarization. We hope this review will facilitate this process, since advances in hyperpolarization will help to overcome existing challenges in magnetic resonance and enable novel applications.

Through the use of an Escherichia coli strain deficient in menaquinone biosynthesis, purified nitrate reductase A (NarGHI)-enriched inner membrane vesicles were titrated and monitored by electron paramagnetic resonance (EPR) spectroscopy, revealing the formation of protein-bound ubisemiquinone (USQ) species. Two-dimensional ESEEM (HYSCORE) experiments on these radicals revealed the same magnetic interaction with an (14)N nucleus as found for menasemiquinone stabilized at the Q(D) site of E. coli NarGHI and assigned to His66 N(delta), a distal heme axial ligand. Moreover, this signature was lost in the NarGHI(H66Y) mutant, which is known to be unable to react with quinols. These findings demonstrate that NarGHI-bound USQ can be formed and detected by EPR. They also provide the first direct experimental evidence for similar binding of natural menasemiquinones and ubisemiquinones within the same protein Q site of NarGHI.

Q -1 “metallic” range (01, = 1.8(2) cm s , the diffusion constant Dl, exhibits a temperature activated behaviour in the low temperature range, at variance with the prediction invoking the model of an intrinsic semiconductor. A low-temperature divergence of the homogenous ESR line width, indicative of one-dimensional fluctuations, was also observed. LOW-DIMENSIONAL ORGANIC conductors have attracted much interest since the discovery of superconductivity in several different crystalline charge transfer (CT) complexes of the Bechgaard-type [l-3]. Up to date no system using pure hydrocarbons (HC) as donor molecules were found to exhibit superconductivity. Instead they are characterized by broad metal-toinsulater (MI) transitions in the temperature range of 120-200 K. In all systems studied so far the low temperature ground state was non-magnetic, thus rendering the HC charge transfer salts ideal candidates for magnetic resonance investigations. However, conventional cw ESR probing susceptibility and line width is of limited value for unravelling the microscopic behaviour of the charge transport. A few systems, exhibiting extremely narrow ESR lines in the range of IO-30mG, have been investigated by pulsed ESR. Especially the CT-complex (fluoranthenyl)sX was utilized as a model compound [4]. In the high-temperature metallic phase the extreme onedimensionality was proven from the analysis of the Hahn-echo decay function with [S] and without [6] the application of an external field gradient. The application of the well-established experimental method to other HC-CT complexes was impeded by electron-spin dephasing times T,, in the sub-MS range. We therefore developed a pulsed ESR apparatus with optimized time resolution, allowing the detection of the echo signal within at least 200ns referenced to the beginning of the first microwave pulse. With such a time resolution not only the spin diffusion constant in the metallic regime can be measured but also the relaxation rates in the low-temperature insulating phase can be investigated. In particular, we were interested, if remnants of SDW fluctuations could be detected via T,, in the low-temperature regime, were susceptibility measurements showed a T-O.” behaviour, indicative of a random exchange Heisenberg antiferromagnetic chain (REHAC) [7].

The electron-spin-echo decay function was determined for the conducting phase of [(C16H10)2]+X- in the temperature range 183-300 K. The decay function exhibited an exp[-(γt)3/2] dependence, characteristic of one-dimensional spin diffusion. The ratio 5×106≤D∥/D⊥≤5×108 for the inner-stack-to-out-of-stack diffusion rates was determined from the characteristic time t0, at which the electron-spin-echo decay function changed to the "conventional" exp(-2γ′τ)) form. The deduced value 2×1013≤D∥≤2×1016 rad/s is consistent with the bulk dc conductivity and with recently determined nuclear spin-lattice relaxation rates. The overall assumption of highly mobile electronic spins was confirmed by an Overhauser-type experiment.