DNP is the technique that is most generally applicable in the production of hyperpolarized molecular probes and the principle of these methods is briefly detailed as follows. DNP hinges on the transfer of electron spin polarization from a free radical to nuclear spins by microwave irradiation [19,22,23]. This transfer is best conducted in amorphous samples that assure the homogenous distribution of electron and nuclear spins. DNP is typically performed at low temperatures (<1.5 K) and at high magnetic fields (>3 T) where the electron spin polarization approaches 100% (Figure 1A). Dedicated instruments for DNP under these conditions achieve solid-state polarizations of NMR active nuclei above 10% and are commercially available as so-called ��polarizers�� (http://www.oxford-instruments.

com [24]). The DNP approach to hyperpolarization has gained broad chemical and biological relevance due to a dissolution setup that harvests a hyperpolarized molecular probe by washing the frozen glass of ~1 K temperature rapidly out of a polarizer with heated buffer [25]. Hyperpolarization losses during this dissolution step can be kept to a minimum and molecular probes with polarizations enhanced by several orders of magnitude can be produced for use in biological assays at ambient temperature and for detection with high-resolution liquid state NMR spectroscopy. A principal limitation of using hyperpolarized molecular probes is the short hyperpolarization lifetime of seconds to a few minutes for non-protonated sites in small molecules.

Hyperpolarized tracers employ a variety of NMR active nuclei with sufficiently slow hyperpolarization loss (determined by the longitudinal T1 relaxation time of the nucleus) to perform assays on the minute time scale (Table 1). In practice, these probes combine isotope enrichment with hyperpolarization in GSK-3 order to achieve up to >106 fold signal enhancement over non-informative cellular background signals due to the combined (multiplicative) effect of isotope enrichment and hyperpolarization. The generation and detection of hyperpolarized NMR signal is particularly useful for the nuclei in Table 1 [15,16,25�C28], as the low magnetogyric ratios relative to 1H leads to small equilibrium polarizations (Figure 1A) and the generation of smaller recorded signal by Faraday induction in the NMR coil (see molar receptivity in Table 1) [29]. At the same time, long relaxation times necessitate long inter-scan recycle delays for some of these nuclei in conventional NMR, thus aggravating their poor utility in conventional NMR detecting nuclear magnetism under conditions of equilibrium spin polarization.Table 1.Nuclei used in hyperpolarized NMR probes.