Dissolution dynamic nuclear polarization (d-DNP) is a powerful method to enhance nuclear magnetic resonance (NMR) signals by several orders of magnitude, notably in 13C-labelled metabolites. The production of 13C-hyperpolarized metabolites has opened the way to a broad range of novel experiments, such as the detection of intermediates in fast chemical reactions the observation of protein folding in real time or the detection and monitoring of cancer in humans. In d-DNP experiments, the 13C metabolites are usually polarized at low temperatures (1.2 - 4.2 K) and moderate fields (usually 3.35 - 6.7 T) either directly6 or indirectly by 1H-13C cross-polarization (CP).
The sample formulation usually consists of a homogeneous aqueous mixture of paramagnetic polarizing agents (PAs) and metabolites sometimes containing a glass-forming agent such as glycerol. The frozen solution is then dynamically polarized by microwave irradiation. The formation of a glass upon freezing is critical for efficient DNP. Alternatively, the PAs can be covalently attached to the surface of mesostructured materials that are impregnated with aqueous solutions of metabolites, in which case one can dispense with glass-forming agents. The PAs may also be generated in situ by ultraviolet irradiation16. However, intimate contact of the nuclear spins with the PA leads to paramagnetic relaxation that is exacerbated at low fields and thus requires dissolution of the sample directly in the cryostat. Hyperpolarized solutions have lifetimes T1(13C)B30–60 s in carboxyl groups that are sufficiently long for immediate imaging or spectroscopy, but not for transport of the sample to a distant user site.
References: Transportable Hyperpolarized Metabolites. Xiao Ji, Aurélien Bornet, Basile Vuichoud, Jonas Milani, David Gajan, Aaron J. Rossini, Lyndon Emsley, Geoffrey Bodenhausen & Sami Jannin. Nature Communications, Article number: 13975 (2017) doi:10.1038/ncomms13975